Publications
2023

156.(2023) Scientific Reports. 13, 1, 1064. Abstract
An addition of longchain, flexible polymers strongly affects laminar and turbulent Newtonian flows. In laminar inertialess viscoelastic channel flow, the supercritical elastic instability of nonnormal eigenmodes of nonHermitian equations at finitesize perturbations leads to chaotic flow. Then three chaotic flow regimes: transition, elastic turbulence (ET), and drag reduction (DR), accompanied by elastic waves, are observed and characterized. Here we show that independently of external perturbation strength and structure, chaotic flows above the instability onset in transition, ET, and DR flow regimes reveal similar scaling of flow properties, universal scaling of elastic wave speed with Weissenberg number, Wi, defined the degree of polymer stretching, and the coherent structure of velocity fluctuations, selforganized into cycling selfsustained process, synchronized by elastic waves. These properties persist over the entire channel length above the instability threshold. It means that only an absolute instability exists in inertialess viscoelastic channel flow, whereas a convective instability, is absent. This unexpected discovery is in sharp contrast with Newtonian flows, where both convective and absolute instabilities are always present in open flows. It occurs due to differences in nonlinear terms in an elastic stress equation, where except for the advective term, two key terms describing polymer stretching along the channel length are present.

155.(2023) Proceedings of the National Academy of Sciences of the United States of America. 120, 28, e230559512. Abstract
Inertialess viscoelastic channel flow displays a supercritical nonnormal mode elastic instability due to finitesize perturbations despite its linear stability. The nonnormal mode instability is determined mainly by a direct transition from laminar to chaotic flow, in contrast to normal mode bifurcation leading to a single fastestgrowing mode. At higher velocities, transitions to elastic turbulence and further drag reduction flow regimes occur accompanied by elastic waves in three flow regimes. Here, we demonstrate experimentally that the elastic waves play a key role in amplifying wallnormal vorticity fluctuations by pumping energy, withdrawn from the mean flow, into wallnormal fluctuating vortices. Indeed, the flow resistance and rotational part of the wallnormal vorticity fluctuations depend linearly on the elastic wave energy in three chaotic flow regimes. The higher (lower) the elastic wave intensity, the larger (smaller) the flow resistance and rotational vorticity fluctuations. This mechanism was suggested earlier to explain elastically driven Kelvin–Helmholtzlike instability in viscoelastic channel flow. The suggested physical mechanism of vorticity amplification by the elastic waves above the elastic instability onset recalls the Landau damping in magnetized relativistic plasma. The latter occurs due to the resonant interaction of electromagnetic waves with fast electrons in the relativistic plasma when the electron velocity approaches light speed. Moreover, the suggested mechanism could be generally relevant to flows exhibiting both transverse waves and vortices, such as Alfven waves interacting with vortices in turbulent magnetized plasma, and Tollmien–Schlichting waves amplifying vorticity in both Newtonian and elastoinertial fluids in shear flows.

154.(2023) Physical Review Fluids. 8, 5, 053301. Abstract
Polymers in nonuniform flows undergo strong deformation, which in the presence of persistent stretching can result in the coilstretch transition. This phenomenon has been characterized by using the formalism of nonequilibrium statistical mechanics. In particular, the entropy of the polymer extension reaches a maximum at the transition. We extend the entropic characterization of the coilstretch transition by studying the differential entropy of the polymer fractional extension in a set of laminar and random velocity fields that are benchmarks for the study of polymer stretching in flow. In the case of random velocity fields, a suitable description of the transition is obtained by considering the entropy of the logarithm of the extension instead of the entropy of the extension itself. Entropy emerges as an effective tool for capturing the coilstretch transition and comparing its features in different flows.
2022

153.(2022) Physical Review Fluids. 7, 8, 080701. Abstract[All authors]
Viscoelastic fluids are a common subclass of rheologically complex materials that are encountered in diverse fields from biology to polymer processing. Often the flows of viscoelastic fluids are unstable in situations where ordinary Newtonian fluids are stable, owing to the nonlinear coupling of the elastic and viscous stresses. Perhaps more surprisingly, the instabilities produce flows with many of the hallmarks of turbulence  even though the effective Reynolds numbers may be O(1) or smaller. We provide perspectives on viscoelastic flow instabilities by integrating the input from speakers at a recent international workshop: historical remarks, characterization of fluids and flows, discussion of experimental and simulation tools, and modern questions and puzzles that motivate further studies of this fascinating subject. The materials here will be useful for researchers and educators alike, especially as the subject continues to evolve in both fundamental understanding and applications in engineering and the sciences.

152.(2022) Physical Review Fluids. 7, 8, L081301. Abstract
We report frictional drag reduction and a complete flow relaminarization of elastic turbulence (ET) at vanishing inertia in a viscoelastic channel flow past an obstacle. We show that the intensity of the observed elastic waves and wallnormal vorticity correlate well with the measured drag above the onset of ET. Moreover, we find that the elastic wave frequency grows with the Weissenberg number, and at sufficiently high frequency it causes a decay of the elastic waves, resulting in ET attenuation and drag reduction. Thus, this allows us to substantiate a physical mechanism, involving the interaction of elastic waves with wallnormal vorticity fluctuations, leading to the drag reduction and relaminarization phenomena at low Reynolds number.

151.(2022) Physical Review Fluids. 7, 7, 073301. Abstract
Twodimensional channel flow simulations of FENEP (finitely extensible nonlinear elasticPeterlin) fluid in the elastoinertial turbulence (EIT) regime reveal distinct regimes ranging from chaos to a steady traveling wave which takes the form of an arrowhead structure. This coherent structure provides insights into the polymer/flow interactions driving EIT. A set of controlled numerical experiments and the study of transfer between elastic and turbulent kinetic energies highlight the role of small and largescale dynamics in the selfsustaining cycle of chaos in EIT flows.

150.(2022) Journal of Fluid Mechanics. 941, Abstract
We examine the response of an inertialess viscoelastic channel flow to localized perturbations. We thus performed an experiment in which we perturbed the flow using a localized velocity pulse and probed the perturbed fluid packet downstream from the perturbation location. While for low Weissenberg numbers the perturbed fluid reaches the measurement location as a single velocity pulse, for sufficiently high Weissenberg numbers and perturbation strengths, a random number of pulses arrive at the measurement location. The average number of pulses observed increases with the Weissenberg number. This observation constitutes a transition to a novel elastic pulsesplitting regime. Our results suggest a possible new direction for studying the elastic instability of viscoelastic channel flows at high elasticity through the growth of localized perturbations.

149.(2022) Low temperature physics (Woodbury, N.Y.). 48, 6, p. 492507 Abstract
We shortly describe the main results on elastically driven instabilities and elastic turbulence in viscoelastic inertialess flows with curved streamlines. Then we describe a theory of elastic turbulence and prediction of elastic waves Re ≪ 1 and Wi ≫ 1, which speed depends on the elastic stress similar to the Alfvén waves in magnetohydrodynamics and in a contrast to all other, which speed depends on medium elasticity. Since the established and testified mechanism of elastic instability of viscoelastic flows with curvilinear streamlines becomes ineffective at zero curvature, so parallel shear flows are proved linearly stable, similar to Newtonian parallel shear flows. However, the linear stability of parallel shear flows does not imply their global stability. Here we switch to the main subject, namely a recent development in inertialess parallel shear channel flow of polymer solutions. In such flow, we discover an elastically driven instability, elastic turbulence, elastic waves, and drag reduction down to relaminarization that contradict the linear stability prediction. In this regard, we discuss briefly normal versus nonnormal bifurcations in such flows, flow resistance, velocity and pressure fluctuations, and coherent structures and spectral properties of a velocity field as a function of Wi at high elasticity number.

148.(2022) Physical Review Fluids. 7, 6, 063901. Abstract
In this paper, we present experimental results and reveal that strong perturbations are not necessary for elastic instability to occur in straightchannel, inertialess, viscoelastic flows at high elasticity. We show that a nonnormalmode bifurcation is followed by chaotic fluctuations, selforganized as streamwise streaks, and elastic waves due to weak disturbances generated by a small cavity at the center of the top channel wall. The chaotic flow persists in the transition, elastic turbulence, and drag reduction regimes, in agreement with previous observations for the case of strong perturbations at the inlet. Furthermore, the observed elastic waves propagate in the spanwise direction, which allows us to confirm the elastic waves' linear dispersion relation directly. In addition, the spanwise propagating elastic wave's velocity depends on Weissenberg number with the same scaling that was previously observed for streamwise propagating waves, although their velocity magnitude is significantly smaller than what was previously observed for the streamwise ones.
2021

147.(2021) Physical Review. E. 103, 31, 033107. Abstract
Polymer molecules in a flow undergo a coilstretch phase transition on an increase of the velocity gradients. Modelindependent identification and characterization of the transition in a random flow has been lacking so far. Here we suggest to use the entropy of the extension statistics as a proper measure due to strong fluctuations around the transition. We measure experimentally the entropy as a function of the local Weisenberg number and show that it has a maximum, which identifies and quantifies the transition. We compare the new approach with the traditional one based on the theory using either linear OldroydB or nonlinear finite extensible nonlinear elastic polymer models.

146.(2021) Annual Review of Fluid Mechanics. 53, p. 2758 Abstract
A viscous solvent laminar flow may be strongly modified by the addition of a tiny amount of long polymer molecules, resulting in a chaotic flow called elastic turbulence (ET). ET is attributed to polymer stretching, which generates elastic stress and its back reaction on the flow. Its properties are analogous to those observed in hydrodynamic turbulence, although the formal similarity does not imply a similarity in physical mechanisms underlining these two types of random motion. Here we review the statistical and spectral properties and the spatial structure of the velocity field, the statistical and spectral properties of pressure fluctuations, and scaling of the friction factor of ET in wallbounded and unbounded flow geometries, as observed in experiments and numerical simulations and described by theory for a wide range of control parameters and polymer concentrations.
2019

145.(2019) Nature Communications. 10, 937. Abstract
Electron transport in twodimensional conducting materials such as graphene, with dominant electronelectron interaction, exhibits unusual vortex flow that leads to a nonlocal currentfield relation (negative resistance), distinct from the classical Ohm's law. The transport behavior of these materials is best described by low Reynolds number hydrodynamics, where the constitutive pressurespeed relation is Stoke's law. Here we report evidence of such vortices observed in a viscous flow of Newtonian fluid in a microfluidic device consisting of a rectangular cavityanalogous to the electronic system. We extend our experimental observations to elliptic cavities of different eccentricities, and validate them by numerically solving biharmonic equation obtained for the viscous flow with noslip boundary conditions. We verify the existence of a predicted threshold at which vortices appear. Strikingly, we find that a twodimensional theoretical model captures the essential features of threedimensional Stokes flow in experiments.

144.(2019) Nature Communications. 10, 652. Abstract
Speed of sound waves in gases and liquids are governed by the compressibility of the medium. There exists another type of nondispersive wave where the wave speed depends on stress instead of elasticity of the medium. A wellknown example is the Alfven wave, which propagates through plasma permeated by a magnetic field with the speed determined by magnetic tension. An elastic analogue of Alfven waves has been predicted in a flow of dilute polymer solution where the elastic stress of the stretching polymers determines the elastic wave speed. Here we present quantitative evidence of elastic Alfven waves in elastic turbulence of a viscoelastic creeping flow between two obstacles in channel flow. The key finding in the experimental proof is a nonlinear dependence of the elastic wave speed c(el) on the Weissenberg number Wi, which deviates from predictions based on a model of linear polymer elasticity.

143.(2019) Physical Review Letters. 123, 23, 234501. Abstract
We report the scaling relations between the exponents of the powerlaw decays of kinetic and elastic energies, pressure, as well as torque fluctuations in elastic turbulence (ET). The relations are derived by estimating that the divergent part of the elastic stress is much larger than its vortical part, and its contribution into the full elastic stress is dominant in the range of the power spectrum amplitudes observed experimentally in ET. The estimate is in line with polymer stretching by flow: the polymers are stretched mostly by the divergent part associated with a strain rate, whereas a rotational, or vortical, flow plays a minor role in the polymer stretching. The scaling relations agree well with the exponent values obtained experimentally and numerically in the ET regime of a viscoelastic fluid in different flow geometries.

