# Binary pusher-puller mixtures of active microswimmers and their collective behavior

## TL;DR

This study investigates how mixtures of pusher and puller microswimmers exhibit collective behavior, revealing that orientational order can be tuned by adjusting the composition of these active particles.

## Contribution

It provides the first detailed analysis of binary pusher-puller microswimmer mixtures, showing how their collective orientation depends on composition and hydrodynamic interactions.

## Key findings

- Puller suspensions spontaneously develop collective order.
- Ordered motion persists up to pusher-puller ratios of about 1:2.
- Pushers surrounded by pullers show enhanced alignment.

## Abstract

Microswimmers are active particles of microscopic size that self-propel by setting the surrounding fluid into motion. According to the kind of far-field fluid flow that they induce, they are classified into pushers and pullers. Many studies have explored similarities and differences between suspensions of either pushers or pullers, but the behavior of mixtures of the two is still to be investigated. Here, we rely on a minimal discrete microswimmer model, particle-resolved, including hydrodynamic interactions, to examine the orientational ordering in such binary pusher-puller mixtures. In agreement with existing literature, we find that our monodisperse suspensions of pushers do not show alignment, whereas those of solely pullers spontaneously develop ordered collective motion. By continuously varying the composition of the binary mixtures, starting from pure puller systems, we find that ordered collective motion is largely maintained up to pusher-puller composition ratios of about 1:2. Surprisingly, pushers when surrounded by a majority of pullers are more tightly aligned than indicated by the average overall orientational order in the system. Our study outlines how orientational order can be tuned in active microswimmer suspensions to a requested degree by doping with other species.