Do hydrodynamically assisted binary collisions lead to orientational ordering of microswimmers?

TL;DR

This study demonstrates that hydrodynamically assisted binary collisions can induce orientational order in microswimmers, with simplified models capturing the essential collective dynamics observed in detailed simulations.

Contribution

The paper introduces a binary collision model that accurately reproduces collective motion in microswimmers, highlighting the role of binary interactions in global alignment.

Findings

Binary collisions can lead to collective orientational order.

The simplified BCM matches DNS results in most cases.

Density fluctuations influence the accuracy of the models.

Abstract

We have investigated the onset of collective motion in systems of model microswimmers, by performing a comprehensive analysis of the binary collision dynamics using three dimensional direct numerical simulations (DNS) with fully resolved hydrodynamics. From this data, we have constructed a simplified binary collision model (BCM) which accurately reproduces the collective behavior obtained from the DNS for most cases. Thus, we show that global alignment can mostly arise solely from binary collisions. Although the agreement between both models (DNS and BCM) is not perfect, the parameter range in which notable differences appear is also that for which strong density fluctuations are present in the system (where pseudo-sound mound can be observed[1]).