142.(2019) Journal of Statistical Physics. 175, 34, p. 664680 Abstract
In this short review we discuss a decisive and some time unexpected role of a thermal noise in spatiotemporal dynamics of three different nonequilibrium systems, distinguished by their spatial scales, namely macroscopic, mesoscopic as well as microscopic, and by corresponding physical parameters relevant to their susceptibility to thermal fluctuations. As examples of such nonequilibrium systems, we choose a spatiotemporal pattern dynamics in a RayleighBenard convection of a fluid in the vicinity of the gasliquid critical point, as a macroscopic system; a spatiotemporal dynamics of a vesicle in shear as well as linear flows, as a mesoscopic system; and a dynamics of long flexible linear polymer molecules in a shear flow, as a microscopic system.
2018

141.(2018) Physical Review Fluids. 3, 10, 103302. Abstract
Creeping flow of polymeric fluid without inertia exhibits elastic instabilities and elastic turbulence accompanied by drag enhancement due to elastic stress produced by flowstretched polymers. However, in inertiadominated flow at high Re and low fluid elasticity El, a reduction in turbulent frictional drag is caused by an intricate competition between inertial and elastic stresses. Here we explore the effect of inertia on the stability of viscoelastic flow in a broad range of control parameters El and (Re, Wi). We present the stability diagram of observed flow regimes in WiRe coordinates and find that the instabilities' onsets show an unexpectedly nonmonotonic dependence on El. Further, three distinct regions in the diagram are identified based on El. Strikingly, for highelasticity fluids we discover a complete relaminarization of flow at Reynolds number in the range of 1 to 10, different from a wellknown turbulent drag reduction. These counterintuitive effects may be explained by a finite polymer extensibility and a suppression of vorticity at high Wi. Our results call for further theoretical and numerical development to uncover the role of inertial effect on elastic turbulence in a viscoelastic flow.

140.(2018) Physical Review Fluids. 3, 10, 103303. Abstract
We report quantitative evidence of mixinglayer elastic instability in a viscoelastic fluid flow between two widely spaced obstacles hindering a channel flow at Re > 1. Two mixing layers with nonuniform shear velocity profiles are formed in the region between the obstacles. The mixinglayer instability arises in the vicinity of an inflection point on the shear velocity profile with a steep variation in the elastic stress. The instability results in an intermittent appearance of small vortices in the mixing layers and an amplification of spatiotemporal averaged vorticity in the elastic turbulence regime. The latter is characterized through scaling of friction factor with Wi and both pressure and velocity spectra. Furthermore, the observations reported provide improved understanding of the stability of the mixing layer in a viscoelastic fluid at large elasticity, i.e., Wi >> 1 and Re

139.(2018) Journal of Turbulence. 19, 8, p. 647663 Abstract
Scrupulous measurements and detailed data analysis of the torque in a swirling turbulent flow driven by counterrotating bladed disks reveal an apparent breaking of the law of similarity. Potentially, such breakdown could arise from several possible factors, including dependence on dimensionless numbers other than Re or velocity coupling to other fields such as temperature. However, careful redesign and calibration of the experiment showed that this unexpected result was due to background errors caused by minute misalignments which lead to a noisy and irreproducible torque signal at low rotation speeds and prevented correct background subtraction normally ascribed to frictional losses. An important lesson to be learnt is that multiple minute misalignments can nonlinearly couple to the torque signal and provide a dc offset that cannot be removed by averaging. That offset can cause the observed divergence of the friction coefficient Cf from its constant value observed in the turbulent regime. By significant modifications of the setup and conducting the experiment with one bladed disk and precisely aligned the disk, torque meter and motor shaft, we are able to conduct precise measurements close to the expected resolution at small torques at low rotation speeds and to confirm the similarity law in a wide range of Re, in particular, in low viscosity fluids.

138.(2018) Physical Review Fluids. 3, 1, 014102. Abstract
Precise measurements of the torque in a von Karman swirling flow between a rotating and a stationary smooth disk in three Newtonian fluids with different dynamic viscosities are reported. From these measurements the dependence of the normalized torque, called the friction coefficient, on Re is found to be of the form Cf = 1.17(+/ 0.03) Re0.46 +/ 0.003 where the scaling exponent and coefficient are close to that predicted theoretically for an infinite, unshrouded, and smooth rotating disk which follows from an exact similarity solution of the NavierStokes equations, obtained by von Karman. An error analysis shows that deviations from the theory can be partially caused by background errors. Measurements of the azimuthal V. and axial velocity profiles along radial and axial directions reveal that the flow core rotates at Vtheta/r Omega similar or equal to 0.22 (up to z approximate to 4 cm from the rotating disk and up to r(0)/R similar or equal to 0.25 in the radial direction) in spite of the small aspect ratio of the vessel. Thus the friction coefficient shows scaling close to that obtained from the von Karman exact similarity solution, but the observed rotating core provides evidence of the Batchelorlike solution [Q. J. Mech. Appl. Math. 4, 29 (1951)] different from the von Karman [Z. Angew. Math. Mech. 1, 233 (1921)] or Stewartson [Proc. Camb. Philos. Soc. 49, 333 (1953)] one.
2017

137.(2017) Physical Review Fluids. 2, 10, 103301. Abstract
We report detailed experimental studies of statistical, scaling, and spectral properties of elastic turbulence (ET) in a von Karman swirling flow between rotating and stationary disks of polymer solutions in a wide, from dilute to semidilute entangled, range of polymer concentrations phi. The main message of the investigation is that the variation of phi just weakly modifies statistical, scaling, and spectral properties of ET in a swirling flow. The qualitative difference between dilute and semidilute unentangled versus semidilute entangled polymer solutions is found in the dependence of the critical Weissenberg number Wi(c) of the elastic instability threshold on phi. The control parameter of the problem, the Weissenberg number Wi, is defined as the ratio of the nonlinear elastic stress to dissipation via linear stress relaxation and quantifies the degree of polymer stretching. The powerlaw scaling of the friction coefficient on Wi/Wi(c) characterizes the ET regime with the exponent independent of phi. The torque Gamma and pressure p power spectra show powerlaw decays with welldefined exponents, which has values independent of Wi and phi separately at 100

136.(2017) Nature Communications. 8, 468. Abstract
Chaotic flows drive mixing and efficient transport in fluids, as well as the associated beautiful complex patterns familiar to us from our every day life experience. Generating such flows at small scales where viscosity takes over is highly challenging from both the theoretical and engineering perspectives. This can be overcome by introducing a minuscule amount of long flexible polymers, resulting in a chaotic flow dubbed 'elastic turbulence'. At the basis of the theoretical frameworks for its study lie the assumptions of a spatially smooth and randomintime velocity field. Previous measurements of elastic turbulence have been limited to twodimensions. Using a novel threedimensional particle tracking method, we conduct a microfluidic experiment, allowing us to explore elastic turbulence from the perspective of particles moving with the flow. Our findings show that the smoothness assumption breaks already at scales smaller than a tenth of the system size. Moreover, we provide conclusive experimental evidence that 'ballistic' separation prevails in the dynamics of pairs of tracers over long times and distances, exhibiting a memory of the initial separation velocities. The ballistic dispersion is universal, yet it has been overlooked so far in the context of small scales chaotic flows.

135.
2016

134.(2016) Physical Review E. 94, 6, Abstract
In this paper we investigate the in vitro dynamics of a single rabbit red blood cell (RBC) in a planar linear flow as a function of a shear stress sigma and the dynamic viscosity of outer fluid eta(o). A linear flow is a generalization of previous studies dynamics of soft objects including RBC in shear flow and is realized in the experiment in a microfluidic fourroll mill device. We verify that the RBC stable orientation dynamics is found in the experiment being the inshearplane orientation and the RBC dynamics is characterized by observed three RBC dynamical states, namely tumbling (TU), intermediate (INT), and swinging (SW) [or tanktreading (TT)] on a single RBC. The main results of these studies are the following. (i) We completely characterize the RBC dynamical states and reconstruct their phase diagram in the case of the RBC inshearplane orientation in a planar linear flow and find it in a good agreement with that obtained in early experiments in a shear flow for human RBCs. (ii) The value of the critical shear stress sigma(c) of the TUTT(SW) transition surprisingly coincides with that found in early experiments in spite of a significant difference in the degree of RBC shape deformations in both the SW and INT states. (iii) We describe the INT regime, which is stationary, characterized by strong RBC shape deformations and observed in a wide range of the shear stresses. We argue that our observations cast doubts on the main claim of the recent numerical simulations that the only RBC spheroidal stressfree shape is capable to explain the early experimental data. Finally, we suggest that the amplitude dependence of both theta and the shape deformation parameter D on sigma can be used as the quantitative criterion to determine the RBC stressfree shape.

133.

132.

131.(2016) Soft Matter. 12, 7, p. 21862191 Abstract
Dilute polymer solutions are known to exhibit purely elastic instabilities even when the fluid inertia is negligible. Here we report the quantitative evidence of two consecutive oscillatory elastic instabilities in an elongation flow of a dilute polymer solution as realized in a Tjunction geometry with a long recirculating cavity. The main result reported here is the observation and characterization of the first transition as a forward Hopf bifurcation resulted in a uniformly oscillating state due to breaking of time translational invariance. This unexpected finding is in contrast with previous experiments and numerical simulations performed in similar ranges of the Wi and Re numbers, where the forward forkbifurcation into a steady asymmetric flow due to the broken spatial inversion symmetry was reported. We discuss the plausible discrepancy between our findings and previous studies that could be attributed to the long recirculating cavity, where the length of the recirculating cavity plays a crucial role in the breaking of time translational invariance instead of the spatial inversion. The second transition is manifested via time aperiodic transverse fluctuations of the interface between the dyed and undyed fluid streams at the channel junction and advected downstream by the mean flow. Both instabilities are characterized by fluid dischargerate and simultaneous imaging of the interface between the dyed and undyed fluid streams in the outflow channel.

130.(2016) Europhysics Letters. 113, 3, p. 38003 (6 pp.)38003 (6 pp.) Abstract
Vesicles are involved in a vast variety of transport processes in living organisms. Additionally, they serve as a model for the dynamics of cell suspensions. Predicting the rheological properties of their suspensions is still an open question, as even the interaction of pairs is yet to be fully understood. Here we analyse the effect of a single vesicle, undergoing tanktreading motion, on its surrounding shear flow by studying the induced disturbance field , the difference between the velocity field in its presence and absence. The comparison between experiments and numerical simulations reveals an impressive agreement. Tracking ridges in the disturbance field magnitude landscape, we identify the principal directions along which the velocity difference field is analysed in the vesicle vicinity. The disturbance magnitude is found to be significant up to about 4 vesicle radii and can be described by a power law decay with the distance d from the vesicle . This is consistent with previous experimental results on the separation distance between two interacting vesicles under similar conditions, for which their dynamics is altered. This is an indication of vesicles longrange effect via the disturbance field and calls for the proper incorporation of longrange hydrodynamic interactions when attempting to derive rheological properties of vesicle suspensions.
2015

129.(2015) Physical Review E. 92, 2, 23001. Abstract
We report unexpected results of a drastic difference in the transition to fully developed turbulent and turbulent drag reduction (TDR) regimes and in their properties in a von Karman swirling flow with counterrotating disks of waterbased polymer solutions for viscous (by smooth disks) as well as inertial (by bladed disks) forcing and by tracking just torque Gamma(t) and pressure p(t). For the viscous forcing, just a single TDR regime is found with the transition values of the Reynolds number (Re) Rec(turb) = Rec(TDR) similar or equal to (4.8 +/ 0.2) x 10(5) independent of phi, whereas for the inertial forcing two turbulent regimes are revealed. The first transition is to fully developed turbulence, and the second one is to the TDR regime with both Rec(turb) and Rec(TDR) depending on polymer concentration phi. Both regimes differ by the values of Cf and Cp, by the scaling exponents of the fundamental turbulent characteristics, by the nonmonotonic dependencies of skewness and flatness of the pressure PDFs on Re, and by the different frequency power spectra of p with the different dependencies of the main vortex peak frequency in the p power spectra on phi and Re. Thus our experimental results show the transition to the TDR regime in a von Karman swirling flow for the viscous and inertial forcings in a sharp contrast to the recent experiments [Phys. Fluids 10, 426 (1998); Phys. Rev. E 47, R28(R) (1993); and J. Phys.: Condens. Matter 17, S1195 (2005)] where the transition to TDR is observed in the same swirling flow with counterrotating disks only for the viscous forcing. The latter result has led its authors to the wrong conclusion that TDR is a solely boundary effect contrary to the inertial forcing associated with the bulk effect, and this conception is currently rather widely accepted in literature.

