Mixtures of self-propelled particles interacting with asymmetric obstacles

TL;DR

This study uses simulations to show that diversity in self-propulsion speeds among active particles enhances rectification effects near asymmetric obstacles, revealing cooperative behaviors and density changes in active matter systems.

Contribution

It demonstrates how self-propulsion speed diversity amplifies rectification and alters collective behavior, emphasizing the importance of considering heterogeneity in active matter models.

Findings

Rectification current increases superlinearly with speed diversity.

Diversity leads to a rich wetting layer of fast particles.

Steady state density profiles deviate from classical lever rule expectations.

Abstract

In the presence of an obstacle, active particles condensate into a surface "wetting" layer due to persistent motion. If the obstacle is asymmetric, a rectification current arises in addition to wetting. Asymmetric geometries are therefore commonly used to concentrate microorganisms like bacteria and sperms. However, most studies neglect the fact that biological active matter is diverse, composed of individuals with distinct self-propulsions. Using simulations, we study a mixture of "fast" and "slow" active Brownian disks in two dimensions interacting with large half-disk obstacles. With this prototypical obstacle geometry, we analyze how the stationary collective behavior depends on the degree of self-propulsion "diversity", defined as proportional to the difference between the self-propulsion speeds, while keeping the average self-propulsion speed fixed. A wetting layer rich in fast particles arises. The rectification current is amplified by speed diversity due to a superlinear dependence of rectification on self-propulsion speed, which arises from cooperative effects. Thus, the total rectification current cannot be obtained from an effective one-component active fluid with the same average self-propulsion speed, highlighting the importance of considering diversity in active matter. Finally, rectification alters particle evaporation and absorption by the layer, making the density of the dilute phase increase with the global density. Therefore, the steady state violates the lever rule, a result which is valid even for systems of identical particles.