Collective motion in a sheet of microswimmers

TL;DR

This study uses numerical simulations to explore how microswimmers behave collectively in a confined, quasi-2D environment, revealing that boundaries significantly alter their collective motion compared to unbounded systems.

Contribution

The paper introduces a minimal model for microswimmers in confined geometries, showing that boundary effects dominate the nature of collective motion, differing from unbounded suspension behaviors.

Findings

Pushers exhibit short-range collective order in confined geometry.

Pullers develop long-wavelength density instabilities leading to clustering.

Hydrodynamic screening by walls is less influential than geometric restrictions.

Abstract

Self-propelled micron-size particles suspended in a fluid, like bacteria or synthetic microswimmers, are strongly non-equilibrium systems where particle motility breaks the microscopic detailed balance, often resulting in large-scale collective motion. Previous theoretical work has identified long-range hydrodynamic interactions as the main driver of collective motion in unbounded dilute suspension of rear-actuated ("pusher") microswimmers. In contrast, most experimental studies of collective motion in microswimmer suspensions have been carried out in quasi-2-dimensional geometries such as in thin films or near solid or fluid interfaces, where both the swimmers' motion and their long-range flow fields become altered due to the proximity of a boundary. Here, we study numerically a minimal model of microswimmers in such a restricted geometry, where the particles move in the midplane between two no-slip walls. For pushers, we demonstrate collective motion with only short-ranged order, in contrast with the long-ranged flows observed in unbounded systems. For front-actuated ("puller") microswimmers, we discover a long-wavelength density instability resulting in the formation of dense microswimmer clusters. Both types of collective motion are fundamentally different from their previously studied counterparts in unbounded domains. Our results illustrate that hydrodynamic screening due to the presence of a wall is subdominant in determining the collective state of the suspension, which is instead dictated by the geometrical restriction of the swimmers' motion.