Publications
2018

139.(2018). von Karman swirling flow between a rotating and a stationary smooth disk: Experiment. Physical Review Fluids. 3:(1) 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.

138.(2018). Precise measurements of torque in von Karman swirling flow driven by a bladed disk. Journal of Turbulence. 19:(8)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.
2017

137.(2017). Polymer concentration and properties of elastic turbulence in a von Karman swirling flow. Physical Review Fluids. 2:(10) 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). On the role of initial velocities in pair dispersion in a microfluidic chaotic flow. Nature Communications. 8. 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.(2017). Elastic wake instabilities in a creeping flow between two obstacles. PHYSICAL REVIEW FLUIDS. 2:(5)
2016

134.(2016). Intermediate regime and a phase diagram of red blood cell dynamics in a linear flow. 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.(2016). Influence of polymer additives on turbulence in von Karman swirling flow between two disks. II. PHYSICS OF FLUIDS. 28:(3)

131.(2016). Oscillatory elastic instabilities in an extensional viscoelastic flow. Soft Matter. 12:(7)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). Longrange hydrodynamic effect due to a single vesicle in linear flow. Europhysics Letters. 113:(3)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). Turbulence and turbulent drag reduction in swirling flow: Inertial versus viscous forcing. Physical Review E. 92:(2) 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). Early turbulence in von Karman swirling flow of polymer solutions. Epl. 109:(1) 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). Wrinkling instability of vesicles in steady linear flow. Epl. 107:(2) 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). Fluid vesicles in flow. Advances in Colloid and Interface Science. 208:129141. 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). Torque and pressure fluctuations in turbulent von Karman swirling flow between two counterrotating disks. I. Physics of Fluids. 26:(5) 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). Complex Dynamics of Compound Vesicles in Linear Flow. Physical Review Letters. 112:(13) 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.(2014). Single Polymer Dynamics in A Random Flow. MACROMOLECULAR SYMPOSIA. 337:(1)3443. Abstract
Single Polymer Dynamics in A Random Flow
2012

122.(2012). Amplification of Thermal Noise by Vesicle Dynamics. 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). Elastically driven surface plumes in rimming flow of a nonNewtonian fluid. 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). Onset and universality of turbulent drag reduction in von Karman swirling flow. 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). Characteristic spatial scale of vesicle pair interactions in a plane linear flow. 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). Statistics and scaling properties of temperature field in symmetrical nonOberbeckBoussinesq turbulent convection. 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). Elastic turbulence in a curvilinear channel flow. Physical Review E. 84:(5) 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). Coagulation cascade of surface plumes in viscoelastic rimming flow. Epl. 96:(2) 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). Dynamics of vesicles in shear and rotational flows: Modal dynamics and phase diagram. Physics of Fluids. 23:(4) 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). Mixing of passive tracers in the decay Batchelor regime of a channel flow. 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). Molecular sensor of elastic stress in a random flow. Epl. 90:(4) 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). Strong symmetrical nonOberbeckBoussinesq turbulent convection and the role of compressibility. Physics of Fluids. 22:(3) 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). Elastic stresses in random flow of a dilute polymer solution and the turbulent drag reduction problem. Comptes Rendus Physique. 10:(8)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.(2009). Concentration dependence of the longest relaxation times of dilute and semidilute polymer solutions. Journal of Rheology. 53:(5)10691085. Abstract
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). Dynamics of a vesicle in general flow. Proceedings of the National Academy of Sciences of the United States of America. 106:(28)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). Phase Diagram of Single Vesicle Dynamical States in Shear Flow. 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.

107.(2009). Power and Pressure Fluctuations in Elastic Turbulence over a Wide Range of Polymer Concentrations. 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.
2008

106.(2008). Critical slowing down in polymer dynamics near the coilstretch transition in elongation flow. 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). Critical dynamics of vesicle stretching transition in elongational flow. 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 <is an element of(c), the vesicle remains tubular but fluctuates, though its steady state extension increases with the strain rate (is an element of)over dot. Above (is an element of)over dot(c), first a shape transition to dumbbell occurs, and then high order shape modes become unstable, leading to a pearling state. We have quantitatively characterized the transition and found a scaling of (is an element of)over dot(c) with the system parameters. A remarkable feature of vesicle tube behavior around the critical point is a slowdown of the vesicle relaxation to the final extended state in the vesicle stretching. Such feature is similar to that found in continuous phase transitions and to the critical effects recently observed for polymer molecules near the coilstretch transition in elongation flow.

