title: Phase separation and coexistence of hydrodynamically interacting microswimmers.
abstract: 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.
authors.affiliation: Institute of Theoretical Physics, Technische Universität Berlin, Hardenbergstr. 36, D-10623 Berlin, Germany. johannes.blaschke@tu-berlin.de holger.stark@tu-berlin.de.
authors.forename: Johannes
authors.identifier: http://orcid.org/0000-0002-6024-3990
authors.initials: J
authors.lastname: Blaschke
authors.affiliation: Department of Physics and Astronomy, University of California, Los Angeles, California 90095, USA.
authors.forename: Maurice
authors.identifier: http://orcid.org/0000-0002-6541-4197
authors.initials: M
authors.lastname: Maurer
authors.affiliation: Institute of Theoretical Physics, Technische Universität Berlin, Hardenbergstr. 36, D-10623 Berlin, Germany. johannes.blaschke@tu-berlin.de holger.stark@tu-berlin.de.
authors.forename: Karthik
authors.initials: K
authors.lastname: Menon
authors.affiliation: The Rudolf Peierls Centre for Theoretical Physics, University of Oxford, 1 Keble Road, Oxford, OX1 3NP, UK.
authors.forename: Andreas
authors.identifier: http://orcid.org/0000-0002-1736-9619
authors.initials: A
authors.lastname: Zöttl
authors.affiliation: Institute of Theoretical Physics, Technische Universität Berlin, Hardenbergstr. 36, D-10623 Berlin, Germany. johannes.blaschke@tu-berlin.de holger.stark@tu-berlin.de.
authors.forename: Holger
authors.initials: H
authors.lastname: Stark