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Systems of active particles or moving microorganisms have emerged
recently as an attractive research direction. Since the systems reside
constantly in non-equilibrium, novel phenomena and collective patterns
are expected. The talk illustrates this with two examples.
The talk first considers theoretically the sedimentation of a dilute suspension of chemically powered colloids, so-called active Brownian particles, under gravity. Experiments show that the sedimentation length increases with the propulsion velocity of the particles [1]. Based on a Smoluchowski equation for non-interacting active Brownian particles including rotational diffusion, one determines the steady sedimentation profile of the suspension [2]. Interestingly, sedimentation is accompanied by polar order of the active particles, with the mean swimming velocity oriented against the gravitational field. The origin of the predicted polar order is purely kinetic. It results from the active motion and is not due to any particle interactions. The same is true for an enhanced orientational ordering at surfaces, which we also predict together with a strong accumulation of particles, as observed in the experiment [1]. In the second example I demonstrate how microswimmers behave in a microchannel. This mimics, for example, a swimming microorganism in blood vessels. I present simulations for a model swimmer called squirmer but also discuss analytical predictions. In particular, I show that for strong Poiseuille flow, the motion of the microswimmer can be mapped onto the dynamics of a non-linear oscillator. [1] J. Palacci, C. Cottin-Bizonne, C. Ybert, and L. Bocquet, Phys. Rev. Lett. 105, 088304 (2010). [2] M. Enculescu and H. Stark, Phys. Rev. Lett. 106, 208103 (2011). |
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