Density shock waves in confined microswimmers

TL;DR

This paper investigates the formation of density shock waves in confined microswimmers under external flow, revealing a transition from subsonic to supersonic regimes influenced by hydrodynamics and confinement.

Contribution

It introduces a novel quasilinear wave model that captures the dependence of shock formation on external flow in microswimmer systems.

Findings

Density shock waves transition from subsonic to supersonic with increasing flow.

Hydrodynamic interactions and confinement critically influence shock behavior.

The model provides insights for controlling density distribution in microfluidic applications.

Abstract

Motile and driven particles confined in microfluidic channels exhibit interesting emergent behavior from propagating density bands to density shock waves. A deeper understanding of the physical mechanisms responsible for these emergent structures is relevant to a number of physical and biomedical applications. Here, we study the formation of density shock waves in the context of an idealized model of microswimmers confined in a narrow channel and subject to a uniform external flow. Interestingly, these density shock waves exhibit a transition from `subsonic' with compression at the back to `supersonic' with compression at the front of the population as the intensity of the external flow increases. This behavior is the result of a non-trivial interplay between hydrodynamic interactions and geometric confinement, and is confirmed by a novel quasilinear wave model that properly captures the dependence of the shock formation on the external flow. These findings can be used to guide the development of novel mechanisms for controlling the emergent density distribution and average population speed, with potentially profound implications on various processes in industry and biotechnology such as the transport and sorting of cells in flow channels.