# Thermal forces from a microscopic perspective

## TL;DR

This paper develops a unified microscopic theory of thermo-osmosis, explaining how thermal gradients induce fluid motion in liquids and gases through two distinct mechanisms, advancing understanding of particle thermophoresis.

## Contribution

It generalizes Linear Response Theory to inhomogeneous systems, providing an exact microscopic framework for thermo-osmotic flow in both gases and liquids.

## Key findings

- Identifies two independent physical mechanisms driving thermo-osmosis.
- Reduces to known expressions in the appropriate limits for gases and liquids.
- Provides a unified microscopic description of thermo-osmotic flow.

## Abstract

Thermal gradients lead to macroscopic fluid motion if a confining surface is present along the gradient. This fundamental nonequilibrium effect, known as thermo-osmosis, is held responsible for particle thermophoresis in colloidal suspensions. A unified approach for thermo-osmosis in liquids and in gases is still lacking. Linear Response Theory is generalised to inhomogeneous systems,leading to an exact microscopic theory for the thermo-osmotic flow showing that the effect originates from two independent physical mechanisms, playing different roles in the gas and liquid phases, reducing to known expressions in the appropriate limits.