Active mixtures in a narrow channel: Motility diversity changes cluster sizes

TL;DR

This study models bacteria in narrow channels to understand how motility diversity influences cluster sizes, revealing that polydispersity significantly affects cluster formation and can be approximated by an effective binary mixture.

Contribution

The paper introduces a minimal 1D lattice model incorporating motility diversity and confinement, providing new insights into cluster size distribution in active matter systems.

Findings

Cluster size increases with motility diversity in the absence of crossing.

Crossing rate reduces cluster size and diminishes the impact of diversity.

An effective theory accurately predicts cluster size distribution and related parameters.

Abstract

The persistent motion of bacteria produces clusters with a stationary cluster size distribution (CSD). Here we develop a minimal model for bacteria in a narrow channel to assess the relative importance of motility diversity (i.e. polydispersity in motility parameters) and confinement. A mixture of run-and-tumble particles with a distribution of tumbling rates (denoted generically by $\alpha$) is considered on a 1D lattice. Particles facing each other cross at constant rate, rendering the lattice quasi-1D. To isolate the role of diversity, the global average $\alpha$ stays fixed. For a binary mixture with no particle crossing, the average cluster size ($L_\text{c}$) increases with the diversity as lower-$\alpha$ particles trap higher-$\alpha$ ones for longer. At finite crossing rate, particles escape from the clusters sooner, making $L_\text{c}$ smaller and the diversity less important, even though crossing can enhance demixing of particle types between the cluster and gas phases. If the crossing rate is increased further, the clusters become controlled by particle crossing. We also consider an experiment-based continuous distribution of tumbling rates, revealing similar physics. Using parameters fitted from experiments with Escherichia coli bacteria, we predict that the error in estimating $L_\text{c}$ without accounting for polydispersity is around $60\%$. We discuss how to find a binary system with the same CSD as the fully polydisperse mixture. An effective theory is developed and shown to give accurate expressions for the CSD, the effective $\alpha$, and the average fraction of mobile particles. We give reasons why our qualitative results are expected to be valid for other active matter models and discuss the changes that would result from polydispersity in the active speed rather than in the tumbling rate.