Issue 48, 2016

Phase separation and coexistence of hydrodynamically interacting microswimmers


A striking feature of the collective behavior of spherical microswimmers is that for sufficiently strong self-propulsion they phase-separate into a dense cluster coexisting with a low-density disordered surrounding. Extending our previous work, we use the squirmer as a model swimmer and the particle-based simulation method of multi-particle collision dynamics to explore the influence of hydrodynamics on their phase behavior in a quasi-two-dimensional geometry. The coarsening dynamics towards the phase-separated state is diffusive in an intermediate time regime followed by a final ballistic compactification of the dense cluster. We determine the binodal lines in a phase diagram of Péclet number versus density. Interestingly, the gas binodals are shifted to smaller densities for increasing mean density or dense-cluster size, which we explain using a recently introduced pressure balance [S. C. Takatori, et al., Phys. Rev. Lett. 2014, 113, 028103] extended by a hydrodynamic contribution. Furthermore, we find that for pushers and pullers the binodal line is shifted to larger Péclet numbers compared to neutral squirmers. Finally, when lowering the Péclet number, the dense phase transforms from a hexagonal “solid” to a disordered “fluid” state.

Graphical abstract: Phase separation and coexistence of hydrodynamically interacting microswimmers

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Article information

Article type
06 Sep 2016
11 Nov 2016
First published
14 Nov 2016
This article is Open Access
Creative Commons BY license

Soft Matter, 2016,12, 9821-9831

Phase separation and coexistence of hydrodynamically interacting microswimmers

J. Blaschke, M. Maurer, K. Menon, A. Zöttl and H. Stark, Soft Matter, 2016, 12, 9821 DOI: 10.1039/C6SM02042A

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