Correlated dynamics of inclusions in a supported membrane

The hydrodynamic theory of heterogeneous fluid membranes is extended to the case of a membrane adjacent to a solid substrate. We derive the coupling diffusion coefficients of pairs of membrane inclusions in the limit of large separation compared to the inclusion size. Two-dimensional compressive stresses in the membrane make the coupling coefficients decay asymptotically as $1/r^2$ with interparticle distance $r$. For the common case, where the distance to the substrate is of sub-micron scale, we present expressions for the coupling between distant disklike inclusions, which are valid for arbitrary inclusion size. We calculate the effect of inclusions on the response of the membrane and the associated corrections to the coupling diffusion coefficients to leading order in the concentration of inclusions. While at short distances the response is modified as if the membrane were a two-dimensional suspension, the large-distance response is not renormalized by the inclusions.Comment: 15 page

Effective Viscosity of Dilute Bacterial Suspensions: A Two-Dimensional Model

Suspensions of self-propelled particles are studied in the framework of two-dimensional (2D) Stokesean hydrodynamics. A formula is obtained for the effective viscosity of such suspensions in the limit of small concentrations. This formula includes the two terms that are found in the 2D version of Einstein's classical result for passive suspensions. To this, the main result of the paper is added, an additional term due to self-propulsion which depends on the physical and geometric properties of the active suspension. This term explains the experimental observation of a decrease in effective viscosity in active suspensions.Comment: 15 pages, 3 figures, submitted to Physical Biolog

Effective Viscosity of a Dilute Suspension of Membrane-bound Inclusions

When particulate suspensions are sheared, perturbations in the shear flows around the rigid particles increase the local energy dissipation, so that the viscosity of the suspension is effectively higher than that of the solvent. For bulk (three-dimensional) fluids, understanding this viscosity enhancement is a classic problem in hydrodynamics that originated over a century ago with Einstein's study of a dilute suspension of spherical particles. \cite{Einstein1} In this paper, we investigate the analogous problem of the effective viscosity of a suspension of disks embedded in a two-dimensional membrane or interface. Unlike the hydrodynamics of bulk fluids, low-Reynolds number membrane hydrodynamics is characterized by an inherent length scale generated by the coupling of the membrane to the bulk fluids that surround it. As a result, we find that the size of the particles in the suspension relative to this hydrodynamic length scale has a dramatic effect on the effective viscosity of the suspension. Our study also helps to elucidate the mathematical tools needed to solve the mixed boundary value problems that generically arise when considering the motion of rigid inclusions in fluid membranes.Comment: 33 pages, 4 figures (preprint); submitted to Physics of Fluid

Numerical study of one-dimensional and interacting Bose-Einstein condensates in a random potential

We present a detailed numerical study of the effect of a disordered potential on a confined one-dimensional Bose-Einstein condensate, in the framework of a mean-field description. For repulsive interactions, we consider the Thomas-Fermi and Gaussian limits and for attractive interactions the behavior of soliton solutions. We find that the disorder average spatial extension of the stationary density profile decreases with an increasing strength of the disordered potential both for repulsive and attractive interactions among bosons. In the Thomas Fermi limit, the suppression of transport is accompanied by a strong localization of the bosons around the state k=0 in momentum space. The time dependent density profiles differ considerably in the cases we have considered. For attractive Bose-Einstein condensates, a bright soliton exists with an overall unchanged shape, but a disorder dependent width. For weak disorder, the soliton moves on and for a stronger disorder, it bounces back and forth between high potential barriers.Comment: 13 pages, 13 figures, few typos correcte

Bandgaps in the propagation and scattering of surface water waves over cylindrical steps

Here we investigate the propagation and scattering of surface water waves by arrays of bottom-mounted cylindrical steps. Both periodic and random arrangements of the steps are considered. The wave transmission through the arrays is computed using the multiple scattering method based upon a recently derived formulation. For the periodic case, the results are compared to the band structure calculation. We demonstrate that complete band gaps can be obtained in such a system. Furthermore, we show that the randomization of the location of the steps can significantly reduce the transmission of water waves. Comparison with other systems is also discussed.Comment: 4 pages, 3 figure

Préparation de couches minces de fer, cobalt et nickel et étude de leur absorption optique

We have studied, in a spectral region from 2 300 to 7 700 Å, the optical absorption of thin iron, cobalt and nickel layers, obtained in a vacuum by a thermal evaporation process which ensured a very high purity. By comparison with the absorption of the bulk metal, three distinct spectral regions can be discerned on the curves for 2νχd/2ν∞ χ∞ against wavelength, and in one of these regions, the absorption by the thin layer is proportional to that of the corresponding bulk metal. We conclude this paper by a tentative explanation of the experimental results.Nous avons étudié, dans un domaine spectral pouvant s'étendre de 2 300 à 7 700 Å, l'absorption optique du fer, du cobalt et du nickel pris en couches minces, obtenues sous vide par un procédé d'évaporation thermique leur garantissant une très grande pureté. Par comparaison avec l'absorption du métal massif, on peut mettre en évidence, sur les courbes donnant 2νχd/2ν ∞ χ∞ en fonction de la longueur d'onde, trois régions spectrales distinctes et montrer que dans l'une d'elles l'absorption par la couche mince est proportionnelle à celle du métal massif correspondant. En conclusion, nous donnons un court aperçu d'une possible signification de ces résultats expérimentaux

RESISTANCE ELECTRIQUE DE FILMS D'ALLIAGES DILUÉS Mg Fe ET Mg Mn DÉPOSÉS À BASSE TEMPÉRATURE

Des films d'alliages dilués de Fe et Mn dans le magnésium déposés sous ultra-vide à basse température, montrent un comportement résistif de type Kondo à une impureté. On étudie l'évolution de la résistance au cours des recuits.The resistance of dilute Mg Fe et Mg Mn alloys, quench condensed under ultra high vacuum conditions, shows a single impurity Kondo behaviour. The variations of the resistance versus annealing temperature are investigated

Fluctuating particle motion during shear induced granular compaction

4 pagesUsing a refractive index matching method, we investigate the trajectories of particles in a three dimensional granular packing submitted to cyclic shear deformation. The particle motion observed during compaction is not diffusive but exhibits transient cage effect, similar to the one observed in colloidal glasses. We precisely study the statistics of the step size between two successive cycles and observe that it is proportional to the shear amplitude. The link between the microscopic observations and the macroscopic evolution of the volume fraction during compaction is discussed