# Surface Tensions between Active Fluids and Solid Interfaces: bare vs dressed

## TL;DR

This paper investigates the complex nature of surface tension at active fluid-solid interfaces, revealing setup-dependent contributions and proposing methods to measure and compute the bare surface tension in active systems.

## Contribution

It demonstrates that surface tension in active fluids includes setup-dependent parts and introduces a way to measure the bare surface tension using a generalized Virial formula.

## Key findings

- Surface tension has both equation-of-state and setup-dependent components.
- Setup-dependent currents 'dress' the measured surface tension.
- A generalized Virial formula can compute the bare surface tension.

## Abstract

We analyze the surface tension exerted at the interface between an active fluid and a solid boundary in terms of tangential forces. Focusing on active systems known to possess an equation of state for the pressure, we show that interfacial forces are of a more complex nature. Using a number of macroscopic setups, we show that the surface tension is a combination of an equation-of-state abiding part and of setup-dependent contributions. The latter arise from generic setup-dependent steady currents which "dress" the measurement of the "bare" surface tension. The former shares interesting properties with its equilibrium counterpart, and can be used to generalize the Young-Laplace law to active systems. We finally show how a suitably designed probe can directly access this bare surface tensions, which can also be computed using a generalized Virial formula.