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Biomedical subjects

A Ajdari

Publications and source records attributed to A Ajdari.

At least 19 recordsLinked to original sources
PubMed2004

Elastic consequences of a single plastic event: a step towards the microscopic modeling of the flow of yield stress fluids.

With the eventual aim of describing flowing elasto-plastic materials, we focus here on the elementary process of such a flow, a plastic event, and compute the long-range perturbation it elastically induces in a medium submitted to a global shear strain. We characterize the effect of a nearby wall on this perturbation, and quantify the importance of finite-size effects. Although most of our explicit formulae refer to 2D situations, our statements hold for 3D situations as well.

Journal Article↗
PubMed2002

Rheological chaos in a scalar shear-thickening model.

We study a simple scalar constitutive equation for a shear-thickening material at zero Reynolds number, in which the shear stress sigma is driven at a constant shear rate gamma; and relaxes by two parallel decay processes: a nonlinear decay at a nonmonotonic rate R(sigma(1)) and a linear decay at rate lambda sigma(2). Here sigma(1,2)(t)= tau(-1)(1,2) integral (t)(0)sigma(t')exp[-(t-t')/tau(1,2)]dt' are two retarded stresses. For suitable parameters, the steady state flow curve is monotonic but unstable; this arises when tau(2)>tau(1) and 0>R'(sigma)>-lambda so that monotonicity is restored only through the strongly retarded term (which might model a slow evolution of the material structure under stress). Within the unstable region we find a period-doubling sequence leading to chaos. Instability, but not chaos, persists even for the case tau(1)-->0. A similar generic mechanism might also arise in shear thinning systems and in some banded flows.

Journal Article↗
PubMed2002

Electrically induced flows in the vicinity of a dielectric stripe on a conducting plane.

We report a theoretical and experimental study of the hydrodynamic flow induced by an a.c. electric field in the vicinity of a dielectric stripe deposited on a conducting plate. In the theoretical part, we model the stripe as a small change of the surface capacitance of the plate, and a perturbative approach is used to perform the calculations. This approach predicts an outwards rectified electro-osmotic slip along the surface that generates two steady counter-rotating rolls, the size of which decreases with the frequency. In the experimental section, we use tracers to determine the structure of the flow and investigate its dependence on the frequency and the amplitude of the applied voltage. The structure and amplitude of the observed flow compares satisfactorily with the theoretical analysis. This could guide the design of surface-controlled flows and help to understand the collective behavior of colloids near electrodes.

Journal Article↗
PubMed2001

Jamming, hysteresis, and oscillation in scalar models for shear thickening.

We investigate shear thickening and jamming within the framework of a family of spatially homogeneous, scalar rheological models. These are based on the "soft glassy rheology" model of Sollich et al. [Phys. Rev. Lett. 78, 2020 (1997)], but with an effective temperature x that is a decreasing function of either the global stress sigma or the local strain l. For appropriate x=x(sigma), it is shown that the flow curves include a region of negative slope, around which the stress exhibits hysteresis under a cyclically varying imposed strain rate (.)gamma.A subclass of these x(sigma) have flow curves that touch the (.)gamma=0 axis for a finite range of stresses; imposing a stress from this range jams the system, in the sense that the strain gamma creeps only logarithmically with time t, gamma(t) approximately ln t. These same systems may produce a finite asymptotic yield stress under an imposed strain, in a manner that depends on the entire stress history of the sample, a phenomenon we refer to as history-dependent jamming. In contrast, when x=x(l) the flow curves are always monotonic, but we show that some x(l) generate an oscillatory strain response for a range of steady imposed stresses. Similar spontaneous oscillations are observed in a simplified model with fewer degrees of freedom. We discuss this result in relation to the temporal instabilities observed in rheological experiments and stick-slip behavior found in other contexts, and comment on the possible relationship with "delay differential equations" that are known to produce oscillations and chaos.

Journal Article↗
PubMed2001

Casimir torques between anisotropic boundaries in nematic liquid crystals.

Fluctuation-induced interactions between anisotropic objects immersed in a nematic liquid crystal are shown to depend on the relative orientation of these objects. The resulting long-range "Casimir" torques are explicitly calculated for a simple geometry where elastic effects are absent. Our study generalizes previous discussions restricted to the case of isotropic walls, and leads to new proposals for experimental tests of Casimir forces and torques in nematics.

Journal Article↗
PubMed2001

Effective-area elasticity and tension of micromanipulated membranes.

We evaluate the effective Hamiltonian governing, at the optically resolved scale, the elastic properties of micromanipulated membranes. We identify floppy, entropic-tense and stretched-tense regimes, representing different behaviors of the effective-area elasticity of the membrane. The corresponding effective tension depends on the microscopic parameters (total area, bending rigidity) and on the optically visible area, which is controlled by the imposed external constraints. We successfully compare our predictions with recent data on micropipette experiments.

Biopolymers↗
PubMed2001

Viscoelasticity of solutions of motile polymers.

