Low Reynolds Number Dynamics of Boundary Driven and “Activated” Microparticles
Lundi 6 juin 2016 14:00
- Duree : 1 heure
Lieu : Conference room - LIPhy - Bât E - 140 Avenue de la Physique - St Martin d’Hères. Accès par interphone, appeler le secrétariat
Orateur : Raphaël JEANNERET (University of Warwick, UK)
In this talk, I will present the results of 2 different experimental projects, both dealing with the dynamics of micron-sized particles at low Reynolds number. The first part will be focused on biomixing at the microscale. This thematic tries to rationalize to which extent motile organisms stir their local environment. If it is generally accepted that macroscopic to mesoscopic organisms do have a big influence on the mixing of oceans, estimation of this effect by micron-sized swimmers is still scarce. Trying to bring experimental estimate to this field, we investigated the dynamics of tracers in bath of swimming microorganisms, more particularly the model swimmer Chlamydomonas Reinhardtii, a 10μm-sized fresh-water unicellular alga. By combining 3 different experiments we have shown that the tracers exhibit enhanced diffusion an order of magnitude bigger than previously thought (corresponding, at low swimmer concentration of φ ≈ 0.5%, to ∼ 40 times the thermal diffusion).
In the second part of the talk, I will focus on a very fundamental aspect of low Reynolds number hydrodynamics : hydrodynamic reversibility. In brief, fluid particle trajectories of viscous flows have the property of being perfectly reversible upon reversal of the strain applied to the boundaries of the system. This property is a trivial consequence of the Stokes equation and has been astonishingly demonstrated experimentally by G.I. Taylor in the 1960s. Here we revisit this property by considering the dynamics of many non-Brownian droplets embedded in a liquid subjected to a periodic driving. We have shown that the dynamics of the particle assembly remains reversible (i.e. all particles do come back to their initial position after one cycle of the periodic driving) only below a certain threshold in the amplitude of driving. This transition appears to be a first order dynamical phase transition accompanied by a structural transition.
Contact : philippe.peyla@univ-grenoble-alpes.fr
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