Hydrodynamic bend instability of motile particles on a substrate

TL;DR

This paper demonstrates that hydrodynamic bend instabilities in active particle suspensions can occur without dipolar stresses, driven solely by self-propulsion forces, leading to complex flow states.

Contribution

It reveals a new mechanism for hydrodynamic instability in active suspensions driven only by self-propulsion, independent of dipolar active stresses.

Findings

Bend instability occurs above a critical self-propulsion force.

Increased self-propulsion leads to disordered flow states.

Analytical and simulation results confirm the instability mechanism.

Abstract

The emergence of hydrodynamic bend instabilities in ordered suspensions of active particles is widely observed across diverse living and synthetic systems, and is considered to be governed by dipolar active stresses generated by the self-propelled particles. Here, using linear stability analyses and numerical simulations, we show that a hydrodynamic bend instability can emerge in the absence of any dipolar active stress and solely due to the self-propulsion force acting on polar active units suspended in an incompressible fluid confined to a substrate. Specifically, we show analytically, and confirm in simulations, that a uniformly ordered state develops bend instability above a critical self-propulsion force. Numerical simulations show that a further increase in the self-propulsion strength leads the system towards a disorderly flow state. The results offer a new route for development of hydrodynamic instabilities in two-dimensional self-propelled materials that are in contact with a substrate, with wide implications in layers of orientationally ordered cells and synthetic active particles.