Abstract

Active motion of microorganisms or artificial microswimmers, such as active colloids, is an appealing subject which has attracted much attention recently. Since these swimmers move constantly in non-equilibrium, they give rise to novel phenomena which, in particular, occur when external fields are applied or when they move collectively.

The talk reviews our recent work on how active Brownian particles behave in external fields and in confinement. For example, they develop orientational order in a gravitational field [1] and exhibit an interesting instability in dense suspensions when they are bottom-heavy. They also create active fluid pumps in a harmonic trap. Finally, under Poiseuille flow they show nonlinear dynamics reminiscent of the nonlinear pendulum where the bounding walls introduce “dissipation” [2].

We also study the collective motion of so-called squirmers in a quasi 2D geometry by means of multi-particle collision dynamics. We observe dynamical clustering, phase separation, and active jamming which is strongly influenced by hydrodynamic near-field interactions. In dense suspensions rotational diffusion is greatly enhanced and the critical volume fraction for crystallisation is different for pushers and pullers.

[1] M. Enculescu and H. Stark, Phys. Rev. Lett. 106, 208103 (2011). [2] A. Zoettl and H. Stark, Phys. Rev. Lett. 108, 218104 (2012).

Co-authors: Marc Hennes (Institute of Theoretical Physics, Technische Universitt Berlin), Katrin Wolff (Institute of Theoretical Physics, Technische Universitt Berlin), Andreas Zoettl (Institute of Theoretical Physics, Technische Universitt Berlin)