Confined binary active particles: Preferential wall accumulation and correspondence between active and fluid systems

Active Brownian particles (ABPs) serve as a generic model for synthetic active matter systems, such as active colloids, distinguished by their ability to self-propel while undergoing Brownian motion. Under confinement, ABPs exhibit propulsion-induced wall accumulation, a behavior that holds potential for microfluidic and lab-on-a-chip applications. This wall accumulation is further influenced in multi-component ABP systems by differences in particle activity, as well as by particle-particle and particle-wall interactions. We investigate two equimolar binary mixtures of ABPs confined in slit pores and examine how their wall accumulation depends on particle activity, particle-particle and particle-wall interactions, and slit width at both gas and liquid densities. We consider either a mixture with equal particle sizes but different softnesses, or one with different particle sizes but identical softnesses. Asymmetry in ABP size and softness plays a key role in preferential wall accumulation and multi-layer structuring, especially at high particle activities and liquid density. This leads to qualitatively new collective behaviors under confinement by coupling wall layering to motility-induced aggregation. The mixture with equal particle sizes exhibits confined motility-induced phase separation (MIPS) under strong confinement, characterized by coexistence between solid-like and dense phases, and surface MIPS in wider slits, characterized by high-density clustering near the walls coexisting with a moderately dense suspension in the slit interior. In contrast, the mixture with different particle sizes forms multi-layer cluster structures near the slit walls, with composition varying from layer to layer, rather than developing confined or surface MIPSs as observed in the equal-size mixture. Finally, we also explore the correspondence between confined binary ABPs and their equilibrium counterparts, confined equimolar binary fluid mixtures.