Self-organised structures in mixed active-passive suspensions due to hydrodynamic interactions

TL;DR

This study explores how hydrodynamic interactions influence the self-organisation of mixed active-passive suspensions, revealing new phase-separation phenomena and microstructures in dense microswimmer systems.

Contribution

It provides the first detailed analysis of self-organised structures in mixed active-passive suspensions considering hydrodynamics using 3D Stokesian dynamics.

Findings

Phase separation is metastable for neutral or puller squirmers at high densities.

Bottom-heavy squirmers can dynamically induce fibrillar phase separation.

Pullers with strong bottom-heaviness form lamellar structures with layered passive and active particles.

Abstract

Microswimmers in suspension exhibit collective swimming behaviour, forming various self-organised structures including ordered, aggregated, and turbulent-like structures. When mixed with passive particles phase-separation is known to occur, but due to the difficulty of accurately handling many-body hydrodynamic interactions, the formation of self-organised structures in mixed suspensions has remained unexplored so far. In this study, we investigate the dynamics of mixed dense suspensions of spherical bottom-heavy squirmers and obstacle spheres using Stokesian dynamics in three dimensions, taking hydrodynamic interactions into account. The results show that without an external orientating mechanism the formation of orientational order is in general disturbed by the presence of passive spheres. An initially phase-separated state is metastable for neutral or puller squirmers at high packing densities. When the squirmers are bottom-heavy, phase-separation can occur dynamically in some cases, notably a fibrillar kind of separation for neutral squirmers and pullers at medium densities. We also observed a novel form of lamellar phase-separation for pullers at high densities with strong bottom-heaviness, with a sandwich-like structure consisting of a layer of passive particles pushed by a layer of swimmers, followed by a gap. These results indicate that microstructure and particle transport undergo significant changes depending on the type of swimmer, highlighting the importance of hydrodynamic interactions. These insights allow for a deeper understanding of the behaviour of active particles in complex fluids and to control them using external torques.