Sorting of binary active–passive mixtures in designed microchannels

Abstract

Mixtures of active and passive particles are ubiquitous at the microscale. Many essential microbial processes involve interactions with dead or immotile cells or passive crowders. When passive objects are immersed in active baths, their transport properties are enhanced and can be tuned by controlling active agents' spatial and orientational distribution. Active-passive mixtures provide a platform to explore fundamental questions about the emergent behaviour of passive objects under simultaneous thermal and active noise and a foundation for technological applications in cargo delivery and bioremediation. In this work, we use computational simulations to study an active-passive mixture confined in microchannels designed with funnel-like obstacles that selectively allow the passage of passive particles. Active particles follow overdamped Langevin translational dynamics and run-and-tumble rotational dynamics. We find that adjusting the tumbling rate of active agents and the microchannel geometry leads to a maximum enhancement of the transport properties of the passive particles (diffusion coefficient and advective velocity) that correlates with the highest mixture sorting efficiency and the shortest response time. We demonstrate that the active drift is the cause of the observed enhanced separation of the mixture in contrast with scenarios where only thermal or active diffusion are present.