128.(2015) EPL. 109, 1, 14006. Abstract
We present quantitative experimental results on the transition to early turbulence in von Karman swirling flow of water and watersugarbased polymer solutions compared to the transition to turbulence in their Newtonian solvents by measurements of solely global quantities as torque Gamma(t) and pressure p(t) with large statistics as a function of Re. For the first time the transition values of Rec(turb) to fully developed turbulence and turbulent drag reduction regime Rec(TDR) are obtained as functions of elasticity El by using the solvents with different viscosities and polymer concentrations phi. Two scaling regions for fundamental turbulent characteristics are identified and they correspond to the turbulent and TDR regimes. Both Rec(turb) and Rec(TDR) are found via the dependence of the friction coefficient Cf and Cp, defined through scaled average torque (Gamma) over bar and rms pressure fluctuations p(rms), respectively, on Re for different El and phi and via the limits of the two scaling regions. Copyright (C) EPLA, 2015
2014

127.(2014) EPL. 107, 2, 28001. Abstract
We present experimental observations and numerical simulations of a wrinkling instability that occurs at sufficiently high strain rates in the trembling regime of vesicle dynamics in steady linear flow. Spectral and statistical analysis of the data shows similarities and differences with the wrinkling instability observed earlier for vesicles in transient elongation flow. The critical relevance of thermal fluctuations for this phenomenon is revealed by a simple model using coupled Langevin equations that reproduces the experimental observations quite well. Copyright (C) EPLA, 2014.

126.(2014) Advances in Colloid and Interface Science. 208, p. 129141 Abstract
We review the dynamical behavior of giant fluid vesicles in various types of external hydrodynamic flow. The interplay between stresses arising from membrane elasticity, hydrodynamic flows, and the ever present thermal fluctuations leads to a rich phenomenology. In linear flows with both rotational and elongational components, the properties of the tanktreading and tumbling motions are now well described by theoretical and numerical models. At the transition between these two regimes, strong shape deformations and amplification of thermal fluctuations generate a new regime called trembling. In this regime, the vesicle orientation oscillates quasiperiodically around the flow direction while asymmetric deformations occur. For strong enough flows, smallwavelength deformations like wrinkles are observed, similar to what happens in a suddenly reversed elongational flow. In steady elongational flow, vesicles with large excess areas deform into dumbbells at large flow rates and pearling occurs for even stronger flows. In capillary flows with parabolic flow profile, single vesicles migrate towards the center of the channel, where they adopt symmetric shapes, for two reasons. First, walls exert a hydrodynamic lift force which pushes them away. Second, shear stresses are minimal at the tip of the flow. However, symmetry is broken for vesicles with large excess areas, which flow offcenter and deform asymmetrically. In suspensions, hydrodynamic interactions between vesicles add up to these two effects, making it challenging to deduce theological properties from the dynamics of individual vesicles. Further investigations of vesicles and similar objects and their suspensions in steady or timedependent flow will shed light on phenomena such as blood flow. (C) 2014 Elsevier B.V. All rights reserved.

125.(2014) Physics of Fluids. 26, 5, 55102. Abstract
We report the experimental studies of the statistical and scaling properties of the fully developed turbulent regime in von Karman swirling flow between counterrotating disks with and without blades using the only global measurements of the spatially averaged torque Gamma and pressure p fluctuations in water and watersugar solutions of different viscosities in the same cell geometry. We show that for all fluids under investigation probability distribution functions (PDFs) of the torque fluctuations delta Gamma/Gamma/(rms) are Gaussian in both the laminar and turbulent regimes and for the both types of the stirrers. On the contrary, PDFs of the pressure fluctuations change from Gaussian in the laminar regime into the skewed shape with the exponential tails toward lowpressure events for both the entrainment methods. Both the friction coefficient Cf and normalized rms of the pressure fluctuations Cp are independent of Re in the fully developed turbulent regime for all fluids under study and found in a good quantitative agreement with the previous results. We also observe that the internal flow variables such as the normalized torque (Gamma) over tilde/ Vprms versus the "internal" Reynolds number Rerms = (p(rms)/rho.)R1/2 rho/eta instead of the global variables Cf, Cp versus Re show sharp transition into the well developed turbulent regime. We find that the scaling exponents of the fundamental characteristics based only on and p measurements in the range of fully developed turbulent flow, namely, the integral, Taylor, and Kolmogorov dissipation lengths, as well as the Taylorbased Reynolds number R., are in rather fair agreement with the predictions. We would like to emphasize that scaling of the main turbulent parameters Rlambda, lambda, eta(d) obtained via the global variables is a very nontrivial result. It is not obvious that measurements based on the global quantitieswill provide the predicted scaling relations. The result on such scaling obtained pr

124.(2014) Physical Review Letters. 112, 13, 138106. Abstract
We report first experimental observations of dynamics of compound vesicles in linear flow realized in a microfluidic fourroll mill. We show that while a compound vesicle undergoes the same main tanktreading, trembling (TR), and tumbling regimes, its dynamics are far richer and more complex than that of unilamellar vesicles. A new swinging motion of the inner vesicle is found in TR in accord with simulations. The inner and outer vesicles can exist simultaneously in different dynamical regimes and can undergo either synchronized or unsynchronized motions depending on the filling factor. A compound vesicle can be used as a physical model to study white blood cell dynamics in flow similar to a unilamellar vesicle used successfully to model anucleate cells.

123.
2012

122.(2012) Physical Review Letters. 109, 26, Abstract
A novel noise amplification mechanism resulting from the interaction of thermal fluctuations and nonlinear vesicle dynamics is reported. It is observed in a timedependent vesicle state called trembling (TR). High spatial resolution and very long time series of TR compared to the vesicle period allow us to quantitatively analyze the generation and amplification of spatial and temporal modes of the vesicle shape perturbations. During a compression part of each TR cycle, a vesicle finds itself on the edge of the wrinkling instability, where thermally excited spatial modes are amplified. DOI: 10.1103/PhysRevLett.109.268103

121.(2012) Physical Review E. 86, 5, Abstract
A polymer solution partially filling a rotating horizontal drum undergoes an elastically driven instability at low Reynolds numbers. This instability manifests itself through localized plumelike bursts, perturbing the free liquid surface. Here we present an expanded experimental account regarding the dynamics of individual plumes and the statistics pertaining to the complex collective interaction between plumes, which leads to plume coagulation. We also present a detailed description of an optical technique that enables the visualization and measurement of surface perturbations in coating flows within a rotating horizontal drum.

120.(2012) EPL. 100, 2, Abstract
We report the results of experiments on turbulent drag reduction (TDR) in swirling flow of water and watersucrose polymer solutions, where Re and Wi as well as polymer concentration phi are varied. The friction coefficients Cf and Cp defined through average torque (Gamma) over bar and rms of pressure fluctuations p(rms) for different elasticity El = Wi/Re and phi vs. Re/Rec collapse onto universal curves in accord with theory, where Rec is Re at TDR onset. The transition lines to the TDR state, Rec  El and Rec  phi, are measured and relevant physics is discussed. Power spectra for Gamma and p at Re/Rec > 1 show a drastic reduction of lowfrequency noise and the emergence of a peak corresponding to the main vortex frequency in accord with TDR. Copyright (C) EPLA, 2012

119.(2012) Physical Review E. 85, 5, Abstract
We report the experimental studies on interaction of two vesicles trapped in a microfluidic fourroll mill, where a plane linear flow is realized. We found that the dynamics of a vesicle in tanktreading motion is significantly altered by the presence of another vesicle at separation distances up to 3.23.7 times of the vesicle effective radius. This result is supported by measurement of a single vesicle backreaction on the velocity field. Thus the experiment provides the upper bound for the volume fraction phi = 0.080.13 of noninteracting vesicle suspensions.

118.(2012) Physics of Fluids. 24, 4, Abstract
The influence of symmetrical nonOberbeckBoussinesq (SNOB) effect on statistical and scaling properties of temperature field in turbulent convection is investigated experimentally in SF6 in the vicinity of its gasliquid critical point. The main conclusion of the studies is that the most of properties of large scale circulation (LSC) flow in SNOB turbulent convection are the same as in the OberbeckBoussinesq (OB) case: (i) the emergence of the main peak frequency f(c) and the second harmonics frequency 2f(c) in the temperature power spectra; (ii) the relation between f(c) and the delay time tau(sh) in the LSC flow between lower and upper regions found from cross correlation functions and their scaling with Ra and Pr agree with those in the OB case and with theoretical predictions. Thus the theory quantitatively describe the properties of the LSC in SNOB turbulent convection as well as in the OB case. (iii) The degree of coherence of the LSC flow oscillations described and the threshold for the onset of the coherent oscillations in the LSC have Pr as well as Ra dependence studied recently in the OB case. However, some differences with the OB case are also identified. Much stronger Ra dependence of the rms of temperature fluctuations normalized by the temperature difference across the cell than in the (OB) case is found. Another distinctive feature observed in the frequency power spectra of the temperature fluctuations is the emergence of strong second harmonics peak at 2f(c) sometimes even higher than the main one that points out on the strong modulation of the coherent oscillations during the LSC cycle. The peak frequency f(p) of the dissipation spectra of the temperature fluctuations and its scaling differ significantly from f(p) in the OB case studied recently. And finally, the dependence of the normalized scaling exponents of the structure functions on the order of the structure functions is qualitatively similar to those for the OB case, and both show stron
2011

117.(2011) Physical Review E. 84, 5, 056325. Abstract
We report detailed quantitative studies of elastic turbulence in a curvilinear channel flow in a dilute polymer solution of high molecular weight polyacrylamide in a high viscosity watersugar solvent. Detailed studies of the average and rms velocity and velocity gradients profiles reveal the emergence of a boundary layer associated with the nonuniform distribution of the elastic stresses across the channel. The characteristic boundary width is independent of the Weissenberg number Wi and proportional to the channel width, which is consistent with the findings our early investigations of the boundary layer in elastic turbulence in different flow geometries. The nonuniform distribution of the elastic stresses across the channel and appearance of the characteristic spatial scales of the order of the boundary layer width of both velocity and velocity gradient in the correlation functions of the velocity and velocity gradient fields in a bulk flow may suggest that excessive elastic stresses, concentrated in the boundary layer, are ejected into the bulk flow similar to jets observed in passive scalar mixing in elastic turbulence observed recently. Finally, the experimental results show that one of the main predictions of the theory of elastic turbulence, namely, the saturation of the normalized rms velocity gradient in the bulk flow of elastic turbulence contradicts the experimental observations both qualitatively and quantitatively in spite of the fact that the theory explains well the observed sharp powerlaw decay of the velocity power spectrum. The experimental findings call for further development of theory of elastic turbulence in a bounded container, similar to what was done for a passive scalar problem.

