Publications
2002
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6.(2002) Nature. 420, 6915, p. 473 Abstract
The aim of aerodynamic design is to reduce the drag experienced by a body, such as a car, in a flowing medium, such as air. But what happens if the body is flexible and bends in response to the flow?
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5.(2002) Europhysics Letters. 60, 5, p. 704-709 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 non-uniform elastic stress distribution and should occur in the region of stable stratification of the hoop elastic stresses in any viscoelastic fluid flow with curvilinear trajectories.
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4.(2002) Physical Review E. 66, 5, p. 13 Abstract
The onset of the wave resistance via the generation of capillary-gravity waves by a small object moving with a velocity V is investigated experimentally. Due to the existence of a minimum phase velocity [formula presented] 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 [formula presented] 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 [formula presented] for various fluids. This observation contradicts the theory of Raphaël 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 capillary-gravity wave pattern and the shape of the wave-generating region are investigated both experimentally and theoretically. Good agreement between the theory and the experimental data is found in this case.
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3.(2002) Macromolecular Chemistry and Physics. 203, 12, p. 1833-1843 Abstract
The synthesis and characterization of ultrahigh-molecular-weight fluorescent partially hydrolyzed polyacrylamide (HPAm) of the type P[Am*]ξ[Am] 85[AA]15-ξ, where Am is an acrylamide unit, Am* is a fluorescent-labeled Am unit, and AA is an acrylic acid unit, are reported. The dansyl probe N-(8-aminooctanyl)-5-dimethylamino-1-naphthalene-sulfonamide [DNSNH(CH2)8NH2], sensitive to conformational changes of the polymer, was covalently bound to the HPAm. The fluorescence of the dansyl-labeled 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 blue-shifted 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 4.7-9.8 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 dansyl-labeled 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*]ξ[Am]85[AA]15-ξ allows the 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 is shown by fluorescence microscopy that the dansyl-labeled polymer is stretched by a circular shear flow.
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2.(2002) Physical review letters. 88, 24, p. 4 Abstract
We present experimental studies of a new pattern sequence observed in non-Boussinesq convection in a compressible fluid near its gas-liquid 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 [Formula presented] cell heights and is attributed to large compressibility of a fluid near the CP.
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1.(2002) Physical review letters. 88, 24, p. 2445031-2445034 244503. AbstractReentrant hexagons in non-Boussinesq Rayleigh-Bénard convection: Effect of compressibility
An investigation of a new pattern sequences observed in non-Boussinesq in a compressible fluid near gas-liquid critical point (CP) was presented. Another hysteretic transition from rolls to hexagons at higher value of the control parameter was found, besides the known hysteretic transitions among conduction state, hexagons and rolls. The results showed that a reentrance phenomenon was observed in a rather narrow range of about 60- to 100-μm cell heights and was attributed to large compressibility of a fluid near the CP.