Abstract
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 and hydrodynamics. These direct interactions are typically anisotropic, and come in different "flavors", such as spherical and elongated particle shapes, pusher and puller flow fields, 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. For spherical particles, the comparison between ABPs and hydrodynamic-squirmer ensembles reveals a suppression of MIPS due to hydrodynamic interactions. Yet, our analysis shows that dynamic clusters exist, with a broad size distribution. 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. The transition to a phase-separated state strongly depends on the nature of the swimmer's flow field-with an increased tendency toward MIPS for pullers, and a reduced tendency for pushers. Thus, hydrodynamic interactions show opposing effects on MIPS for spherical and elongated microswimmers, and details of the propulsion mechanism of biological microswimmers may be very important to determine their collective behavior.
Highlights
Motile bacteria at interfaces exhibit intriguing collective phenomena,[1,2] such as cluster formation, observed for Myxococcus xanthus[3] or Thiovulum majus,[4] as well as swarming, swirling, raft formation,[5,6,7,8,9,10] and the emergence of mesoscale turbulence,[8,11,12] observed for E. coli[13] or Bacillus subtilis.[14]
We have studied the collective dynamics of spherical and spheroidal squirmers confined in a narrow slit by mesoscale hydrodynamic simulations (MPC)
We find that hydrodynamics suppresses phase separation for spherical microswimmers, in contrast to ref. 35, but in accordance with ref. 28
Summary
Motile bacteria at interfaces exhibit intriguing collective phenomena,[1,2] such as cluster formation, observed for Myxococcus xanthus[3] or Thiovulum majus,[4] as well as swarming, swirling, raft formation,[5,6,7,8,9,10] and the emergence of mesoscale turbulence,[8,11,12] observed for E. coli[13] or Bacillus subtilis.[14] experiments on self-phoretic artificial spherical microswimmers, such as Janus particles, self-propelled liquid droplets, and photo-activated colloids, exhibit cluster formation and phase separation despite their isotropic shape and purely repulsive interactions.[15,16,17,18,19,20,21] The various active agents are propelled by different mechanisms and may exhibit different steric and propulsion-related interactions It is a priori not evident which processes govern structure formation in the rather distinct systems. The intuitive explanation for the emergence of MIC and MIPS is a positive feedback between blocking of persistent particle motion by steric interactions, and an enhanced probability of collisions with further particles at sufficiently large concentrations and activities.[21,28,29] in self-phoretic systems, the phoretic field has been shown to significantly contribute to clustering through field-induced attractive interactions.[30,31]
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