Rheotaxis of spheroidal squirmers in microchannel flow: Interplay of shape, hydrodynamics, active stress, and thermal fluctuations

Microswimmers exposed to microchannel flows exhibit an intriguing coupling between propulsion, shape, hydrodynamics, and flow which gives rise to distinct swimming behaviors. We employ a generic coarse-grained model of prolate spheroidal microswimmers, denoted as squirmers, exposed to channel flow to shed light onto their transport properties. The embedding fluid is implemented by the multiparticle collision dynamics approach (MPC), a particle-based mesoscale simulation method, which includes thermal fluctuations. Specifically, the influence of swimmer shape-spherical vs spheroidal-, active stress-pusher, ciliate, puller-, and thermal fluctuations on their rheotactic behavior is analyzed. The microswimmers accumulate at the confining walls at very low flow rates. With increasing flow strength, squirmers are depleted from the walls, and at high flow rates are also depleted from the channel center. The squirmers show pronounced cross-channel swimming between the confining walls with mixed oscillating and rotational motions due to thermal fluctuations. This strongly affects their rheotactic behavior. In particular, spherical pullers and ciliates swim upstream, whereas spherical pushers essentially swim downstream. The anisotropic shape of spheroidal squirmers enhances wall and center depletion and the alignment of the propulsion direction parallel to the flow, which leads to preferred downstream swimming for all active stresses. This emphasizes the importance of swimmer shape and hydrodynamic wall interactions on the transport properties of a microswimmer such as Volvox and Opalina, for example.