116.(2011) EPL. 96, 2, 28004. Abstract
We report the experimental observation of Kolmogorovtype steady coagulation cascade in an elastically driven instability occurring at the liquid front of a dilute polymer solution partially filling a rotating horizontal drum. Surface plumes are found to dominate the flow in the vicinity of the front at low Reynolds numbers. Plume merging leads to an area cascade with a timeindependent mass flux. The coalescence process results in a powerlaw decay of the stationary plume area distribution, with an exponent which agrees with that of the Smoluchowski coagulation model. Copyright (C) EPLA, 2011

115.(2011) Physics of Fluids. 23, 4, 041905. Abstract
Despite the recent upsurge of theoretical reduced models for vesicle shape dynamics, comparisons with experiments have not been accomplished. We review the implications of some of the recently proposed models for vesicle dynamics, especially the tumblingtrembling domain regions of the phase plane, and show that they all fail to capture the essential behavior of real vesicles for excess areas Delta greater than 0.4. We emphasize new observations of shape harmonics and the role of thermal fluctuations. (C) 2011 American Institute of Physics. [doi:10.1063/1.3556439]
2010

114.(2010) Physics of Fluids. 22, 12, Abstract
We report detailed quantitative studies of passive scalar mixing in a curvilinear channel flow, where elastic turbulence in a dilute polymer solution of high molecular weight polyacrylamide in a high viscosity watersugar solvent was achieved. For quantitative investigation of mixing, a detailed study of the profiles of mean longitudinal and radial components of the velocity in the channel as a function of Wi was carried out. Besides, a maximum of the average value as well as a rms of the longitudinal velocity was used to determine the threshold of the elastic instability in the channel flow. The rms of the radial derivatives of the longitudinal and radial velocity components was utilized to define the control parameters of the problem, the Weissenberg Wi(loc) and the Peclet Pe numbers. The main result of these studies is the quantitative test of the theoretical prediction about the value of the mixing length in the decay Batchelor regime. The experiment shows large quantitative discrepancy, more than 200 times in the value of the coefficient C, which appears in the theoretical expression for the mixing length, but with the predicted scaling relation. There are two possible reasons to this discrepancy. First is the assumption made in the theory about the deltacorrelated velocity field, which is in odds with the experimental observations. Second, and probably a more relevant suggestion for the significantly increased mixing length and thus reduced mixing efficiency, is the observed jets, the rare, localized, and vigorous ejection of the scalar trapped near the wall, which protrudes into the peripheral region as well as the bulk. They are first found in the recent numerical calculations and then observed in the experiment reported. The jets definitely strongly reduce the mixing efficiency in particular in the peripheral region and so can lead to considerable increase of the mixing length. We hope that this result will initiate further numerical calculations of the

113.(2010) EPL. 90, 4, 44002. Abstract
A single fluorescently stained T4DNA molecule with known elastic properties is used as a stress sensor in a random flow. Using measured polymer stretching statistics and its known elastic properties, the elastic stress in elastic turbulence, created by the same unlabeled molecules, is directly obtained for the first time as a function of rotation speed and T4DNA concentration in watersaccharose solvents. It is found that i) the value of elastic stress is by at least two orders of magnitude larger than theoretically predicted that disproves the existing theory of elastic turbulence, and ii) the normalized elastic stress in the elasticturbulence regime linearly depends on the local Weissenberg number, which defines the polymer stretching, for all used polymer concentrations and saturates at its highest values. Copyright (C) EPLA, 2010

112.(2010) Physics of Fluids. 22, 3, 35108. Abstract
Strong nonOberbeckBoussinesq (OB) effects in turbulent convection were investigated experimentally in SF6 in the vicinity of its gasliquid critical point (CP). The temperature and density dependencies of the thermodynamic and kinetic properties of SF6 near its CP and at the average critical density lead to strong but symmetric vertical variations of the main physical properties, which enter into the control parameters of turbulent convection. This produces an updown symmetry in the temperature drops across the upper and lower half of the cell, while the temperature in the middle of the cell remains equal to the average value. Thus, in spite of the strong variations of the fluid properties across the cell height, the updown symmetry remains like in the OB case. The distinctive feature of the symmetric nonOB turbulent convection is that the heat transport scales with the Rayleigh number Ra like in the OB turbulent convection. At the same time, it shows a much stronger dependence on the Prandtl number Pr. We singled out the influence of the nonOB effect on the heat transport and found that, for the same Pr, an eightfold larger nonOB effect does not alter either the value of the Nusselt number, Nu, nor its scaling with respect to the Rayleigh number, Nu(proportional to)Ra(gamma). The conclusion is that the strong symmetric nonOB effect by itself is not responsible for the strong Pr dependence of the heat transport near CP. The possible source of this Pr dependence is the strongly enhanced isothermal compressibility in the vicinity of CP, which can affect the dynamics of plumes and so the heat transport close to the CP, and manifests itself in a dependence of Nu on Pr much steeper than in the OB case. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3358462]
2009

111.(2009) Comptes Rendus Physique. 10, 8, p. 728739 Abstract
In this short review I argue that the progress in our understanding the mechanism of turbulent drag reduction is conditioned by obtaining experimental data on dynamics and statistics of polymer stretching and elastic stresses in inertial turbulence at high Reynolds numbers that is a technically challenging task. The suggested way out of the currently unresolved technical problem is to collect the same data in elastic turbulence, which is a smooth random flow similar to that found in inertial turbulence below the dissipation scale. Since the polymer stretching and elastic stresses in inertial turbulence are influenced only by small scales, it is appropriate to use information on the polymer stretching and elastic stresses obtained in elastic turbulence. The experimental data on the statistics of the polymer stretching, the coilstretch transition, and elastic stresses together with spatial distribution and values of the rms of the velocity gradients were collected in elastic turbulence for the last several years. This information serves a basis for a new hypothesis of turbulent drag reduction. To cite this article: V Steinberg, C R. Physique 10 (2009). (C) 2009 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.

110.Concentration dependence of the longest relaxation times of dilute and semidilute polymer solutions
The longest relaxation times of polymer Solutions of semiflexible T4 DNA and flexible 18 M molar mass polyacrylamide (PAAm) in dilute and semidilute concentration range are studied by the polymer extension relaxation of stretched single DNA molecules and by the stress relaxation of PAAm solutions measurements. For both polymer solutions, the longest relaxation time normalized by the value at infinite dilution with the same solvent viscosity tau/tau(0) increases with increasing concentration. In the dilute regime, the longest relaxation time increases just slightly with increasing concentration as tau/tau(0)=[1 + cA  root 2(cA)(1.5) + 2(cA)(2)] as well as the empirical relation of tau/tau(0)=exp(cA) up to c similar to 3c* with A approximate to 0.5[eta], where c* is the overlap concentration, in accord with the theory and previous experiments. For the semidilute solutions, the scaling of tau/tau(0) with concentration shows two different exponents in two concentration regions, corresponding to the unentangled and entangled regimes. The exponents are consistent with those expected by the theory of dynamical scaling for semidilute polymer solutions. The crossover concentration from the unentangled to entangled regime c(e) is found to be similar to 9c* in a good solvent, in accord with other experiments based on the relaxation of a single molecule, the diffusion coefficient, and the solution viscosity measurements. We also test the universality of the concentration dependence of the longest relaxation time for both flexible and semiflexible polymers in both good and Theta solvents in the dilute and semidilute regimes by analyzing literature data sets. (C) 2009 The Society of Rheology. [DOI: 10.1122/1.3160734]

109.(2009) Proceedings of the National Academy of Sciences of the United States of America. 106, 28, p. 1144411447 Abstract
An approach to quantitatively study vesicle dynamics as well as biologicallyrelated microobjects in a fluid flow, which is based on the combination of a dynamical trap and a control parameter, the ratio of the vorticity to the strain rate, is suggested. The flow is continuously varied between rotational, shearing, and elongational in a microfluidic 4roll mill device, the dynamical trap, that allows scanning of the entire phase diagram of motions, i.e., tanktreading (TT), tumbling (TU), and trembling (TR), using a single vesicle even at lambda = eta(in)/eta(out) = 1, where eta(in) and eta(out) are the viscosities of the inner and outer fluids. This cannot be achieved in pure shear flow, where the transition between TT and either TU or TR is attained only at lambda > 1. As a result, it is found that the vesicle dynamical states in a general are presented by the phase diagram in a space of only 2 dimensionless control parameters. The findings are in semiquantitative accord with the recent theory made for a quasispherical vesicle, although vesicles with large deviations from spherical shape were studied experimentally. The physics of TR is also uncovered.

108.(2009) Physical Review Letters. 102, 12, Abstract
Injected power P and pressure p fluctuations in a swirling flow of polymer solutions in a wide range of polymer concentrations c in elastic turbulence regime show nonGaussian statistics that strongly resemble statistical behavior of P and p in hydrodynamic turbulence. Together with this fact, weak dependence of the statistics of rescaled variables on c may suggest that there are universal mechanisms determining the intermittent statistics of P and p. We also show that the study of the statistics of p provides a way to study statistics of the elastic stresses in elastic turbulence otherwise currently unattainable.

107.(2009) Physical Review Letters. 102, 11, Abstract
We report the first experimental phase diagram of vesicle dynamical states in a shear flow presented in a space of two dimensionless parameters suggested recently by V. Lebedev et al. To reduce errors in the control parameters, 3D geometrical reconstruction and determination of the viscosity contrast of a vesicle in situ in a plane Couette flow device prior to the experiment are developed. Our results are in accord with the theory predicting three distinctly separating regions of vesicle dynamical states in the plane of just two selfsimilar parameters.
2008

106.(2008) Physical Review E. 78, 4, Abstract
We present experimental results on relaxation dynamics of lambdaDNA and T4 polymer molecules toward a steady state in elongation flow. Strong critical slowing down (similar to the wellknown effect in continuous phase transitions) in polymer relaxation near the coilstretch transition (CST) is quantitatively investigated and found to be in good accord with predictions. For polymers with a small number of Kuhn segments the maximum of the relaxation time vs the strain rate provides precise information about the location of the CST and serves as its criterion.

105.(2008) Physical Review Letters. 101, 4, Abstract
We present results on the stretching of single tubular vesicles in an elongation flow toward dumbbell shapes, and on their relaxation. A critical strain rate (is an element of)over dot(c) exists; for strain rates (is an element of)over dot

104.(2008) Physical Review Letters. 100, 25, Abstract
We present a shear instability, which can be triggered in compressible fluids with densitydependent viscosity at shear rates above critical. The instability mechanism is generic: It is based on densitydependent viscosity, compressibility, as well as flow two(three)dimensionality that provides coupling between streamwise and transversal velocity components and density variations. The only factor stabilizing the instability is fluid elasticity. The corresponding eigenvalue problem for a plane Couette flow is solved analytically in the limiting cases of large and small wave numbers.

103.(2008) EPL. 82, 5, Abstract
We studied the dynamics of isolated vesicles as well as vesicle interactions in semidilute vesicle suspensions subjected to a shear flow. We found that the longrange hydrodynamic interactions between vesicles give rise to strong fluctuations of vesicle shape and inclination angle, empty set, though the functional dependence of empty set and the transition path to tumbling motion is preserved. The dependence of the suspension viscosity on the viscosity ratio between inner and outer fluids, lambda, was found to be nonmonotonic and surprisingly growing with lambda at the fixed outer fluid viscosity for lambda
2007

102.(2007) Physical Review Letters. 99, 17, Abstract
We present experimental results on the relaxation dynamics of vesicles subjected to a timedependent elongation flow. We observed and characterized a new instability, which results in the formation of higherorder modes of the vesicle shape (wrinkles), after a switch in the direction of the velocity gradient. This surprising generation of membrane wrinkles can be explained by the appearance of a negative surface tension during the vesicle deflation, which tunes itself to alternating stress. Moreover, the formation of buds in the vesicle membrane was observed in the vicinity of the dynamical transition point.

101.(2007) Physics of Fluids. 19, 5, Abstract
We discuss the role of elastic stress in the statistical properties of elastic turbulence, realized by the flow of a polymer solution between two disks. The dynamics of the elastic stress are analogous to those of a smallscale fast dynamo in magnetohydrodynamics, and to those of the turbulent advection of a passive scalar in the Batchelor regime. Both systems are theoretically studied in the literature, and this analogy is exploited to explain the statistical properties, the flow structure, and the scaling observed experimentally. The following features of elastic turbulence are confirmed experimentally and presented in this paper: (i) The rms of the vorticity (and that of velocity gradients) saturates in the bulk of the elastic turbulent flow, leading to the saturation of the elastic stress. (ii) The rms of the velocity gradients (and thus the elastic stress) grows linearly with Wi in the boundary layer, near the driving disk. The rms of the velocity gradients in the boundary layer is one to two orders of magnitude larger than in the bulk. (iii) The PDFs of the injected power at either constant angular speed or torque show skewness and exponential tails, which both indicate intermittent statistical behavior. Also the PDFs of the normalized accelerations, which can be related to the statistics of velocity gradients via the Taylor hypothesis, exhibit wellpronounced exponential tails. (iv) A new length scale, i.e., the thickness of the boundary layer, as measured from the profile of the rms of the velocity gradient, is found to be relevant for the boundary layer of the elastic stresses. The velocity boundary layer just reflects some of the features of the boundary layer of the elastic stresses (rms of the velocity gradients). This measured length scale is much smaller than the vessel size. (v) The scaling of the structure functions of the vorticity, velocity gradients, and injected power is found to be the same as that of a passive scalar advected by an elastic t

100.(2007) Physical Review Letters. 98, 14, Abstract
Investigations of shear flows in threedimensional complexplasma fluids produced in a dc discharge were carried out. The shear was induced either by an inhomogeneous gas flow or by a laser beam. The viscosity of complex plasmas was measured over a broad range of shear rates, up to the hydrodynamic limit when the discreteness becomes important. Analysis of the measurements reveals nonNewtonian behavior of complex plasmas accompanied by substantial shear thinning.

