  The spatiotemporal dynamics in systems of active self-propelled particles is
controlled by the propulsion mechanism in combination with various direct
interactions, such as steric repulsion, hydrodynamics, and chemical fields.
Yet, these direct interactions are typically anisotropic, and come in different
'flavors', such as spherical and elongated particle shapes for steric
repulsion, pusher and puller flow fields for hydrodynamics, etc. The
combination of the various aspects is expected to lead to new emergent
behavior. However, it is a priori not evident whether shape and hydrodynamics
act synergistically or antagonistically to generate motility-induced clustering
(MIC) and phase separation (MIPS). We employ a model of prolate spheroidal
microswimmers - called squirmers - in quasi-two-dimensional confinement to
address this issue by mesoscale hydrodynamic simulations. For comparison,
non-hydrodynamic active Brownian particles (ABPs) are considered to elucidate
the contribution of hydrodynamic interactions on MIC and MIPS. For spherical
particles, the comparison between ABP and hydrodynamic-squirmer ensembles
reveals a suppression of MIPS due to hydrodynamic interactions. The fundamental
difference between ABPs and squirmers is attributed to an increased
reorientation of squirmers by hydrodynamic torques during their collisions. In
contrast, for elongated squirmers, hydrodynamics interactions enhance MIPS.
Thus, hydrodynamic interactions show opposing effects on MIPS for spherical and
elongated microswimmers.
