Binary pusher–puller mixtures of active microswimmers and their collective behaviour

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 behaviour 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.

GRAPHICAL ABSTRACT

KEYWORDS:

• Microswimmers
• active suspensions
• binary mixture
• pushers and pullers
• orientational order

Acknowledgments

This paper is dedicated to Daan Frenkel on the occasion of his 70th birthday.

Disclosure statement

No potential conflict of interest was reported by the authors.

ORCID

Andreas M. Menzel   http://orcid.org/0000-0003-0713-4979

Additional information

Funding

The authors thank M. Puljiz and C. Hoell for helpful discussions and the Deutsche Forschungsgemeinschaft for support of this work through the SPP 1726, grant numbers LO 418/17 and ME 3571/2.