99.(2007) Macromolecules. 40, 6, p. 21722176 Abstract
The longest relaxation times of double and singlestranded lambda DNA (ds lambdaDNA, ss lambdaDNA) were studied by viscosity measurements in an oscillatory flow and by stress relaxation measurements. The results for ds lambdaDNA agreed well with chain retraction experiments of stretched single molecules; both were close to the relaxation time calculated from the Zimm model. The relaxation time of ss lambdaDNA is too small to be accurately measured by the stress relaxation experiments even in a very viscous solution. Such a small relaxation time makes it difficult to be studied in experiments of single molecule dynamics. For ds lambdaDNA in the intermediate time scale, the stress relaxes in a power law manner toward the long time scale with an exponent of 0.95, which is in good agreement with the Brownian dynamics simulations.

98.(2007) Experiments in Fluids. 42, 2, p. 291299 Abstract
This work is an attempt to test the concept of the hydrodynamic charge (analogous to the electric charge in electromagnetism) in the simple case of a coherent structure such as the Burgers vortex. We provide experimental measurements of both the socalled Lamb vector and its divergence (the charge) by twodimensional particles images velocimetry. In addition, we perform a HelmholtzHodge decomposition of the Lamb vector in order to explore its topological features. We compare the charge with the wellknown Qcriterion in order to assess its interest in detecting and characterizing coherent structure. Usefulness of this concept in studies of vortex dynamics is demonstrated.
2006

97.(2006) Physical Review Letters. 96, 21, Abstract
The role of elastic stress in statistical and scaling properties of elastic turbulence in a polymer solution flow between two disks is discussed. The analogy with a smallscale magnetodynamics and a passive scalar turbulent advection in the Batchelor regime is used to explain the experimentally observed statistical properties, the flow structure, and the scaling of elastic turbulence. The emergence of a new length scale, namely, the boundary layer thickness, is observed and studied.

96.(2006) Physical Review Letters. 96, 3, Abstract
We present experimental results on statistics of polymer orientation angles relative to the shear plane and tumbling times in shear flow with thermal noise. The strong deviation of the probability distribution functions (PDFs) of the orientation angles from Gaussian PDFs was observed in good accord with theory. A universal exponential PDF tail for the tumbling times and its predicted scaling with Wi (that is, the dimensionless shear rate normalized by the polymer relaxation time) are also tested experimentally against numerics. The scaling relations of PDF widths for both angles as a function of Wi are verified and compared with numerics.

95.(2006) Physical Review Letters. 96, 3, Abstract
Experimental results on the tanktreadingtumbling transition in the dynamics of a vesicle subjected to a shear flow as a function of a vesicle excess area, viscosity contrast, and the normalized shear rate are presented. Good agreement on the transition curve and scaling behavior with theory and numerical simulations was found. A new type of unsteady motion at a large degree of vesicle deformability was discovered and described as follows: a vesicle trembles around the flow direction, while the vesicle shape strongly oscillates.

94.(2006) Journal Of Statistical MechanicsTheory And Experiment. Abstract
Continuous separation and size sorting of particles and blood cells suspended in a microchannel flow due to an acoustic force are investigated both numerically and experimentally. Good agreement in the measured particle trajectories in a microchannel flow subjected to the acoustic force with those obtained by the numerical simulations up to the fitting parameter is found. High separation efficiency, particularly in a threestage microdevice (up to 99.975%), for particles and blood cells leads us to believe that the device can be developed into commercially useful setup. The novel particle size sorting microdevice provides an opportunity to replace rather expensive existing devices based on specific chemical bonding with an ultrasound cell size sorter that can be considerably improved by adding many stages for multistage size sorting.
2005

93.(2005) Physical Review Letters. 95, 25, Abstract
Experimental results on mean inclination angle and its fluctuation due to thermal noise in tanktreading motion of a vesicle in shear flow as a function of vesicle excess area, normalized shear rate, viscosity, and viscosity contrast between inner and outer fluids, epsilon, are presented. Good quantitative agreement with theory made for epsilon=1 was found. At epsilon>1 the dependence is altered significantly. Dependence of the vesicle shape on shear rate is consistent with theory. A tanktreading velocity of the vesicle membrane is found to be a periodic function close to that predicted by theory.

92.(2005) Physics of Fluids. 17, 10, Abstract
The validity of the Taylor frozen flow hypothesis in a chaotic flow of a dilute polymer solution in a regime of elastic turbulence is investigated experimentally. By accurate timedependent measurements of the flow field we study the velocity coherence between pairs of points displaced both in time and space and quantify the degree of applicability of the Taylor hypothesis. Alternatively, the frozen flow assumption is assessed by comparison of the measured velocity structure functions with the ones derived by a frozen flow assumption. The breakdown of the Taylor hypothesis is further discussed in both the context of strong velocity fluctuations and longrange spatial correlations, which are the result of the flow smoothness and lack of scale separation. Different choices of the advection velocity are tested and discussed. (c) 2005 American Institute of Physics.

91.(2005) Europhysics Letters. 71, 2, p. 221227 Abstract
By quantitative studies of statistics of polymer stretching in an elastic turbulence and the statistical properties of this random flow itself that are characterized by the average Lyapunov exponents of particle pair separations, (lambda) over bar, we demonstrate that the stretching of polymer molecules in a 3D random flow occurs rather sharply via the coilstretch transition. The experimental value of the onset of the coilstretch transition, (lambda) over bar (cr) . tau = 0.77 +/ 0.20, where tau is the polymer relaxation time, is found to be rather close to the theoretically predicted one.

90.(2005) Physical Review E. 71, 4, Abstract
By performing soundscattering measurements with a detector array consisting of 62 elements in a flow between two counterrotating disks we obtain the energy and vorticity power spectra directly in both spatial and temporal domains. Fastaccumulated statistics and a large signaltonoise ratio allow us to get highquality data rather effectively and to test scaling laws in details.
2004

89.(2004) Europhysics Letters. 68, 4, p. 529535 Abstract
We investigate experimentally the statistics of a chaotic flow of a dilute polymer solution in a regime of elastic turbulence by using the Lagrangian coordinates approach. We show that due to flow smoothness at small scales the Finite Time Lyapunov Exponent (FTLE) technique can be successfully used to investigate the statistics of particle pair separations at different scales. We compare the measured FTLE with the characteristics of statistical description in the Eulerian coordinate presentation, namely the velocity correlation times and the average velocity gradients. We characterize the flow intermittency by measuring highorder moments of the statistics of the particle pair separations.

88.(2004) Physical Review E. 69, 6, Abstract
Microscopic flows are almost universally linear, laminar, and stationary because the Reynolds number, Re, is usually very small. That impedes mixing in microfluidic devices, which sometimes limits their performance. Here, we show that truly chaotic flow can be generated in a smooth microchannel of a uniform width at arbitrarily low Re, if a small amount of flexible polymers is added to the working liquid. The chaotic flow regime is characterized by randomly fluctuating threedimensional velocity field and significant growth of the flow resistance. Although the size of the polymer molecules extended in the flow may become comparable to the microchannel width, the flow behavior is fully compatible with that in a macroscopic channel in the regime of elastic turbulence. The chaotic flow leads to quite efficient mixing, which is almost diffusion independent. For macromolecules, mixing time in this microscopic flow can be three to four orders of magnitude shorter than due to molecular diffusion.

87.(2004) Physics of Fluids. 16, 5, p. 15871602 Abstract
Sound scattering by a finite width beam on a single rigid body rotation vortex flow is detected by a linear array of transducers (both smaller than a flow cell), and analyzed using a revised scattering theory. Both the phase and amplitude of the scattered signal are obtained on 64 elements of the detector array and used for the analysis of velocity and vorticity fields. Due to averaging on many pulses the signaltonoise ratio of the phases difference in the scattered sound signal can be amplified drastically, and the resolution of the method in the detection of circulation, vortex radius, vorticity, and vortex location becomes comparable with that obtained earlier by timereversal mirror method [P. Roux, J. de Rosny, M. Tanter, and M. Fink, Phys. Rev. Lett. 79, 3170 (1997)]. The revised scattering theory includes two crucial steps, which allow overcoming limitations of the existing theories. First, the Huygens construction of a farfield scattering signal is carried out from a signal obtained at any intermediate plane. Second, a beam function that describes a finite width beam is introduced, which allows using a theory developed for an infinite width beam for the relation between a scattering amplitude and the vorticity structure function. Structure functions of the velocity and vorticity fields deduced from the sound scattering signal are compared with those obtained from simultaneous particle image velocimetry measurements. Good quantitative agreement is found. (C) 2004 American Institute of Physics.

86.(2004) Physical Review Letters. 92, 16, Abstract
By using high molecular weight fluorescent passive tracers with different diffusion coefficients and by changing the fluid velocity we study the dependence of a characteristic mixing length on the Peclet number, Pe, which controls the mixing efficiency. The mixing length is found to be related to Pe by a power law, L(mix)proportional toPe(0.26+/0.01), and increases faster than expected for an unbounded chaotic flow. The role of the boundaries in the mixing length abnormal growth is clarified. The experimental findings are in good quantitative agreement with recent theoretical predictions.

85.

84.(2004) New Journal of Physics. 6, Abstract
Following our first report (A Groisman and V Steinberg 2000 Nature 405 53), we present an extended account of experimental observations of elasticityinduced turbulence in three different systems: a swirling flow between two plates, a CouetteTaylor (CT) flow between two cylinders, and a flow in a curvilinear channel (Dean flow). All three setups had a high ratio of the width of the region available for flow to the radius of curvature of the streamlines. The experiments were carried out with dilute solutions of highmolecularweight polyacrylamide in concentrated sugar syrups. High polymer relaxation time and solution viscosity ensured prevalence of nonlinear elastic effects over inertial nonlinearity, and development of purely elastic instabilities at low Reynolds number (Re) in all three flows. Above the elastic instability threshold, flows in all three systems exhibit features of developed turbulence. They include: (i) randomly fluctuating fluid motion excited in a broad range of spatial and temporal scales and (ii) significant increase in the rates of momentum and mass transfer (compared with those expected for a steady flow with a smooth velocity profile). Phenomenology, driving mechanisms and parameter dependence of the elastic turbulence are compared with those of the conventional highRe hydrodynamic turbulence in Newtonian fluids. Some similarities as well as multiple principal differences were found. In two out of three systems (swirling flow between two plates and flow in the curvilinear channel), power spectra of velocity fluctuations decayed rather quickly, following power laws with exponents of about 3.5. It suggests that, being random in time, the flow is rather smooth in space, in the sense that the main contribution to deformation and mixing (and, possibly, elastic energy) is coming from flow at the largest scale of the system. This situation, random in time and smooth in space, is analogous to flows at small scales (below the Kolmogorov diss
2003

83.(2003) New Journal of Physics. 5, Abstract
Interaction of magnetic particles with each other and with a magnetic field was studied experimentally in a complex plasma. Monodisperse plastic microspheres with magnetic filler were suspended in an rf symmetrically driven discharge to form a multilayer dust cloud. The magnetic field induced a magnetic moment in the grains. The particles were pulled upward in the direction of the magnetic field gradient and their levitation height increased. This was used as a new diagnostic method to calculate the particle charge and the thickness of the plasma sheath. It was demonstrated that the particle weight can be compensated for. Some particles formed agglomerates due to magnetic attraction between the grains. Analysis of the particle interaction forces showed that at intermediate magnetic fields (used in the experiment) the particles can agglomerate only if their kinetic energy is high enough to overcome the barrier in the interaction potential. The possibility of magnetically induced formation of a plasma crystal was discussed.
2002

82.

