# Hydrodynamic interactions dominate the structure of active swimmers' pair distribution functions

## TL;DR

This study shows that hydrodynamic interactions significantly influence the pair distribution functions of microswimmer suspensions, reducing overall structure and creating orientation-dependent patterns, advancing understanding of active matter systems.

## Contribution

It provides the first detailed analysis of how hydrodynamics affect the statistical structure of microswimmer liquids through particle-based simulations.

## Key findings

- Hydrodynamic interactions alter the orientation-dependent pair distribution function.
- Hydrodynamics decrease the overall liquid structure compared to passive models.
- At higher densities, a stronger first neighbor shell forms in specific directions.

## Abstract

Microswimmers often exhibit surprising patterns due to the nonequilibrium nature of their dynamics. Collectively, suspensions of microswimmers appear as a liquid whose properties set it apart from its passive counterpart. To understand the impact of hydrodynamic interactions on the basic statistical features of a microswimmer's liquid, we investigate its structure by means of the pair distribution function. We perform particle-based simulations of microswimmers that include steric effects, shape anisotropy, and hydrodynamic interactions. We find that hydrodynamic interactions considerably alter the orientation-dependent pair distribution function compared to purely excluded-volume models like active Brownian particles, and generally decrease the structure of the liquid. Depletion regions are dominant at lower filling fractions, while at larger filling fraction the microswimmer liquid develops a stronger first shell of neighbors in specific directions, while losing structure at larger distances. Our work is a first step towards a statistico-mechanical treatment of the structure of microswimmer suspensions.