# Hydrodynamic correlations in isotropic fluids and liquid crystals simulated by multi-particle collision dynamics

## TL;DR

This paper analyzes multi-particle collision dynamics for simulating isotropic fluids and liquid crystals, focusing on hydrodynamic fluctuations, relaxation, and the extension of the method to nematic liquid crystals.

## Contribution

It extends the multi-particle collision dynamics method to include nematic liquid crystals and analyzes hydrodynamic and orientational fluctuations.

## Key findings

- Hydrodynamic fluctuations relax towards equilibrium as described by a linearized theory.
- Flow fluctuations can induce orientation fluctuations in liquid crystals.
- Observable effects on velocity correlations occur only at certain simulation parameters.

## Abstract

Multi-particle collision dynamics is an appealing numerical technique aiming at simulating fluids at the mesoscopic scale. It considers molecular details in a coarse-grained fashion and reproduces hydrodynamic phenomena. Here, the implementation of multi-particle collision dynamics for isotropic fluids is analysed under the so-called Andersen-thermostatted scheme, a particular algorithm for systems in the canonical ensemble. This method gives rise to hydrodynamic fluctuations that spontaneously relax towards equilibrium. This relaxation process can be described by a linearized theory and used to calculate transport coefficients of the system. The extension of the algorithm for nematic liquid crystals is also considered. It is shown that thermal fluctuations in the average molecular orientation can be described by an extended linearized scheme. Flow fluctuations induce orientation fluctuations. However, orientational changes produce observable effects on velocity correlation functions only when simulation parameters exceed their values from those used in previous applications of the method. Otherwise, the flow can be considered to be independent of the orientation field.