Rheotaxis of microswimmer in colloid-laden channel flow

Microswimmers are often found in heterogeneous and crowded environments within narrow conduits under external flow conditions, exhibiting interesting translational and rotational motions, such as upstream swimming, wall-following, and oscillatory motion. To investigate the rheotactic behavior of microswimmers in a channel flow with the presence of hard, monodisperse spherical colloids, we use the spherical squirmer model coupled with a mesoscale simulation framework, multi-particle collision dynamics. We find that the presence of colloids significantly modifies the rheotactic behavior of the squirmer. At low flow strengths, pushers preferentially occupy the channel center with a dominant upstream orientation, whereas pullers move away from the center toward the walls. In contrast, in a colloid-free channel, pushers exhibit near-wall motion with a downstream orientation dominance, and pullers near the channel center with upstream orientation dominance. At sufficiently high flow strengths, the imposed flow dominates the squirmer-induced hydrodynamics, leading pushers, neutrals, and pullers to adopt a common upstream-oriented state. We also observe that, for a constant colloidal packing fraction, the streamwise velocity of the puller is less than that of the pusher.