Biomedical subjects

Armand Ajdari

Publications and source records attributed to Armand Ajdari.

7 recordsLinked to original sources

PubMed2002

Fluctuations of fluctuation-induced casimir-like forces.

The force experienced by objects embedded in a correlated medium undergoing thermal fluctuations-the so-called fluctuation-induced force-is actually itself a fluctuating quantity. Using a scalar field model, we compute the corresponding probability distribution and show that it is a Gaussian centered on the well-known Casimir force, with a nonuniversal standard deviation that can be typically as large as the mean force itself. The relevance of these results to the experimental measurement of fluctuation-induced forces in soft condensed matter is discussed, as well as the influence of the finite temporal resolution of the measuring apparatus.

Journal Article↗

PubMed2002

Simple model for heterogeneous flows of yield stress fluids.

Various experiments evidence spatial heterogeneities in sheared yield stress fluids. To account for heterogeneities in the velocity gradient direction, we use a simple model corresponding to a nonmonotonic local flow curve and study a simple shear geometry. Different types of boundary conditions are considered. Under controlled macroscopic shear stress Sigma, we find homogeneous flow in the bulk and a hysteretic macroscopic stress-shear-rate curve. Under controlled macroscopic shear rate Gamma;, shear banding is predicted within a range of values of Gamma;. For small shear rates, stick-slip can also be observed. These qualitative behaviors are robust to changes in the boundary conditions.

Journal Article↗

PubMed2002

Patterning flows using grooved surfaces.

Through a simple analytical description we quantify how pressure-driven flows over grooved surfaces develop transverse components, which, for shallow grooves, can be modeled with simple anisotropic effective boundary conditions. Helical recirculation results in channels or capillaries with grooved walls. An experimental validation of our model is presented. Our analysis provides a workable guide for the design of 3D flows with simple patterns of grooved regions, e.g., to control the position of streams in the cross section of a channel or to promote mixing. Potential applications in microfluidics are outlined.

Algorithms↗

PubMed2002

Electrically induced interactions between colloidal particles in the vicinity of a conducting plane.

We address the problem of two-dimensional (2D) colloidal aggregation driven by an ac electrical field, by observing an aqueous dispersion of latex microspheres in contact with a conducting surface. Using micron-sized carboxylated polystyrene particles, we have systematically investigated the aggregation process, as a function of particle size and charge, and of the applied electric field amplitude and frequency. A low-density 2D phase is observed at high frequency (typically above 1 kHz), while at low frequency (below a "contact frequency" nu(c)) the collection of particles collapses into disconnected compact aggregates of crystalline (hexagonal) structure. We argue that this scenario is governed by the competition between an attractive force, of electrohydrodynamic nature, and a repulsive force, basically an electrical dipole-dipole interaction. Both contributions are revealed and analyzed in independent experiments on isolated particle pairs, using optical manipulation and dynamometry.

Journal Article↗

PubMed2002

Dynamic response of adhesion complexes: beyond the single-path picture.

We analyze the response of molecular adhesion complexes to increasing pulling forces (dynamic force spectroscopy) when dissociation can occur along either one of two alternative trajectories in the underlying multidimensional energy landscape. A great diversity of behaviors (e.g., nonmonotonicity) is found for the unbinding force and time as a function of the rate at which the pulling force is increased. In particular we identify a class of "harpoon" stickers that bind easily but resist strong pulling efficiently. Using existing data, we also demonstrate the consequent difficulty of unambiguously determining the features of the energy landscape from such single-molecule pulling experiments.

Journal Article↗

PubMed2002

Chaotic mixer for microchannels.

It is difficult to mix solutions in microchannels. Under typical operating conditions, flows in these channels are laminar-the spontaneous fluctuations of velocity that tend to homogenize fluids in turbulent flows are absent, and molecular diffusion across the channels is slow. We present a passive method for mixing streams of steady pressure-driven flows in microchannels at low Reynolds number. Using this method, the length of the channel required for mixing grows only logarithmically with the Péclet number, and hydrodynamic dispersion along the channel is reduced relative to that in a simple, smooth channel. This method uses bas-relief structures on the floor of the channel that are easily fabricated with commonly used methods of planar lithography.

Chemical Phenomena↗

PubMed2001

Transverse electrokinetic and microfluidic effects in micropatterned channels: lubrication analysis for slab geometries.

Off-diagonal (transverse) effects in micropatterned geometries are predicted and analyzed within the general frame of linear-response theory, relating applied pressure gradient and electric field to flow and electric current. These effects could contribute to the design of pumps, mixers, or flow detectors. Shape and charge-density modulations are proposed as a means to obtain sizeable transverse effects, as demonstrated by focusing on simple geometries and using the lubrication approximation.

Journal Article↗