8th Liquid Matter Conference September 6-10, 2011 Wien, Austria ...

P10.32Tue 611:23-14:00Active colloidal suspensions exhibit polar order undergravityHolger Stark 1 and Mihaela Enculescu 11 Technische Universität Berlin, Sekr. EW7-1, Hardenbergstraße 36 10623, Berlin,GermanyWe consider theoretically the sedimentation of a dilute suspension of chemically powered colloids,so-called active Brownian particles, under gravity. This system has been studied experimentallyand it was found that the sedimentation length increases with the propulsion velocity of theparticles [1]. We derive a Smoluchowski equation for non-interacting active Brownian particlessubject to gravity and also include rotational besides translational diffusion [2]. We determine thesteady sedimentation profile of the suspension both by perturbation analysis and by numericallysolving the Smoluchowsky equation. We show that sedimentation is accompanied by polar orderof the active particles, with the mean swimming velocity oriented against the gravitational field.We suggest realistic parameter values to observe this ordering which increases strongly with theparticle radius. The origin of the predicted polar order is purely kinetic. It results from the activemotion and is not due to any particle interactions. The same is true for an enhanced orientationalordering at surfaces, which we also predict together with a strong accumulation of particles,as observed in the experiment [1]. Finally, we extend our analysis to bottom-heavy active particles.[1] J. Palacci et al., Phys. Rev. Lett. 105, 088304 (2010)[2] M. Enculescu and H. Stark, submitted to Phys. Rev. Lett.32