Spontaneous Circulation of Confined Active Suspensions

TL;DR

This paper investigates how confinement induces spontaneous circulation in active suspensions, revealing activity thresholds, stable circulating states, and oscillatory behaviors through stability analysis and numerical simulations.

Contribution

It demonstrates that confinement enables stable auto-circulation in active suspensions and identifies activity thresholds and dynamic regimes.

Findings

Confinement stabilizes circulating states in active suspensions.

A critical activity threshold triggers spontaneous circulation.

High activity leads to oscillatory flow regimes.

Abstract

Many active fluid systems encountered in biology are set in total geometric confinement. Cytoplasmic streaming in plant cells is a prominent and ubiquitous example, in which cargo-carrying molecular motors move along polymer filaments and generate coherent cell-scale flow. When filaments are not fixed to the cell periphery, a situation found both in vivo and in vitro, we observe that the basic dynamics of streaming are closely related to those of a non-motile stresslet suspension. Under this model, it is demonstrated that confinement makes possible a stable circulating state; a linear stability analysis reveals an activity threshold for spontaneous auto-circulation. Numerical analysis of the long-time behavior reveals a phenomenon akin to defect separation in nematic liquid crystals, and a high-activity bifurcation to an oscillatory regime.