We explore the linear viscoelastic response of an entangled, isotropic solution of polar semiflexible polymers with active, motile centers which generate longitudinal motion. Because of the activity of these centers, the short-time modulus displays two novel power-law regimes: Initially G(t) proportional to t(-1/8), then the response is "Rouse-like" with G(t) proportional to t(-1/2). At longer times we find accelerated relaxation due to directed reptation, resulting in a reduced low frequency viscosity.

Actins↗
PubMed2001

Averaging rheological quantities in descriptions of soft glassy materials.

Many mean-field models have been introduced to describe the mechanical behavior of glassy materials. They often rely on averages performed over distributions of elements or states. We here underline that averaging is a more intricate procedure in mechanics than in more classical situations such as phase transitions in magnetic systems. This leads us to modify the predictions of the recently proposed soft glassy rheology model for soft glassy materials, for which we suggest that the viscosity should diverge at the glass transition temperature T(g) with an exponential form eta - exp[A/(T-T(g))].

Journal Article↗
PubMed2000

Pumping liquids using asymmetric electrode arrays

Following a general symmetry argument, I suggest using locally asymmetric electric geometries to pump liquid in channels or drive droplets on surfaces. This strategy, which requires no global gradient in the pumping direction, should be of interest for microfluidic devices and micro-electro-mechanical systems. A practical realization consists in using polar periodic arrays of electrodes addressed by an ac voltage difference. A simple electro-osmotic model provides an estimate of the pumping velocities achievable.

Journal Article↗
PubMed1999

Electroosmotic Flows Created by Surface Defects in Capillary Electrophoresis.

We compute the electroosmotic flow in nonuniformly charged planar and cylindrical capillaries for the limit of low-Reynolds-number flows and thin Debye layers. Analytical formulae for the velocity field are provided for the general case of an arbitrary surface inhomogeneity but we also focus on various specific defect geometries. Many important features can be obtained from the simple lubrication approximation. The pressure jump induced by the presence of such surface defects is calculated and the possible occurrence of recirculating flows is discussed, as are effects of the flow perturbations on dispersion in capillary electrophoresis. Copyright 1999 Academic Press.

Journal Article↗
PubMed1999

Energy transduction of isothermal ratchets: generic aspects and specific examples close to and far from equilibrium.

We study the energetics of isothermal ratchets which are driven by a chemical reaction between two states, and operate in contact with a single heat bath of constant temperature. We discuss generic aspects of energy transduction such as Onsager relations in the linear response regime as well as the efficiency and dissipation close to and far from equilibrium. In the linear response regime where the system operates reversibly, the efficiency is in general nonzero. Studying the properties for specific examples of energy landscapes and transitions, we observe in the linear response regime that the efficiency can have a maximum as a function of temperature. Far from equilibrium in the fully irreversible regime, we find a maximum of the efficiency with values larger than in the linear regime for an optimal choice of the chemical driving force. We show that the corresponding efficiencies can be of the order of 50%. A simple analytic argument allows us to estimate the efficiency in this irreversible regime for small external forces.

Energy Metabolism↗
PubMed1999

Moving droplets on asymmetrically structured surfaces.

It is shown theoretically and experimentally that a liquid droplet can move on a surface structured with a locally asymmetric pattern when a breathing of the drop is induced by external means. Two different situations can be envisioned: a drop whose volume is modulated and a drop whose equilibrium contact angle is switched between two extreme values. This last case was experimentally investigated using electric fields acting on water droplets in castor oil. The main trends of the theory are verified although a quantitative analysis would necessitate either a simpler experimental geometry or a more elaborate model. The results are discussed with a miniaturization of the setup in mind which would have important potential applications in the field of integrated analysis systems.

Journal Article↗
PubMed1996

Electrophoretic mobility of composite objects in free solution: application to DNA separation.

We propose here a simple procedure to estimate the electrophoretic mobility of composite objects obtained by linking two charged subunits, stressing that this electrophoretic mobility is in general different from the ratio of the total charge to the total friction coefficient. We focus especially on the situation where at least one of the subunits is a polyelectrolyte. Our remarks in particular correct the existing theoretical analysis of separation capabilities of devices in which a buoy (sphere, protein, polymer) is attached to a DNA chain. We also predict that, in some cases, the direction of electrophoretic motion can be reversed by increasing the electric field amplitude.

DNA↗
PubMed1994

Directional motion of brownian particles induced by a periodic asymmetric potential.

Structures possessing spatial asymmetry should act as pumps in the presence of dissipation alone, without the need for macroscopic forces or temperature differences to drive vectorial motion. It has been shown theoretically that particles subjected to an asymmetric periodic potential can display net directional motion even if the space-averaged force is zero. Here we demonstrate such behaviour experimentally. We have studied the behaviour of colloidal particles suspended in solution and exposed to a sawtooth dielectric potential which is turned on and off periodically. The particles exhibit net motion with a velocity that depends on their size, suggesting applications in separation processes for objects in the size range 0.1-5 microns--a range that includes biological structures such as viruses, cells and chromosomes. We furthermore point out the analogy between our device and motor protein assemblies.

Colloids↗