David Saintillan (University of Illinois at Urbana-Champaign - USA)

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7 janvier 2011 11:00 » 12:00 — Amphi Langevin

Collective dynamics in particulate suspensions.

Dispersions of small particles in viscous fluids are ubiquitous in both natural and technological processes. A major difficulty in modeling and simulating these systems arises from the slow decay of hydrodynamic disturbances at low Reynolds number, which leads to long-ranged interactions and often results in strong velocity fluctuations and large-scale correlated motions. In this talk, I will discuss two problems of interest in which such effects arise : (i) the emergence of large-scale flows and collective motions in biologically active suspensions, and (ii) the dynamics in dispersions of polarizable particles under electric fields.

Biologically active suspensions, of which a bath of swimming microorganisms is a paradigmatic example, are known to undergo complex dynamics as a result of multi-body interactions : these motions are biologically relevant as they impact mean particle transport, mixing and diffusion, with possible consequences for nutrient uptake and the spreading of bacterial infections. To analyze these effects, a kinetic theory is first developed and applied to study the pattern formation in these systems. A stability analysis and nonlinear continuum simulations both predict the emergence of large-scale flows and concentration patterns beyond a certain system size and suspension volume fraction, resulting in complex chaotic dynamics and efficient fluid mixing. All these predictions from the kinetic theory are then tested using direct particle simulations, where the conclusions of the stability analysis are all confirmed.

The ability to manipulate small particles at the micro- and nanoscale plays a central role in a wide range of technological applications, from self-assembly to sorting to biochemical analysis. To this end, electric fields offer a low-cost and efficient method of controlling particle motions. Yet, understanding and modeling the dynamics that result when many particles are present remains a challenge in many situations. In this work, I investigate the dynamics that result from two nonlinear electrokinetic effects in large-scale suspensions : dielectrophoresis (which arises for any type of particle in an electrolyte) and induced-charge electrophoresis (which arises only for polarizable particles). Both effects are shown to cause significant particle-particle interactions and the formation of complex patterns, which are analyzed in detail using large-scale numerical simulations. I then conclude by discussing applications of this study to the assembly of colloidal structures by electrophoretic deposition.

Un séminaire organisé par le Laboratoire de Physique et mécanique des milieux hétérogènes (PMMH)

10 Rue Vauquelin, 75005 Paris