Confinement-induced accumulation and spontaneous de-mixing of microscopic active-passive mixtures

TL;DR

This study investigates how confinement influences the distribution and segregation of active and passive microscopic particles, revealing non-monotonic accumulation and spontaneous de-mixing driven by active matter dynamics.

Contribution

It demonstrates experimentally and theoretically how spatial confinement induces complex particle distributions and spontaneous de-mixing in active-passive mixtures, advancing control over microscale transport.

Findings

Confinement causes non-monotonic steady-state particle distributions.

Active dynamics can be modeled by a space-dependent Poisson process.

Spontaneous de-mixing of active and passive particles is achievable.

Abstract

Understanding the out-of-equilibrium properties of noisy microscale systems and the extent to which they can be modulated externally, is a crucial scientific and technological challenge. It holds the promise to unlock disruptive new technologies ranging from targeted delivery of chemicals within the body to directed assembly of new materials. Here we focus on how active matter can be harnessed to transport passive microscopic systems in a statistically predictable way. Using a minimal active-passive system of weakly Brownian particles and swimming microalgae, we show that spatial confinement leads to a complex non-monotonic steady-state distribution of colloids, with a pronounced peak at the boundary. The particles' emergent active dynamics is well captured by a space-dependent Poisson process resulting from the space-dependent motion of the algae. Based on our findings, we then realise experimentally the spontaneous de-mixing of the active-passive suspension, opening the way for manipulating colloidal objects via controlled activity fields.