Collective dynamics of microswimmer suspensions in power-law fluids confined in a channel

Abstract

In this study, a two-dimensional lattice Boltzmann method is employed to numerically investigate the collective dynamics of squirmer–passive particle mixtures in a channel filled with power-law fluids. Particular attention is paid to the effects of the power-law index n, propulsion type β, and squirmer fraction c on the spatial distribution, local microstructure, squirmer orientation and transport properties of the system. The results show that n significantly alters the collective mode of the system. In shear-thinning fluids (n< 1), particles exhibit pronounced near-wall accumulation and cluster coalescence, which promotes the formation of large-scale aggregates. In Newtonian fluids (n = 1), particles mainly form finite-sized local aggregates near the channel walls. In shear-thickening fluids (n > 1), near-wall accumulation and cluster coalescence are largely suppressed; instead, particles are distributed over a wider region, while squirmers are more likely to develop stable orientational alignment. The dependence of local packing and orientational organization on β varies with n. In particular, puller-type squirmers (β> 0) are more prone to near-wall accumulation and local packing in shear-thinning fluids, whereas pushers (β < 0) exhibit stronger orientational alignment in shear-thickening fluids. The squirmer fraction c further regulates the transport properties of the mixture: reducing c enhances the near-wall retention and accumulation of passive particles and shifts the velocity distribution towards lower velocities, whereas increasing c helps sustain stronger active rearrangement and higher transport velocities. These findings indicate that squirmer aggregation does not necessarily correlate positively with orientational order. Rather, the organizational state of the mixed system emerges from the combined effects of n, β and c.