# Hydrodynamic interactions in dense active suspensions: from polar order to dynamical clusters

## TL;DR

This paper investigates how hydrodynamic interactions influence the collective behavior of dense active suspensions, revealing the importance of near-field forces in phase separation and polar order formation.

## Contribution

It introduces a new framework to distinguish hydrodynamic effects at different scales and demonstrates the significant role of lubrication forces in active particle dynamics.

## Key findings

- Lubrication forces are as crucial as long-range interactions in many-body behavior.
- Near-field interactions suppress motility-induced phase separation, leading to gel-like clusters.
- A polar ordered phase emerges for neutral swimmers, driven by collision-induced alignment.

## Abstract

We study the role of hydrodynamic interactions in the collective behaviour of collections of microscopic active particles suspended in a fluid. We introduce a novel calculational framework that allows us to separate the different contributions to their collective dynamics from hydrodynamic interactions on different length scales. Hence we are able to systematically show that lubrication forces when the particles are very close to each other play as important a role as long-range hydrodynamic interactions in determining their many-body behaviour. We find that motility-induced phase separation is suppressed by near-field interactions, leading to open gel-like clusters rather than dense clusters. Interestingly, we find a globally polar ordered phase appears for neutral swimmers with no force dipole that is enhanced by near field lubrication forces in which the collision process rather than long-range interaction dominates the alignment mechanism.