81.(2002) Europhysics Letters. 60, 5, p. 704709 Abstract
The existence of internal viscoelastic waves below the elastic instability threshold in the circular Couette flow of a polymer solution is predicted. We present results of calculations of the dispersion relation of the waves. These weakly attenuating waves are sustained by a nonuniform elastic stress distribution and should occur in he region of stable strati cation of the hoop elastic stresses in any viscoelastic fluid flow with curvilinear trajectories.

80.(2002) Physical Review E. 66, 5, Abstract
The onset of the wave resistance via the generation of capillarygravity waves by a small object moving with a velocity V is investigated experimentally. Due to the existence of a minimum phase velocity V(c) for surface waves, the problem is similar to the generation of rotons in superfluid helium near their minimum. In both cases, waves or rotons are produced at V>V(c) due to Cherenkov radiation. We find that the transition to the wave drag state is continuous: in the vicinity of the bifurcation the wave resistance force is proportional to rootVV(c) for various fluids. This observation contradicts the theory of Raphael and de Gennes. We also find that the reduced wave drag force for different fluids and different ball size may be scaled in such a way that all the data collapse on a single curve. The capillarygravity wave pattern and the shape of the wavegenerating region are investigated both experimentally and theoretically. Good agreement between the theory and the experimental data is found in this case.

79.(2002) Macromolecular Chemistry and Physics. 203, 12, p. 18331843 Abstract
The synthesis and characterization of ultrahigh molecularweight fluorescent partially hydrolyzed polyacrylamide (HPAm) of the type P[Am*](x)[Am](85)[AA](15x), where Am is an acrylamide unit, Am* is a fluorescentlabeled Am unit, and AA is an acrylic acid unit, are reported. The dansyl probe N(8aminooctanyl)5dimethylamino1naphthalene sulfonamide [DNSNH(CH2)(8)NH2], sensitive to conformational changes of the polymer, was covalently bound to the HPAm. The fluorescence of the dansyllabeled polymer and the influence of various external stimuli, such as pH, NaCl and metal ions, are described. The dansyl probe acts as a hydrophobic pendant on the chain of HPAm, and shows a blueshifted emission in comparison with the unattached dye, suggesting that the dansyl probe may cause a coiling effect toward a more compact structure of the polymer. Fluorescence emission studies reveal significant compaction of the polymer chain for pH in the range 4.79.8 and extension for pH below 3.7 or above 11.0. Increased fluorescence intensity with respect to the fluorescence signal of the free dansyl probe in NaCl, is emitted by the dansyllabeled polymer. Metal ions,such as Cd(II), Cu(II) and Co(II), modulate the fluorescence emission. Viscosity measurements yield further evidence of structural changes of HPAm, following the attachment of the dansyl probe. Fluorescence imaging of p[Am*](x)[Am](85)[AA](15x) allows observation of the dynamic behavior and the kinetics of conformational changes of the labeled polymer. Single polymer visualization is routinely achieved on DNA molecules, however, to our knowledge, nobody has so far extended this technique to synthetic polymers. It ius shown by fluorescence microscopy that the dansyllabeled polymer is stretched by a circular shear flow.

78.(2002) Physical Review Letters. 88, 24, Abstract
We present experimental studies of a new pattern sequence observed in nonBoussinesq convection in a compressible fluid near its gasliquid critical point (CP). Besides the known hysteretic transitions among conduction state, hexagons, and rolls, another hysteretic transition from rolls to hexagons at higher values of the control parameter is found. This reentrance phenomenon is observed in a rather narrow range of about 60 to 100mum cell heights and is attributed to large compressibility of a fluid near the CP.
2001

77.(2001) Europhysics Letters. 56, 6, p. 808814 Abstract
By using a heterodyne twocrossbeam configuration, we develop a new light scattering technique to study critical fluctuations. This optical arrangement gives the possibility to measure the critical dependence of both the intensity linewidth and the correlation function of the phase time derivative, in SF6 near its gasliquid critical point. We showed both experimentally and theoretically that the critical behaviour of the phase time derivative correlation function is determined by the thermodiffusive mode critical dependence, similar to the intensity linewidth. A good agreement between the experimental results and the theory is found.

76.(2001) Physical Review B. 64, 5, Abstract
We report experimental studies of parametric excitation of second sound (SS) by first sound (FS) in superfluid helium in a resonance cavity. The results on several topics in this system are presented: (i) The linear properties of the instability, namely, the threshold, its temperature and geometrical dependencies, and the spectra of SS just above the onset were measured. They were found to be in good quantitative agreement with the theory. (ii) It was shown that the mechanism of SS amplitude saturation is due to the nonlinear attenuation of SS via three wave interactions between the SS waves. Strong lowfrequency amplitude fluctuations of SS above the threshold were observed. The spectra of these fluctuations had a universal shape with exponentially decaying tails. Furthermore, the spectral width grew continuously with the FS amplitude. The role of three and four wave interactions are discussed with respect to the nonlinear SS behavior. The first evidence of Gaussian statistics of the wave amplitudes for the parametrically generated wave ensemble was obtained. (iii) The experiments on simultaneous pumping of the FS and independent SS waves revealed several effects. Below, the instability threshold, the SS phase conjugation as a result of three wave interactions between the FS and SS waves was observed. Above the threshold two interesting effects were found: a giant amplification of the SS wave intensity and strong resonance oscillations of the SS wave amplitude as a function of the FS amplitude. Qualitative explanations of these effects are suggested.

75.

74.(2001) Nature. 410, 6831, p. 905908 Abstract
Mixing in fluids is a rapidly developing area in fluid mechanics(13), being an important industrial and environmental problem. The mixing of liquids at low Reynolds numbers is usually quite weak in simple flows, and it requires special devices to be efficient. Recently, the problem of mixing was solved analytically for a simple case of random flow, known as the Batchelor regime(48). Here we demonstrate experimentally that very viscous liquids containing a small amount of highmolecularweight polymers can be mixed quite efficiently at very low Reynolds numbers, for a simple flow in a curved channel. A polymer concentration of only 0.001% suffices. The presence of the polymers leads to an elastic instability(9) and to irregular flow(10), with velocity spectra corresponding to the Batchelor regime(48). Our detailed observations of the mixing in this regime enable us to confirm several important theoretical predictions: the probability distributions of the concentration exhibit exponential tails(6,8), moments of the distribution decay exponentially along the flow 8, and the spatial correlation function of concentration decays logarithmically.

73.(2001) Physical Review Letters. 86, 12, p. 25572560 Abstract
The onset of the wave resistance, via generation of capillarygravity waves of a small object moving with velocity V, is investigated experimentally. Because of the existence of a minimum phase velocity V, for surface waves, the problem is similar to the generation of rotons in superfluid helium near their minimum. In both cases waves or rotons are produced at V > Vc due to Cherenkov radiation. We find that the transition to the wave drag state is continuous: in the vicinity of the bifurcation the wave resistance force is proportional to rootVVc for various fluids.

72.(2001) Physical Review Letters. 86, 5, p. 934937 Abstract
Behavior of a dilute polymer solution in a random threedimensional flow with an average shear is studied experimentally. Polymer contribution to the shear stress is found to be more than 2 orders of magnitude higher than in a laminar shear flow. The results indicate that the polymer molecules are strongly stretched by the random motion of the fluid.
2000

71.

70.(2000) Nature. 405, 6782, p. 5355 Abstract
Turbulence is a ubiquitous phenomenon that is not fully understood. It is known that the flow of a simple, newtonian fluid is likely to be turbulent when the Reynolds number is large (typically when the velocity is high, the viscosity is low and the size of the tank is large(1,2)). In contrast, viscoelastic fluids(3) such as solutions of flexible longchain polymers have nonlinear mechanical properties and therefore may be expected to behave differently. Here we observe experimentally that the flow of a sufficiently elastic polymer solution can become irregular even at low velocity, high viscosity and in a small tank. The fluid motion is excited in a broad range of spatial and temporal scales, and we observe an increase in the flow resistance by a factor of about twenty. Although the Reynolds number may be arbitrarily low, the observed flow has all the main features of developed turbulence. A comparable state of turbulent flow for a newtonian fluid in a pipe would have a Reynolds number as high as 10(5) (refs 1, 2). The low Reynolds number or 'elastic' turbulence that we observe is accompanied by significant stretching of the polymer molecules, resulting in an increase in the elastic stresses of up to two orders of magnitude.

69.(2000) Physical Review E. 61, 2, p. 18901898 Abstract
Observations show that plasma crystals, suspended in the sheath of a radiofrequency discharge, rotate under the influence of a vertical magnetic field. Depending on the discharge conditions, two different cases are observed: a rigidbody rotation (al the particles move with a constant angular velocity) and sheared rotation (the angular velocity of particles has a radial distribution). When the discharge voltage is increased sufficiently, the particles may even reverse their direction of motion. A simple analytical model is used to explain qualitatively the mechanism of the observed particle motion and its dependence on the confining potential and discharge conditions. The model takes into account electrostatic, ion drag, neutral drag, and effective interparticle interaction forces. For the special case of rigidbody rotation, the confining potential is reconstructed. Using data for the radial dependence of particle rotation velocity, the shear stresses are estimated. The critical shear stress at which shearinduced melting occurs is used to roughly estimate the shear elastic modulus of the plasma crystal. The latter is also used to estimate the viscosity contribution due to elasticity in the plasma liquid. Further development is suggested in order to quantitatively implement these ideas.
1999

68.(1999) Physical Review Letters. 83, 23, p. 47604763 Abstract
Direct measurements of the velocity in turbulent convection of SF6 near its gasliquid critical point by light scattering on the critical density fluctuations were conducted. The temperature, velocity, and cross frequency power spectra in a wide range of the Rayleigh and Prandtl numbers show scaling behavior with indices, which are rather close to the BolgianoObukhov scaling in the wave number domain. The statistics of the velocity fluctuations remain Gaussian up to the Reynolds numbers of 10(5).

67.(1999) Physical Review Letters. 83, 18, p. 36413644 Abstract
SF6 in the vicinity of its critical point was used to study turbulent convection up to exceptionally high Rayleigh numbers, Ra (up to 5 x 10(14)) and to verify for the first time the generalized scaling laws for the heat transport and the large scale circulation velocity as a function of Ra and the Prandtl number, Pr, in a very wide range of these parameters. Both scaling laws obtained are consistent with theoretical predictions by Shraiman and Siggia [Phys. Rev. A 42, 3650 (1900)].
1998

66.(1998) Physical Review Letters. 81, 26, p. 58125815 Abstract
We present the first experimental observation of phase conjugate (PC) second sound (SS) waves superfluid helium. The main feature of a PC wave is that its phase is complex conjugate to the Incident wave phase. It is generated and amplified as a result of nonlinear interaction between an incident SS wave and a first sound (FS) pumping wave. At FS amplitudes larger than the threshold value, a parametric instability, i.e., spontaneous decay of a FS wave into two SS waves, takes place. Three main, theoretically predicted, features of phase conjugate waves were verified experimentally.

