Pressure is not a state function for generic active fluids

TL;DR

This paper demonstrates that for many active fluids, pressure depends on particle-wall interactions and lacks an equation of state, contrasting with equilibrium systems where pressure depends solely on bulk properties.

Contribution

It reveals that active fluids generally lack an equation of state for pressure, highlighting the importance of interactions in non-equilibrium active matter systems.

Findings

Pressure depends on particle-wall interactions in active fluids.

Active fluids often lack an equation of state for pressure.

Some specific active models do exhibit an equation of state.

Abstract

Pressure is the mechanical force per unit area that a confined system exerts on its container. In thermal equilibrium, it depends only on bulk properties (density, temperature, etc.) through an equation of state. Here we show that in a wide class of active systems the pressure depends on the precise interactions between the active particles and the confining walls. In general, therefore, active fluids have no equation of state, their mechanical pressures exhibit anomalous properties that defy the familiar thermodynamic reasoning that holds in equilibrium. The pressure remains a function of state, however, in some specific and well-studied active models that tacitly restrict the character of the particle-wall and/or particle-particle interactions.