104.(2008). Shear instability in fluids with a densitydependent viscosity. 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). Dynamics of interacting vesicles and rheology of vesicle suspension in shear flow. 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 <1, at odds with recent predictions made for a dilute suspension of noninteracting vesicles. Copyright (c) EPLA, 2008.
2007

102.(2007). Vesicle dynamics in timedependent elongation flow: Wrinkling instability. 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). Elastic turbulence in von Karman swirling flow between two disks. 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). NonNewtonian viscosity of complexplasma fluids. 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). Longest relaxation times of doublestranded and singlestranded DNA. Macromolecules. 40:(6)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). On the Lamb vector and the hydrodynamic charge. Experiments in Fluids. 42:(2)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). Role of elastic stress in statistical and scaling properties of elastic turbulence. 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). Statistics of tumbling of a single polymer molecule in shear flow. 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). Transition to tumbling and two regimes of tumbling motion of a vesicle in shear flow. 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). Continuous particle size separation and size sorting using ultrasound in a microchannel. 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). Orientation and dynamics of a vesicle in tanktreading motion in shear flow. 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). Validity of the Taylor hypothesis in a random spatially smooth flow. 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). Singlepolymer dynamics: Coilstretch transition in a random flow. Europhysics Letters. 71:(2)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). Spatial and temporal turbulent velocity and vorticity power spectra from sound scattering. 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). Statistics of particle pair separations in the elastic turbulent flow of a dilute polymer solution. Europhysics Letters. 68:(4)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). Chaotic flow and efficient mixing in a microchannel with a polymer solution. 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). Flow induced ultrasound scattering: Experimental studies. Physics of Fluids. 16:(5)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). Mixing by polymers: Experimental test of decay regime of mixing. 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.(2004). Highly resolved fluid flows: "Liquid plasmas" at the kinetic level. PHYSICAL REVIEW LETTERS. 92:(17)

84.(2004). Elastic turbulence in curvilinear flows of polymer solutions. 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). Levitation and agglomeration of magnetic grains in a complex (dusty) plasma with magnetic field. 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.(2002). Internal viscoelastic waves in a circular Couette flow of a dilute polymer solution. Europhysics Letters. 60:(5)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.

81.

80.(2002). Wave drag due to generation of capillarygravity surface waves. 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). Fluorescent ultrahighmolecularweight polyacrylamide probes for dynamic flow systems: Synthesis, conformational behavior and imaging. Macromolecular Chemistry and Physics. 203:(12)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). Reentrant hexagons in nonBoussinesq RayleighBenard convection: Effect of compressibility. 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). New light scattering technique based on phase time derivative correlation function. Europhysics Letters. 56:(6)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). Parametric generation of second sound in superfluid helium: Linear stability and nonlinear dynamics. 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.(2001). Vertical pairing of identical particles suspended in the plasma sheath. PHYSICAL REVIEW LETTERS. 86:(20)45404543.

74.(2001). Efficient mixing at low Reynolds numbers using polymer additives. Nature. 410:(6831)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). Onset of wave drag due to generation of capillarygravity waves by a moving object as a critical phenomenon. Physical Review Letters. 86:(12)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). Stretching of polymers in a random threedimensional flow. Physical Review Letters. 86:(5)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.(2000). Nonlinear vertical oscillations of a particle in a sheath of a rf discharge. PHYSICAL REVIEW LETTERS. 85:(19)40604063.

70.(2000). Elastic turbulence in a polymer solution flow. Nature. 405:(6782)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). Rigid and differential plasma crystal rotation induced by magnetic fields. Physical Review E. 61:(2)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). Spectra and statistics of velocity and temperature fluctuations in turbulent convection. Physical Review Letters. 83:(23)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). High Rayleigh number turbulent convection in a gas near the gasliquid critical point. Physical Review Letters. 83:(18)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). Acoustic phase conjugation in superfluid helium. Physical Review Letters. 81:(26)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). Mechanism of elastic instability in Couette flow of polymer solutions: Experiment. Physics of Fluids. 10:(10)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). Elastic versus inertial instability in CouetteTaylor flow of a polymer solution: review. Philosophical Magazine BPhysics Of Condensed Matter Statistical Mechanics Electronic Optical And Magnetic Properties. 78:(2)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). Elastic vs. inertial instability in a polymer solution flow. Europhysics Letters. 43:(2)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). Surface gravity waves due to a moving body: Onset of wave resistance as a critical phenomenon. Physical Review Letters. 79:(21)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 <Vc. For arbitrary shape of a moving disturbance the bifurcation becomes imperfect. It results in a small nonzero R usually observed at 0 <V <Vc. Using this idea, we have succeeded in describing quantitatively experimental data on the wave resistance of ship models obtained by D.W. Taylor in 1908.

