# Casimir stresses in active nematic films

## TL;DR

This paper extends the concept of Casimir stresses to active nematic fluids, revealing how activity alters the stress behavior, including divergence and sign changes near instability thresholds.

## Contribution

It introduces a stochastic hydrodynamic framework to calculate Casimir stresses in active nematic films, generalizing equilibrium results to active systems.

## Key findings

- Active Casimir stress diverges logarithmically near instability for contractile activity.
- For extensile activity, the stress remains finite and repulsive.
- Active stresses differ significantly from equilibrium counterparts in magnitude and sign.

## Abstract

We calculate the Casimir stresses in a thin layer of active fluid with nematic order. By using a stochastic hydrodynamic approach for an active fluid layer of finite thickness $L$, we generalize the Casimir stress for nematic liquid crystals in thermal equilibrium to active systems. We show that the active Casimir stress differs significantly from its equilibrium counterpart. For contractile activity, the active Casimir stress, although attractive like its equilibrium counterpart, diverges logarithmically as $L$ approaches a threshold of the spontaneous flow instability from below. In contrast, for small extensile activity, it is repulsive, has no divergence at any $L$ and has a scaling with $L$ different from its equilibrium counterpart.