Abstract

Active Brownian particles (ABPs) serve as a generic model for synthetic active matter systems such as active colloids which can self-propel while undergoing Brownian motion. Under confinement, ABPs exhibit propulsion-induced wall accumulation [1], a fundamentally non-equilibrium behaviour with potential for practical applications such as microfluidics or lab-on-chip devices. In multi-component systems, the wall accumulation is further influenced by differences in particle activities which introduce demixing and spatial heterogeneity.

We use overdamped Langevin dynamics to study equimolar binary ABP mixtures confined in slit pores, varying slit width, propulsion strength, and interaction asymmetry. Two mixtures are considered: equal size particles with different softness (B1 mixture), and equal softness ones with different sizes (B2 mixture), at both gas-like and liquid-like densities.

The ratio of slit width to persistence length (related to propulsion strength) governs the wall accumulation behaviour. In B1 mixtures, overlapping wall-density peaks induce strong local crowding at higher activities, driving confined or surface motility-induced phase separation (MIPS). In B2 mixtures, spatially shifted density peaks of smaller and larger particles suppress MIPS in favour of compositionally distinct multi-layer structures, with smaller particles preferentially occupying the innermost wall layer.

Direct comparison with equilibrium Lennard-Jones fluid mixtures shows that ABP wall enrichment requires no attractive particle-wall interactions, is tunable by propulsion, and generates substantially larger wall pressures than those in adsorbed fluids. These results advance understanding of demixing and self-organisation in confined active colloidal systems, with implications for microfluidic particle sorting.

References

[1] K. Šindelka, A. Gadermeteva, M. Lísal, Soft Matter, 21, 7544-7564 (2021).