61.(1997). Universal broadening of frequency spectra in parametrically driven systems. Physical Review Letters. 78:(23)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). On an analog of the AharonovBohm effect in superfluid helium. Europhysics Letters. 38:(4)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.

59.(1997). Largescale flow and spiral core instability in RayleighBenard convection. Physical Review E. 55:(5)R4877R4880. Abstract
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.

58.(1997). Solitary vortex pairs in viscoelastic Couette flow. Physical Review Letters. 78:(8)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). Spinup and nucleation of vortices in superfluid He4. Physical Review B. 54:(18)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). Dynamics of crack propagation in brittle materials. JOURNAL DE PHYSIQUE II. 6:(10)14931516.

55.(1996). CouetteTaylor flow in a dilute polymer solution. Physical Review Letters. 77:(8)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.(1996). Parametric generation of second sound by first sound in superfluid helium. PHYSICAL REVIEW LETTERS. 76:(12)21052108.

53.(1996). Observation of coexisting upflow and downflow hexagons in Boussinesq RayleighBenard convection. Physical Review Letters. 76:(5)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.

52.(1996). Stability of multicharged vortices in a model of superflow. Physical Review B. 53:(1)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.
1995

51.(1995). SCATTERING OF 2ND SOUNDWAVES BY QUANTUM VORTICITY. Physical Review Letters. 75:(6)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). BURGERSEQUATION AND THE EVOLUTION OF NONLINEAR 2ND SOUND. Physica D. 84:(4Mar)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.(1995). EXPERIMENTALSTUDY OF THE INSTABILITY OF CRACKPROPAGATION IN BRITTLE MATERIALS. EUROPHYSICS LETTERS. 30:(6)337342.
1994

48.(1994). BURST AND COLLAPSE IN TRAVELINGWAVE CONVECTION OF A BINARYFLUID. Physical Review E. 50:(5)37123722.

47.(1994). PHASE GRADIENT MECHANISM OF SELFFOCUSING AND COLLAPSE IN NONLINEAR DISPERSIVE TRAVELING WAVES. Europhysics Letters. 28:(4)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). PHASESEPARATION OF A CRITICAL BINARY MIXTURE SUBJECTED TO A TEMPERATUREGRADIENT. Physica A. 208:(4Mar)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). COMPETING STATES IN A COUETTETAYLOR SYSTEM WITH AN AXIALFLOW. Physical Review E. 49:(5)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). EXPERIMENTAL EVALUATION OF THE INTRINSIC NOISE IN THE COUETTETAYLOR SYSTEM WITH AN AXIALFLOW. Physical Review E. 49:(2)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). ABSOLUTE AND CONVECTIVE INSTABILITIES AND NOISESUSTAINED STRUCTURES IN THE COUETTETAYLOR SYSTEM WITH AN AXIALFLOW. Physical Review E. 49:(2)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 PTV's appear first at the outlet at the absolute instability transition. At Re > 1 the PTV's are also sustained in the convectively unstable region, but the properties of the PTV's in the absolutely and convectively unstable regions differ distinctively. In both regions the PTV's are characterized by the existence of an interface separating the pattern state from the CouettePoiseuille flow. The interface is stationary in the absolutely unstable region and fluctuates in the convectively unstable region. The distance from the inlet to the interface changes as both control parameters epsilonBAR and Re are varied, where epsilonBAR is the distance from the convective line. This dependence is, however, different in both regions. In the absolutely unstable region the healing length is scaled with the PTV's group velocity at all values of epsilonBAR and Re, and diverges at the absolute instability transition line. In the convectively unstable region the healing length does not obey the general scaling but is about inversely proportional to epsilonBAR. The most distinctive difference in the PTV's behavior in the two regions is a different sensitivity to noise. A timedependent spatial profile of the PTV's leads to a broadband power spectrum of the velocity in the convectively unstable region near the outlet. The PTV's velocity power spectrum in the absolutely unstable region is, on the other hand, noisefree. The different sensitivity to noise was used as an experimental criterion to locate the absolute instability line for Re > 1. The wavenumber selection is also found to be differen