65.(1998) Physics of Fluids. 10, 10, p. 24512463 Abstract
Experiments on flow stability and pattern formation in Couette flow between two cylinders with highly elastic polymer solutions are reported. It is found that the flow instabilities are determined by the elastic Deborah number, De, and the polymer concentration only, while the Reynolds number becomes completely irrelevant. A mechanism of such ''purely elastic'' instability was suggested a few years ago by Larson, Shaqfeh, and Muller [J. Fluid Mech. 218, 573 (1990)], referred to as LMS. It is based on the OldroydB rheological model and implies a certain functional relation between De at the instability threshold and the polymer contribution to the solution viscosity, eta(p)/eta, that depends on the polymer concentration. The elastic force driving the instability arises when perturbative elongational flow in radial direction is coupled to the strong primary azimuthal shear. This force is provided by the "hoop stress" that develops due to stretching of the polymer molecules along the curved streamlines. It is found experimentally that the elastic instability leads to a strongly nonlinear flow transition. Therefore, the linear consideration by LMS is expanded to include finite amplitude velocity perturbations. It is shown that the nature of the elastic force implies major asymmetry between inflow and outflow in finite amplitude secondary Rows. This special feature is indeed exhibited by the experimentally observed flow patterns. For one of the flow patterns it is also shown that the suggested elastic force should be quite efficient in driving: it, which is important evidence for the validity of the mechanism proposed by LMS. Further, the predicted relation between De and eta(p)/eta is tested. At fixed eta(p)/eta the elastic instability is found to occur at constant Deborah number in a broad range of the solution relaxation times in full agreement with the theoretical prediction. The experimentally found dependence of the Deborah number on eta(p)/eta also agrees with

64.(1998) Philosophical Magazine BPhysics Of Condensed Matter Statistical Mechanics Electronic Optical And Magnetic Properties. 78, 2, p. 253263 Abstract
We present a review of our recent experimental results on the stability and pattern selection in the CouetteTaylor (CT) flow of dilute polymer solutions. We succeeded in varying the solution elasticity by three orders of magnitude in a controlled way and have obtained a diagram of flow states in the whole elasticity range. It made it possible to study the role of elastic versus inertial effects in destabilization of the CT flow and pattern selection above the instability threshold. We investigated transformation of the flow instabilities from purely inertial to purely elastic and studied the properties of new elastic and inertioelastic modes. By changing the rheological properties of the polymer solution, we verified the suggested criterion of elastic instability.

63.(1998) Europhysics Letters. 43, 2, p. 165170 Abstract
The interrelation between elastic and inertial effects in destabilizing the flow of a polymer solution is studied experimentally. To achieve this goal, solution elasticity is varied by three orders of magnitude and a diagram of the flow states in a CouetteTaylor system is obtained. The regions of purely elastic and purely inertial flow instabilities and a crossover region between them are characterized. The main feature of the elastic instability, constant Deborah number at the instability threshold, is verified. An analogy between inertial and elastic flow transitions and dynamics is found and the concept of viscoelastic similarity is introduced.
1997

62.(1997) Physical Review Letters. 79, 21, p. 41784181 Abstract
The onset of the wave resistance, R, via generation of surface gravity waves by a long body moving with a velocity V, is considered. We demonstrate that for a special shape of the disturbance moving uniformly with velocity V, the wave resistance R(V) has a welldefined threshold (perfect bifurcation) at the critical value Vc, i.e., R similar to root V  Vc for V > Vc, and equals zero for V

61.(1997) Physical Review Letters. 78, 23, p. 43834386 Abstract
We report the experimental observation of the broadening of the frequency spectra of parametrically generated second sound waves by first sound in superfluid He4. The first experimental evidence of Gaussian statistics of amplitudes of parametrically generated waves is presented. Using the kinetic theory of parametric excitation we suggest that the broadening and exponential tails of spectra result from fourwave resonance interaction in a close analogy with similarevidence in parametrically generated spin waves. Universality of the exponential tails in the frequency spectra is suggested.

60.(1997) Physical Review E. 55, 5, p. R4877R4880 AbstractLargescale flow and spiral core instability in RayleighBenard convection
The spiral core instability, observed in large aspectratio RayleighBenard convection, is studied numerically in the framework of the SwiftHohenberg equation coupled to a largescale flow. It is shown that the instability leads to nontrivial core dynamics and is driven by the selfgenerated vorticity. Moreover, the recently reported transition from spirals to hexagons near the core is shown to occur only in the presence of a nonvariational nonlinearity, and is linked to the spiral core instability. Qualitative agreement between the simulations and the experiments is demonstrated.

59.(1997) Europhysics Letters. 38, 4, p. 297300 Abstract
We propose a hydrodynamic analog of the AharonovBohm experiment in which secondsound waves are scattered on a quantized vortex in superfluid helium. The time lag that develops between the two sides of a secondsound plane wave impinging on a vortex from the side would be compared. Estimates for these experiments show that for reasonable experimental conditions the wavefront splitting can be readily measured at reduced temperatures of 10(6). The advantages of doing this experiment near the superfluid transition is discussed. This experiment would compliment the three other AharanovBohmtype experiments which involve all four possible combinations of classical/quantum waves and classical/quantum vortices. This technique would provide a noninvasive means of investigating the dynamics of quantum vortex nucleation and their stability as well as addressing the problem of multiply quantized.

58.(1997) Physical Review Letters. 78, 8, p. 14601463 Abstract
We report experimental observation of a localized structure, which is of a new type for dissipative systems. It appears as a solitary vortex pair (''diwhirl'') in Couette Bow with highly elastic polymer solutions. In contrast to the usual solitons the diwhirls are stationary. It is also a new object in fluid dynamicsa pair of vortices that build a single entity. The diwhirls rise as a result of a purely elastic instability through a hysteretic transition at negligible Reynolds numbers. It is suggested that the vortex flow is driven by the same forces that cause the Weissenberg effect.
1996

57.(1996) Physical Review B. 54, 18, p. 1307213082 Abstract
It is shown that the GinzburgLandau model corrected for the normal component describes adequately the spinup problem for the superfluid liquid helium. An analysis of the Eckhaus instability in an inhomogeneous rotationally invariant system is presented. It has been found that the number of vortices which can be nucleated at the threshold of instability scales with the radius of the container as R(3/4). Th, effect of excitation of the vortex loops by thermal fluctuations is considered, and the barrier and the nucleation rate are evaluated.

56.(1996) Journal de Physique II. 6, 10, p. 14931516 AbstractDynamics of crack propagation in brittle materials

55.(1996) Physical Review Letters. 77, 8, p. 14801483 Abstract
We present experimental evidence of the striking influence of small additions of high molecular weight polymers on stability and pattern selection in CouetteTaylor flow. Two novel oscillatory flow patterns were observed. One of them is essentially due to the fluid elasticity. The other results from inertial instability modified by the elasticity.

54.

53.(1996) Physical Review B. 53, 1, p. 7578 Abstract
In the framework of the nonlinear Schrodinger equation (NLSE) as a model of superflow we found that multicharged vortices are very longliving objects, contrary to the accepted opinion. The lifetime of these entities is inversely proportional to the dissipation rate which can be incorporated phenomenologically into NLSE. We calculated unstable eigenvalues and corresponding eigenfunctions. The nonlinear stage of the instability is studied numerically. We discuss the implications of our observation in the context of spinup, flow past a body, and turbulence experiments in a superfluid helium.

52.(1996) Physical Review Letters. 76, 5, p. 756759 Abstract
We experimentally observed patterns consisting of domains of upflow hexagons coexisting with domains of downflow hexagons in Boussinesq RayleighBenard convection. The sequence of patterns, as the control parameter epsilon = Delta T/Delta Tc  1 is increased, is from ordered or disordered rolls at onset to extended targets and spirals at epsilon  1, to coexisting hexagons at epsilon similar to 3. Hexagons occur for all values of the Prandtl number 2.8 less than or equal to P less than or equal to 28 investigated. Surprisingly, they appear in a range where only rolls were known to be stable, and their wave number differs substantially from the roll wave number they coexist with.
1995

51.(1995) Physical Review Letters. 75, 6, p. 11021105 Abstract
A new method of detection and measurement of quantum vorticity by scattering second sound off quantized vortices in superfluid helium is suggested. Theoretical calculations of the relative amplitude of the scattered second sound waves from a single quantum vortex, a vortex lattice, and bulk vorticity are presented. The relevant estimates show that an experimental verification of the method is feasible. Moreover, it can even be used for the detection of a single quantum vortex.

50.(1995) PHYSICA D. 84, 4Mar, p. 635644 Abstract
A systematic, experimental and numerical search for subharmonic generation and/or amplification was conducted at intermediate times and moderate Reynolds numbers in nonlinear second sound near the superfluid transition. We found that the nonlinear acoustic waves are dynamically monotonic in the sense that only energy cascades to smaller and smaller scales (until the dissipation scale) exist. There is no indication of a decay of monochromatic waves to waves of lower wave numbers. This precludes the existence of a decay instability in Burgers' equation as has been discussed in the literature. We thus extend the theoretical proof of Sinai concerning the absence of subharmonics in the solutions of Burger's equation to intermediate times.

49.
1994

48.

47.(1994) Europhysics Letters. 28, 4, p. 237243 Abstract
We present experimental, theoretical and numerical results of a new mechanism of selffocusing and collapse of travellingwave pulses originating from large nonlinear dispersion. We discovered a transition between the mechanism of Bretherton and Spiegel (Bretherton C. S. and Spiegel E. A., Phys, Lett. A, 96 (1983) 152) and the new mechanism, as a function of the initial pulse width. These dynamics have been observed in travellingwave convection in binary mixtures and are described in the framework of the complex GinzburgLandau equation. We also present, for the first time, a quantitative experimental study of the role of phase winding in renormalizing the amplitude growth rate.

46.(1994) Physica A. 208, 4Mar, p. 373393 Abstract
We present experimental results on phase separation of a binary mixture of isobutyric acid and water in a thin horizontal, extended layer at the critical concentration, x(c), and in the vicinity of the consolute temperature, T(c), subjected to a vertical temperature gradient spanning the critical temperature. For relatively small temperature gradients, spinodal decompositionlike patterns are stabilized. A bubble pattern appears for slightly larger temperature gradients, surprisingly always near the hotter boundary, even when T(hot) > T(c). For still larger temperature gradients, polygon morphologies are observed. Their boundaries are probably formed by some kind of surface tension driven instability caused by the nonuniform surface tension along the bubble's interface. However, hydrodynamic instabilities alone have not been able to explain the novel morphologies. The average area of the cellular patterns varies strongly with T(c)  T(cold) and DELTAT across the fluid layer, whereas the mean area of the bubble like patterns changes just slightly.

45.(1994) Physical Review E. 49, 5, p. 40774086 Abstract
We present experimental results on novel pattern states which were observed in the CouetteTaylor flow subjected to an axial flow, in a wide range of the control parameters. Propagating Taylor vortices (PTV's), stationary spirals (SSP's), and moving spirals (MS's) were found as a result of a different symmetry breaking. These modes exhibit different wavenumber selection. Novel states originating from an interaction between these patterns were also found. A ''mixed phase'' of PTV's and SSP's was identified. A ''modecompetition'' state, in which the PTV's and MS's are alternated in the column, is also described. Finally, a ''disorderedTaylorvortices'' state was observed and characterized.

44.(1994) Physical Review E. 49, 2, p. 13091319 Abstract
The intrinsic noise in the CouetteTaylor system with axial flow is evaluated experimentally by several methods, which include a comparison of experimental data with numerical simulations of the amplitude equation with a noise term and the application of an external source of stochastic perturbations at the inlet. The intensity of the intrinsic noise is found in our system to be dependent on the throughflow velocity in the following manner: for large enough throughflow velocities (Reynolds number Re > 2) the intensity of the noise drastically increases with Re, whereas for small Re the noise amplitude is independent of Re and reaches a constant value of congruentto 0.02 mum/s, which is of the order of magnitude of the theoretically estimated value for the thermal noise. The amplitude of the intrinsic noise at large throughflow velocities (Re congruentto 3) is found in our system to be larger than the thermal noise by more than one order of magnitude. Its origin is suggested to be associated with the perturbations of the flow at the inlet boundary.

