Holger Stark1

1, Technical University Berlin, Berlin, , Germany

Microswimmers show strikingly different emergent collective behavior than passive systems in equilibrium. The talk reviews some of our work, where we study how hydrodynamic flow fields and diffusiophoretic coupling via self-generated chemical fields influence the collective motion of active particles.

Using multi-particle collision dynamics, we simulate hydrodynamic flow fields generated by squirmer model swimmers. In quasi-two-dimensional geometry they show motility-induced phase separation [1,2]. However, the binodals depend on the mean squirmer density, a clear signature of the non-equilibrium. Under gravity squirmers exhibit a very dynamic sedimentation profile with dense layering at the bottom and exponential decay towards the top, where large-scale convective flow arises [3]. Finally, a single layer of squirmers under strong gravity exhibits different collective structures including "hydrodynamic Wigner crystals'' and swarming.

I also present results of Brownian dynamic simulations of active colloids interacting via self-generated chemical fields. Translational and rotational diffusiophoretic coupling gives rise to dynamic clustering, pulsating clusters, and a chemotactic collapse [4,5].

[1] A. Zöttl and H. Stark, Phys. Rev. Lett. 112, 118101 (2014).
[2] J. Blaschke, M. Maurer, K. Menon, A. Zöttl, and H. Stark, Soft Matter 12, 9821 (2016).
[3] J.-T. Kuhr, J. Blaschke, F. Rühle, and H. Stark, Soft Matter 13, 7548 (2017).
[4] O. Pohl and H. Stark, Phys. Rev. Lett. 112, 238303 (2014).
[5] O. Pohl and H. Stark, Eur. Phys. J. E 38, 93 (2015).