42.(1994). TRANSITION BETWEEN SPIRAL AND TARGET STATES IN RAYLEIGHBENARD CONVECTION. Nature. 367:(6461)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). PHASE SLIPPAGE, NONADIABATIC EFFECT, AND DYNAMICS OF A SOURCE OF TRAVELING WAVES. Physical Review Letters. 71:(20)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). MEASUREMENT OF REFLECTION OF TRAVELING WAVES NEAR THE ONSET OF BINARYFLUID CONVECTION. Physical Review E. 48:(2)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 <psi <0.0096. We conjecture that the variations in concentration near the cell's end walls due to the traveling waves are responsible for the discrepancy.

39.(1993). RAYLEIGHBENARD CONVECTION NEAR THE GASLIQUID CRITICALPOINT. Physical Review Letters. 70:(25)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.(1993). ROTATING RAYLEIGHBENARD CONVECTION  ASYMMETRIC MODES AND VORTEX STATES. JOURNAL OF FLUID MECHANICS. 249:135159.

37.(1993). THERMALCONVECTION AND THERMALCONDUCTIVITY OF NONDILUTE HE3HE4 MIXTURES. Journal of Low Temperature Physics. 90:(6May)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). TRANSITION FROM CONFINED TO EXTENDED TRAVELING WAVES IN A CONVECTIVE BINARY MIXTURE. Physical Review A. 46:(6)R2996R2999. Abstract
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). NOISEMODULATED PROPAGATING PATTERN IN A CONVECTIVELY UNSTABLE SYSTEM. Physical Review Letters. 67:(24)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.(1991). ASYMMETRIC MODES AND THE TRANSITION TO VORTEX STRUCTURES IN ROTATING RAYLEIGHBENARD CONVECTION. PHYSICAL REVIEW LETTERS. 67:(18)24732476.

33.(1991). ROTATING RAYLEIGHBENARD CONVECTION  KUPPERSLORTZ TRANSITION. PHYSICA D. 51:(3Jan)596607. Abstract
ROTATING RAYLEIGHBENARD CONVECTION  KUPPERSLORTZ TRANSITION

32.(1991). MECHANISM OF TRANSITION TO A WEAK TURBULENCE IN EXTENDED ANISOTROPIC SYSTEMS. Europhysics Letters. 15:(6)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). LONGRANGE AND SHORTRANGE INTERACTIONS OF DEFECTS IN ANISOTROPIC HYDRODYNAMICAL SYSTEMS. Europhysics Letters. 15:(2)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.

30.(1991). ECKHAUS INSTABILITY AND DEFECT NUCLEATION IN 2DIMENSIONAL ANISOTROPIC SYSTEMS. Physical Review A. 43:(10)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.

29.(1991). CONVECTIVE VS ABSOLUTE INSTABILITY IN COUETTETAYLOR FLOW WITH AN AXIALFLOW. Europhysics Letters. 14:(4)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). STATIONARY CONVECTION IN A BINARY MIXTURE. Physical Review A. 43:(2)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

27.(1990). EXPERIMENTAL STUDIES OF DEFECT DYNAMICS AND INTERACTION IN ELECTROHYDRODYNAMIC CONVECTION. Physical Review A. 42:(10)59986008.

26.(1990). WEAKLY NONLINEAR STATES AS PROPAGATING FRONTS IN CONVECTING BINARYMIXTURES. Physical Review A. 41:(10)57435746.
1989

25.(1989). INTERACTIONS AND DYNAMICS OF TOPOLOGICAL DEFECTS  THEORY AND EXPERIMENTS NEAR THE ONSET OF WEAK TURBULENCE. Physical Review Letters. 63:(12)12371240.

24.(1989). PATTERN SELECTION AND TRANSITION TO TURBULENCE IN PROPAGATING WAVES. Physica D. 37:(3Jan)359383.