43.(1994) Physical Review E. 49, 2, p. 12911308 Abstract
A detailed study of the CouetteTaylor system with axial flow in the range of Reynolds number Re up to 4.5, which is characterized by the propagating Taylorvortices (PTV's) state, is presented. Two methods to measure the convective instability line are described. Comparative studies of the PTV's in the absolutely and convectively unstable regions are given. It was found that at Re 1. The wavenumber selection is also found to be differen

42.(1994) Nature. 367, 6461, p. 345347 Abstract
RAYLEIGHBENARD convection14, which occurs when a shallow fluid layer is heated from below, is commonly regarded as a paradigm for pattern formation under nonequilibrium conditions. The formation of hexagonal arrays of Benard cells is well known, but more complex patterns such as targets5 and spirals511 have also been reported. Similar patterns have been seen in electrohydrodynamical convection12,13, oscillatory chemical reactions1419 and biological systems19,20. In general, the spiral and target states are found for different experimental conditions. Here we report the observation of a continuous transition between states containing many spirals and many targets, in a fluid undergoing RayleighBenard convection near the gasliquid critical point. Whether spirals or targets are observed depends on the Prandtl number, the ratio between the thermal and viscous timescales in the fluid. Neither of these states seems to be predicted by the hydrodynamic equations that describe the fluid motions14, 21. The fact that the transformation of one pattern into the other is continuous, and that under some conditions they can coexist, suggests that they may be generated by the same or a similar mechanism.
1993

41.(1993) Physical Review Letters. 71, 20, p. 32913294 Abstract
Experimental studies of the source dynamics of counterpropagating waves in a convecting binary fluid are presented. The source motion is controlled by side heating. There exists a locking band inside which the source is pinned by the underlying roll structure. Outside this band the source commences to move abruptly by steps of half a wavelength. A distortion of the phase field and the resulting nonlocal flow are essential ingredients responsible for the abrupt transition to the sliding state. Strong similarity to switching charge density waves dynamics is discussed.

40.(1993) Physical Review E. 48, 2, p. R661R664 Abstract
We present direct experimental measurements of the reflection coefficient r and of the group velocity s for travelingwave convection in a binary fluid. We measure the dependence of r and s on the separation ratio psi. Theory predicts that, for small enough \psi\, r should vary as \psi\1/2. This dependence was not found. Instead, the value of r is almost constant over the range 0.136

39.(1993) Physical Review Letters. 70, 25, p. 38883891 Abstract
We present experimental results on RayleighBenard convection in SF6 near the gasliquid critical point. We measured the critical temperature difference for the onset of convection, DELTAT0, as a function of the reduced average temperature tau = (TBAR  T(c))/T(c) and found DELTAT0 = 525 x tau1.89, which is close to the expected power law behavior. The strong temperature dependence of the physical properties is used to scan the Prandtl number in a wide range. A new, many ''target'' pattern state, initiated by a defect instability, was observed.

38.

37.(1993) Journal of Low Temperature Physics. 90, 6May, p. 343354 Abstract
We present measurements of the critical temperature difference for the onset of thermal convection and the effective thermal conductivity in two He3superfluidHe4 mixtures. The mixtures were 6.8% and 9.8% by molar volume of He3 in He4 and the measurements were made from 0.65 K to just above the superfluid transition temperature for each mixture. The measurements were made as part of an effort to visualize convective flow patterns in helium mixtures using optical shadowgraph techniques. We discuss the implications of our results for this effort.
1992

36.(1992) Physical Review A. 46, 6, p. R2996R2999 AbstractTRANSITION FROM CONFINED TO EXTENDED TRAVELING WAVES IN A CONVECTIVE BINARY MIXTURE
Experimental evidence of two distinctive mechanisms of transition from localized (LTW) to extended (ETW) traveling waves in convecting binary mixtures is presented. Both are related to the convectively unstable nature of LTW, and reflect its different manifestations. In short cells the mechanism which is responsible for the LTW instability is related to the transition from convective to absolute instability. In long cells and negative enough values of the separation ratio psi, transition from LTW to ETW occurs due to the interaction of convectively growing perturbations and LTW. Crossover between the two mechanisms is demonstrated.
1991

35.(1991) Physical Review Letters. 67, 24, p. 33923395 Abstract
A noisesustained and noisemodulated structure of propagating Taylor vortices (PTV) in a convectively unstable regime of CouetteTaylor flow with a superimposed axial flow was observed and studied. The structure differs drastically from PTV in an absolutely unstable regime in its noisy power spectrum and in the noisy temporal dynamics of the front. Interaction of perturbations with the interface causes an amplitude modulation near the front and a noisy phase further to the outlet. The dependence of the front location on the control parameter indicates that perturbations from the inlet are responsible for the PTV existence.

34.

33.(1991) PHYSICA D. 51, 3Jan, p. 596607 AbstractROTATING RAYLEIGHBENARD CONVECTION  KUPPERSLORTZ TRANSITION
ROTATING RAYLEIGHBENARD CONVECTION  KUPPERSLORTZ TRANSITION

32.(1991) Europhysics Letters. 15, 6, p. 597602 Abstract
Experimental results on the mechanism of transition to defectmediated turbulence which occurs in a narrow long strip of electroconvecting nematics are presented for the first time. The scenario consists of a secondary bifurcation from stationary convection of normal rolls (Williams domain (WD)) to tilted WD, with spontaneous parity breaking, and then a transition to spatiotemporally disordered state via defect nucleation. The central point of the mechanism presented here is the discovery of fluctuations of a soft longitudinal mode in tilted WD which together with finite transverse modes lead to the defect nucleation.

31.(1991) Physical Review A. 43, 10, p. 57285731 Abstract
We present an experimental study of the Eckhaus instability and the spatialtemporal evolution of the roll structure in twodimensional (2D) anisotropic systems. In spite of the fact that the instability in 2D systems is manifested via the nucleation of topological defects, contrary to the case of 1D systems, we find no differences in the basic instability mechanism. Longitudinal, longwavelength modulation is responsible for the instability, exactly as in 1D systems, and thus the stability boundary and the spatialtemporal evolution dynamics are identical for 1D and 2D roll patterns.

30.(1991) Europhysics Letters. 15, 2, p. 167172 Abstract
The experimental data on the mutual interaction of a pair of topological defects in the roll pattern of electroconvecting nematics at a very small or negligible external stress are presented. It is shown that the mutual interaction of defects in a pair becomes significant only below a characteristic length, which is universal for both climbing and gliding and about an order of magnitude larger than the corresponding coherence length.

29.(1991) Europhysics Letters. 14, 4, p. 331336 Abstract
CouetteTaylor flow between cylinders with a superimposed axial flow is studied experimentally. The axial flow suppresses the basic stationary instabilty and leads to propagating Taylor vortices through a forward oscillatory bifurcation. While the throughflow velocity increases the propagating vortices are pushed downstream to the outlet so that at the velocity which corresponds to the absolute instability limit, the pattern is > out of the system. The surprising coexistence of steady Ekman and propagating Taylor vortices close to the inlet and outlet boundaries was discovered. The wave number selection mechanism, similar to that existing in the frontpropagating case, is also identified.

28.(1991) Physical Review A. 43, 2, p. 707722 Abstract
We present an experimental study of stationary convection in a binary mixture at positive values of a separation ratio. The interplay between the RayleighBenard and the Soret mechanisms of instability and the corresponding boundary conditions gives us the possibility to observe a transition from large to smallscale structures as well as a transition between patterns with different symmetries. We also investigate an influence of lateral boundaries and the cell geometry on the pattern selection.
1990
1989

25.

24.

23.(1989) PHYSICA D. 37, 13, p. 341358 Abstract
We present an experimental evidence of the existence of a largescale global circulation flow in nonlinear traveling wave states in convecting binary mixtures. We covered three of the known TW patterns: linear counterpropagating waves (CPW), spatially and temporally modulated TW and spatially modulated TW. We found that the linear CPW that occur in transient regime do not carry mass while both nonlinear states do, and the masstransport onset coincides with the transition to nonlinear TW.

22.(1989) Physica. D. 37, 13, p. 359383 Abstract
We present a study of oscillatory convection in two experimental systems: ethanolwater mixtures in a rectangular container heated from below and a thin layer of nematic liquid crystals under low frequency ac voltage. In both systems the first bifurcation is the transition to travelling waves (TW) with finite wave vector and frequency. We report experimental observations of a sequence of spatial structures and dynamical behaviour of nonlinear TW in a regime of a weak nonlinearity. Most of the rich variety of spatial and dynamical behaviour which we observe in onedimensional finite geometries has been reproduced by numerical simulations based on a simple model of coupled GinzburgLandau equations which considers only the combination of translation and finite geometry. More complicated spatiotemporal behaviour of TW in cells with twodimensional geometry which initiated by defect nucleation is attributed to the mechanism of modulational instability of TW.

21.
1988

20.

19.(1988) Physical Review A. 38, 9, p. 49394942 Abstract
We investigate the transition from weakly nonlinear to nonlinear travelingwave states. Pattern selection due to a transition from convective to absolute instability conditions is found, in good agreement with theory. While the linear properties depend on the (boundarydependent) threshold of convection, the weakly nonlinear properties refer back to the threshold of an infinite system.

18.(1988) Physical Review Letters. 61, 7, p. 838841 Abstract
A new state composed of quasiperiodic traveling waves has been observed in very close vicinity of the convection onset along a stable branch. This state precedes the recently discovered confined travelingwave state, and consists of left and rightgoing traveling waves which periodically alternate between either side of the cell. A timedependent wavenumber spectrum is a characteristic feature of this pattern. Effects of pattern translation and modulational instability can qualitatively explain the observed dynamical behavior.

17.

16.(1988) Physical Review Letters. 60, 20, p. 20302033 Abstract
We present an experimental observation of convective transport in oscillatory convection of a binary mixture. The results show the existence of mass transfer in the direction of propagation of nonlinear traveling waves (TW) as well as that of a backflow. Good agreement with a simple model for particle trajectories in TW was found. We found that the two previously observed oscillatory states differ in mass transfer. In linear counterpropagating waves no mass transfer is detected, and the masstransport onset coincides with the transition to nonlinear TW.

15.
1987

14.

13.(1987) Physical Review A. 35, 6, p. 27572760 Abstract
A novel localized structure of traveling waves (TW’s) was observed at convection onset in ethanolwater mixtures. It is one of various possible flow patterns of TW’s which were observed. We find that for almostonedimensional cell geometry unique dynamics of transient behavior (in the form of two counterpropagating TW’s) can lead to various stable states.

12.
1986

11.

10.(1986) Physical Review Letters. 57, 16, p. 20182021 Abstract
Pattern observations and heattransport measurements of convection in ethanolwater mixtures at positive values of the separation ratio

9.(1986) Physical Review A. 34, 1, p. 693696 Abstract
Flow visualization, heattransprot measurements, and the lightintensity profile as a function of time have been used to study nonlinear propagating waves in ethanolwater mixtures heated from below. The experimental results reveal the main features of the travelling waves predicted by recent theory.

8.
1985

7.

6.Pattern formation and wavenumber selection by rayleighbénard convection in a cylindrical container
We describe an apparatus and procedures for simultaneous heat transport measurements and computer enhanced shadowgraph flowpattern imaging in a shallow horizontal layer of fluid heated from below. The heat transport measurements have a resolution of better than 0.1%, and the shadowgraph technique can detect the flow field for ε ≡ (RR_{c})/R_{c} as small as 10^{2} (R_{c} is the critical value of the Rayleigh number R for onset of convection). The apparatus and procedures were used to study pattern and wave number evolution in a cylindrical layer of water with radiustoheight ratio L = 7.5 and Prandtl number σ = 6.1. We found that dynamic sidewall forcing during the early thermal transients after a change in the heat current from a subcritical to a supercritical value establishes a cylindrical flow pattern. Once created, this pattern is stable in our apparatus over the wide range 0.16 ≳ ε ≳ 8 even after the transients have decayed. With changing ε, adjustment in the wave number k takes place discontinuously by hysteretic changes at the cell center in the number of convection roll pairs. When ε is increased, the discontinuous changes at the cell center are towards smaller k and are preceded by a continuous loss of cylindrical symmetry (the middle roll pair moves off center). The selected wave numbers coincide neither with the zigzag instability of the infinite system, as once suggested, nor with a linear extrapolation to ε = O(1) of the recent prediction to lowest order in ε of Manneville and Piquemal and of Cross. Comparison of the selected k with measurements by others reveals no dependence upon L and σ. For ε

5.
1984

4.(1984) Physical Review A. 30, 5, p. 25482561 Abstract
An amplitude equation is derived for thermal convection in a binary fluid mixture in a porous medium and in bulk, in the vicinity of the intersection point of the lines of stationary and oscillatory instabilities. Slow spatial modulations are included in the amplitude equation near this codimension2 bifurcation. The experimental realizability in binary fluid mixtures is discussed with use of data for specific systems such as alcoholwater mixtures and normalfluid He3He4 mixtures. Analogies are drawn to other physical systems which are easily accessible to experiment, such as the convective instability in nematic liquid crystals in an external magnetic field.

3.

2.

1.