Symmetry-breaking, motion and bistability of active drops through polarization-surface coupling

TL;DR

This paper introduces a new mechanism for active drop motility driven by polarization variations, highlighting symmetry-breaking and bistability in active polar droplets, expanding understanding beyond traditional splay and bend effects.

Contribution

The study presents a thermodynamic model showing active stresses induce motion via polarization strength variations, revealing a novel motility mechanism in active polar droplets.

Findings

Active stresses cause drop motion through polarization strength variations.

Stable moving and resting states can coexist, showing bistability.

The mechanism involves competition between elastic energy and surface polarization control.

Abstract

Cell crawling crucially depends on the collective dynamics of the acto-myosin cytoskeleton. However, it remains an open question to what extent cell polarization and persistent motion depend on continuous regulatory mechanisms and autonomous physical mechanisms. Experiments on cell fragments and theoretical considerations for active polar liquids have highlighted that physical mechanisms induce motility through splay and bend configurations in a nematic director field. Here, we employ a simple model, derived from basic thermodynamic principles, for active polar free-surface droplets to identify a different mechanism of motility. Namely, active stresses drive drop motion through spatial variations of polarization strength. This robustly induces parity-symmetry breaking and motility even for liquid ridges (2D drops) and adds to splay- and bend-driven pumping in 3D geometries. Intriguingly, then, stable polar moving and axisymmetric resting states may coexist, reminiscent of the interconversion of moving and resting keratocytes by external stimuli. The identified additional motility mode originates from a competition between the elastic bulk energy and the polarity control exerted by the drop surface. As it already breaks parity-symmetry for passive drops, the resulting back-forth asymmetry enables active stresses to effectively pump liquid and drop motion ensues.