Biomedical subjects

H Kellay

Publications and source records attributed to H Kellay.

At least 19 recordsLinked to original sources

PubMed2003

Self-similar dynamic quasi-two-dimensional sand fronts.

We report on a study of advancing quasi-two-dimensional sand fronts on an inclined flat and thin strip confined between two vertical plates. These fronts form when a thin initial stream of sand running down the flat obstacle gets trapped at some distance from the injection point. Right after this trapping, the front starts to advance upstream and grow in time. The shapes at successive times are found to be self-similar in time. The stability conditions for the obtained fronts are also outlined. A simple model for interface dynamics gives reasonable predictions for the observed shapes.

Journal Article↗

PubMed2002

Polymers in 2D turbulence: suppression of large scale fluctuations.

Small quantities of a long chain molecule or polymer affect two-dimensional turbulence in unexpected ways. Their presence inhibits the transfers of energy to large scales causing their suppression in the energy density spectrum. This also leads to the change of the spectral properties of a passive scalar which turns out to be highly sensitive to the presence of energy transfers.

Journal Article↗

PubMed2002

Thickness fluctuations in turbulent soap films.

Rapidly flowing soap films provide a simple and attractive system to study two-dimensional hydrodynamics and turbulence. By measuring the rapid fluctuations of the thickness of the film in the turbulent regime, we find that the statistics of these fluctuations closely resemble those of a passive scalar field in a turbulent flow. The scalar spectra are well described by Kolmogorov-like scaling while the high-order moments show clear deviations from regular scaling just like dye or temperature fluctuations in 3D turbulent flows.

Journal Article↗

PubMed2001

Fluctuation and dissipation in liquid-crystal electroconvection.

In this experiment a steady-state current is maintained through a liquid-crystal thin film. When the applied voltage is increased through a threshold, a phase transition is observed to a convective state characterized by the chaotic motion of rolls. Above the threshold, an increase in power consumption is observed that is manifested by an increase in the mean conductivity. A sharp increase in the ratio of the power fluctuations to the mean power dissipated is observed above the transition. This ratio is compared to the predictions of the fluctuation theorem of Gallavotti and Cohen using an effective temperature associated with the rolls' chaotic motion.

Journal Article↗

PubMed2001

Noncoalescing drops.

A pure water drop coalesces almost immediately with a pure water surface. Minute amounts of surfactant can alter this process dramatically. When the drop is released towards the surface of the solution from a certain height smaller than a well defined critical height, the drop of surfactant solution either remains on the surface for a specific time or coalesces immediately. The statistics of the residence time are systematically measured along with the critical heights necessary for coalescence. It turns out that the surface elasticity controls coalescence in such a situation.

Journal Article↗

PubMed2001

Dynamic sand dunes.

When sand falling in the spacing between two plates goes past an obstacle, a dynamic dune with a parabolic shape and an inner triangular region of nonflowing or slowly creeping sand forms. The angle of the triangular zone increases with the height of the dune and saturates at a value determined by the geometry of the cell. The width of the dune, related to the radius of curvature at the tip, shows universal features versus its height rescaled by geometrical parameters. The velocity profile in the flowing part is determined and found to be nonlinear. The parabolic shape can be accounted for using a simple driven convection-diffusion equation for the interface.

Journal Article↗

PubMed2001

Inhibition of the finite-time singularity during droplet fission of a polymeric fluid.

When a drop of fluid detaches from a capillary, singular behavior ensues. We show that the addition of very small amounts of polymer inhibits this singularity in an abrupt way and gives rise, after a period of self-similar dynamics as for simple liquids, to long-lived cylindrical necks or filaments which thin exponentially in time. This abrupt change occurs when the elongation rate epsilon* becomes comparable to the inverse of the polymer relaxation time leading to a large elongational viscosity eta(E) of the dilute polymer solution.

Journal Article↗

PubMed2001

Dynamic light scattering from lyotropic lamellar phases subjected to a flow field.

Dynamic light scattering experiments on lyotropic lamellar phases of brine and surfactant subjected to a flow field have been realized. The obtained results reveal that the relaxation times measured depend strongly on the velocity of the flow. This dependence is indicative of an increase of the effective elasticity modulus K and a decrease of the effective compressibility modulus (-)B of the lamellar phase with the flow velocity. This leads to the conclusion that the shear can induce a suppression of the undulation fluctuations of the bilayers of the lamellar phase. Our results show also that the rigidity of the membranes decreases as the salt concentration of the sample increases.

Biophysics↗

PubMed2001

Shear-induced first-order sponge-to-lamellar transition in a lyotropic surfactant system.

We report a shear-induced sponge (L3) to lamellar (L(alpha)) transition in a surfactant system. Under a constant shear rate, after a delay time t(n) we observe random nucleation and subsequent growth of the L(alpha) phase, demonstrating that the shear-induced transition is first order. A simple argument for the energy of a two-dimensional nucleus accounts for the observed delay and its shear-rate dependence.

Journal Article↗

PubMed2000

Observations of the collapse of dilute lyotropic lamellar phases under shear flow

Experimental evidence of the collapse of dilute lamellar phases due to shear flow is presented. Two systems are used: one composed of brine and an ionic surfactant, and another composed of water, a nonionic surfactant, and cosurfactant. We observe this transition for a range of lamellar spacings and brine salinity. The results are in reasonable agreement with recent theory in which the suppression of fluctuations by shear plays an important role.

Journal Article↗

PubMed2000

Probability density functions of the enstrophy flux in two dimensional grid turbulence

Probability density functions of the enstrophy flux in two dimensional grid turbulence are found to be strongly non-Gaussian and can be mimicked by stretched exponential functions. Evidence of this behavior is found in experiments using turbulent soap films and numerical simulations. The enstrophy flux itself is found to be constant for a range of scales corresponding to the enstrophy cascade.

Journal Article↗

PubMed2000

Coupling between flow and structure for a lamellar surfactant phase

The flow-structure relation of lamellar phases is studied using rheometry and cross-polarized microscopy under flow. The equilibrium phases show different defects. Low salinities lead to very viscous, "onion" phases, whereas at high salinity, a low viscosity plane lamellar phase is found. Under shear, the latter shows a sudden transition to a viscoelastic gel, with a texture and viscosity very similar to that of the onions. Gelation occurs after a certain delay time, increasing rapidly with salinity, by the nucleation of onions. This allows one to relate the delay time to the defect energy.

Journal Article↗

PubMed1999

Numerical study of grid turbulence in two dimensions and comparison with experiments on turbulent soap films.

Numerical simulations of two dimensional channel flow behind an array of cylinders are carried out for high Reynolds numbers. Results for the energy density and enstrophy spectra, as well as for velocity and vorticity differences, are presented. The results compare favorably with recent experiments carried out with turbulent soap films. Some marked deviations from expected behavior are found for the enstrophy spectrum and for moments of vorticity increments.

Journal Article↗

PubMed1998

Delayed fracture of an inhomogeneous soft solid

The spontaneous fracture of polymer gels was studied. Contrary to crystalline solids, where fracture usually happens instantaneously at a well-defined breaking strength, the fracture of a polymer gel can occur with a delay. When a constant force was applied, the cracks nucleated and started to propagate after a delay that can be as long as 15 minutes, depending on the force. This phenomenon can be understood by calculating the activation energy for crack nucleation in arbitrary dimension and accounting for the inhomogeneity of the gel network in terms of its fractal dimension.

Journal Article↗

PubMed1995