Hydrodynamic control of clustering and transport in squirmer–passive suspensions

This study examines how mixture composition and total concentration affect collective organization and passive-particle diffusion in squirmer–passive suspensions. Lattice Boltzmann simulations are performed for two-dimensional periodic suspensions of equal-sized squirmers and passive particles at a low Reynolds number (Re). Using pure squirmer suspensions as a reference, we investigate the effects of the hydrodynamic mode (β), passive-particle fraction (c), and total area fraction (ψ). At an intermediate area fraction, increasing the passive-particle fraction is associated with more pronounced local ordering in strong puller mixtures, whereas the spatial structure of strong pusher mixtures changes comparatively little. At this area fraction, the relative ranking of passive-particle transport metrics across swimming modes also varies with mixture composition: as the passive-particle fraction increases, the mode yielding the highest metric values changes from neutral squirmers to strong pushers. For strong pushers, the area-fraction-weighted passive diffusivity varies nonmonotonically with the total area fraction and reaches its highest value at the intermediate area fraction among the concentrations examined. These results connect changes in local particle organization with composition- and concentration-dependent transport, showing that the swimming mode, mixture composition, and total concentration jointly influence particle aggregation and passive-particle transport in the two-dimensional suspensions studied.