23.(1989). SPATIALLY AND TEMPORALLY MODULATED TRAVELINGWAVE PATTERN IN CONVECTING BINARYMIXTURES  REPLY. Physical Review Letters. 63:(5)579579.

22.

21.(1989). TRAVELING WAVES AND DEFECTINITIATED TURBULENCE IN ELECTROCONVECTING NEMATICS. Physical Review Letters. 62:(7)756759.
1988

20.

19.(1988). NONLINEAR PATTERN AND WAVENUMBER SELECTION IN CONVECTING BINARYMIXTURES. Physical Review A. 38:(9)49394942.

18.(1988). SPATIALLY AND TEMPORALLY MODULATED TRAVELINGWAVE PATTERN IN CONVECTING BINARYMIXTURES. Physical Review Letters. 61:(7)838841.

17.(1988). UNIVERSALITY OF PHYSICALPROPERTIES OF DISORDERED ALLOYS. Physical Review Letters. 60:(23)24022405.

16.(1988). MASSTRANSPORT IN PROPAGATING PATTERNS OF CONVECTION. Physical Review Letters. 60:(20)20302033.

15.(1988). DEPRESSION OF THE SUPERFLUID TRANSITIONTEMPERATURE IN HE4 BY A HEAT CURRENT. PHYSICAL REVIEW LETTERS. 60:(15)15221525.
1987

14.(1987). VORTEXFRONT PROPAGATION IN RAYLEIGHBENARD CONVECTION. Physical Review Letters. 58:(13)13321335.

13.(1987). MULTISTABILITY AND CONFINED TRAVELINGWAVE PATTERNS IN A CONVECTING BINARY MIXTURE. Physical Review A. 35:(6)27572760.

12.(1987). SINGULARITY IN THE KAPITZA RESISTANCE BETWEEN GOLD AND SUPERFLUID HE4 NEAR TLAMBDA. PHYSICAL REVIEW LETTERS. 58:(4)377380.
1986

11.(1986). PHASEDIAGRAM OF EXTERNALLY MODULATED RAYLEIGHBENARD SYSTEM NEAR THE CODIMENSION2 POINT. Physical Review A. 34:(5)41714180.

10.(1986). COMPETING PATTERNS IN A CONVECTIVE BINARY MIXTURE. Physical Review Letters. 57:(16)20182021.

9.(1986). FLOW PATTERNS AND NONLINEAR BEHAVIOR OF TRAVELING WAVES IN A CONVECTIVE BINARY FLUID. Physical Review A. 34:(1)693696.

8.(1986). MULTICRITICALITY IN VISCOELASTIC FLUIDS HEATED FROM BELOW. Physical Review A. 33:(2)14541457.
1985

7.(1985). CHAOTIC BEHAVIOR IN EXTERNALLY MODULATED HYDRODYNAMIC SYSTEMS. Physical Review A. 32:(1)702705.

6.(1985). PATTERNFORMATION AND WAVE NUMBER SELECTION BY RAYLEIGHBENARD CONVECTION IN A CYLINDRICAL CONTAINER. PHYSICA SCRIPTA. T9:97110.

5.(1985). TIMEDEPENDENCE OF FLOW PATTERNS NEAR THE CONVECTIVE THRESHOLD IN A CYLINDRICAL CONTAINER. Physical Review Letters. 54:(13)13731376.
1984

4.(1984). CODIMENSION2 BIFURCATIONS FOR CONVECTION IN BINARY FLUID MIXTURES. Physical Review A. 30:(5)25482561.

3.(1984). AMPLITUDE EQUATIONS FOR THE ONSET OF CONVECTION IN A REACTIVE MIXTURE IN A POROUSMEDIUM. JOURNAL OF CHEMICAL PHYSICS. 80:(1)431435.

2.(1984). ANALOG OF THE BENJAMINFEIR INSTABILITY NEAR THE ONSET OF CONVECTION IN BINARY FLUID MIXTURES. PHYSICAL REVIEW A. 29:(4)23032304.

1.(1984). CROSSOVER FROM CRITICAL TO TRICRITICAL BEHAVIOR IN A NONEQUILIBRIUM SYSTEM  THE CONVECTIVE INSTABILITY IN A BINARY FLUID MIXTURE. Physical Review A. 30:(6)33663368.