# Microscopic analysis of thermo-orientation in systems of off-centre Lennard-Jones particles

## TL;DR

This paper develops a theoretical framework for thermo-orientation in fluids of off-centre Lennard-Jones particles, linking molecular asymmetry to orientation behavior under temperature gradients, and validates it with numerical simulations.

## Contribution

It introduces a simple local equipartition-based theory for thermo-orientation in off-centre Lennard-Jones particles, extending previous results and explaining energy-orientation coupling.

## Key findings

- The theory recovers known thermo-orientation results.
- Simulations show deviations from local equipartition increase with off-centre displacement.
- Significant deviations occur in linear response regimes.

## Abstract

When fluids of anisotropic molecules are placed in temperature gradients, the molecules may align themselves along the gradient: this is called thermo-orientation. We discuss the theory of this effect in a fluid of particles that interact by a spherically symmetric potential, where the particles' centres of mass do not coincide with their interaction centres. Starting from the equations of motion of the molecules, we show how a simple assumption of local equipartition of energy can be used to predict the thermo-orientation effect, recovering the result of Wirnsberger et al. [Phys. Rev. Lett. 120, 226001 (2018)]. Within this approach, we show that for particles with a single interaction centre, the thermal centre of the molecule must coincide with the interaction centre. The theory also explains the coupling between orientation and kinetic energy that is associated with this non-Boltzmann distribution. We discuss deviations from this local equipartition assumption, showing that these can occur in linear response to a temperature gradient. We also present numerical simulations showing significant deviations from the local equipartition predictions, which increase as the centre of mass of the molecule is displaced further from its interaction centre.