Active particles with soft and curved walls: Equation of state, ratchets, and instabilities

TL;DR

This paper derives an equation of state for active fluids with curved boundaries, revealing how pressure, instabilities, and forces relate to particle interactions and boundary shapes in two dimensions.

Contribution

It provides a first-principles derivation of pressure and force relations in active matter with complex boundaries, including effects of interactions and boundary-induced currents.

Findings

Equation of state for active fluids with curved walls

Spontaneous shear stresses and ratchet currents due to asymmetry

Instabilities and motion of flexible obstacles in active baths

Abstract

We study, from first principles, the pressure exerted by an active fluid of spherical particles on general boundaries in two dimensions. We show that, despite the non-uniform pressure along curved walls, an equation of state is recovered upon a proper spatial averaging. This holds even in the presence of pairwise interactions between particles or when asymmetric walls induce ratchet currents, which are accompanied by spontaneous shear stresses on the walls. For flexible obstacles, the pressure inhomogeneities lead to a modulational instability as well as to the spontaneous motion of short semi-flexible filaments. Finally, we relate the force exerted on objects immersed in active baths to the particle flux they generate around them.