# Active particles under confinement and effective force generation among surfaces

## TL;DR

This paper investigates how geometric confinement affects the behavior of a one-dimensional active suspension with thermal noise, revealing the formation of polar order and a Casimir-like force due to active forces and thermal fluctuations.

## Contribution

It combines numerical and analytical methods to analyze active suspension behavior under confinement, highlighting the emergence of polar order and active force-induced Casimir-like forces.

## Key findings

- Active suspension forms a polar ordered layer near walls.
- Thermal noise and active forces influence system structure.
- Active forces generate a Casimir-like attractive force.

## Abstract

We consider the effect of geometric confinement on the steady-state properties of a one-dimensional active suspension subject to thermal noise. The random active force is modeled by an Ornstein-Uhlenbeck process and the system is studied both numerically, by integrating the Langevin governing equations, and analytically by solving the associated Fokker-Planck equation under suitable approximations. The comparison between the two approaches displays a fairly good agreement and in particular, we show that the Fokker-Planck approach can predict the structure of the system both in the wall region and in the bulk-like region where the surface forces are negligible. The simultaneous presence of thermal noise and active forces determines the formation of a layer, extending from the walls towards the bulk, where the system exhibits polar order. We relate the presence of such ordering to the mechanical pressure exerted on the container's walls and show how it depends on the separation of the boundaries and determines a Casimir-like attractive force mediated by the active suspension.