Instability of expanding bacterial droplets

Andrey Sokolov,Igor S Aranson,John F Brady,Leonardo Dominguez Rubio

doi:10.1038/s41467-018-03758-z

Abstract

Suspensions of motile bacteria or synthetic microswimmers, termed active matter, manifest a remarkable propensity for self-organization, and formation of large-scale coherent structures. Most active matter research deals with almost homogeneous in space systems and little is known about the dynamics of strongly heterogeneous active matter. Here we report on experimental and theoretical studies on the expansion of highly concentrated bacterial droplets into an ambient bacteria-free fluid. The droplet is formed beneath a rapidly rotating solid macroscopic particle inserted in the suspension. We observe vigorous instability of the droplet reminiscent of a violent explosion. The phenomenon is explained in terms of continuum first-principle theory based on the swim pressure concept. Our findings provide insights into the dynamics of active matter with strong density gradients and significantly expand the scope of experimental and analytic tools for control and manipulation of active systems.

Highlights

Suspensions of motile bacteria or synthetic microswimmers, termed active matter, manifest a remarkable propensity for self-organization, and formation of large-scale coherent structures
Recent studies demonstrated that while the thermodynamic analogy has limitations, many aspect of active suspension dynamics can be properly captured in the terms of swim pressure and corresponding equation of state[16,17,18,19,20,21]
Nothing is known about the dynamics of active matter with strong density heterogeneities

Summary

Introduction

Suspensions of motile bacteria or synthetic microswimmers, termed active matter, manifest a remarkable propensity for self-organization, and formation of large-scale coherent structures. Recent studies demonstrated that while the thermodynamic analogy has limitations, many aspect of active suspension dynamics can be properly captured in the terms of swim pressure and corresponding equation of state[16,17,18,19,20,21]. Nothing is known about the dynamics of active matter with strong density heterogeneities These conditions can be achieved, for example, by submersing a rapidly rotating solid macroscopic particle into bacterial suspension. We capture the onset of the instability in terms of a swim pressure concept generalized to rod-like particles such as bacteria. The model is derived in the limit when the Peclet number based on the swim diffusivity is small

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Results

Conclusion