# Pressure, surface tension and curvature in active systems: A touch of equilibrium

## TL;DR

This study investigates how active particles exert pressure on curved surfaces and relates this to surface tension, using simulations and effective equilibrium models to understand the influence of activity and curvature on interfacial properties.

## Contribution

It introduces a method to connect active particle pressure with surface tension through effective equilibrium models and extends classical sum rules to active systems at small activity levels.

## Key findings

- Analytical calculation of curvature effects in ideal-gas active systems shows good agreement with simulations.
- Linear pressure correction is model-specific and relates to planar adsorption.
- Effective potential approximation captures density effects on pressure-curvature dependence.

## Abstract

We explore the pressure of active particles on curved surfaces and its relation to other interfacial properties. We use both direct simulations of the active systems as well as simulations of an equilibrium system with effective (pair) interactions designed to capture the effects of activity. Comparing the active and effective passive systems in terms of their bulk pressure, we elaborate that the most useful theoretical route to this quantity is via the density profile at a flat wall. This is corroborated by extending the study to curved surfaces and establishing a connection to the particle adsorption and integrated surface excess pressure (surface tension). In the ideal-gas limit, the effect of curvature on the mechanical properties can be calculated analytically in the passive system with effective interactions, and shows good (but not exact) agreement with simulations of the active models. It turns out that even the linear correction to the pressure is model specific and equals the planar adsorption in each case, which means that a known equilibrium sum rule can be extended to a regime at small but nonzero activity. In turn, the relation between the planar adsorption and the surface tension is reminiscent of the Gibbs adsorption theorem at an effective temperature. At finite densities, where particle interactions play a role, the presented effective-potential approximation captures the effect of density on the dependence of the pressure on curvature.