A Twist On Active Membranes: Odd Mechanics, Spontaneous Flows and Shape Instabilities

TL;DR

This paper explores how chiral active components in membranes generate odd elastic forces and flows, leading to shape instabilities that could explain biological morphologies across scales.

Contribution

It introduces a coordinate-free framework for chiral active membrane mechanics, highlighting odd elastic effects and their role in membrane shape instabilities.

Findings

Chiral active stresses induce tangential flows around membrane curvature maxima and minima.

Normal viscous forces can amplify perturbations, causing shape instabilities.

Examples include spheroids, tubes, and helicoids with implications for biological systems.

Abstract

Living systems are chiral on multiple scales, from constituent biopolymers to large scale morphology, and their active mechanics is both driven by chiral components and serves to generate chiral morphologies. We describe the mechanics of active fluid membranes in coordinate-free form, with focus on chiral contributions to the stress. These generate geometric `odd elastic' forces in response to mean curvature gradients but directed perpendicularly. As a result, they induce tangential membrane flows that circulate around maxima and minima of membrane curvature. When the normal viscous force amplifies perturbations the membrane shape can become linearly unstable giving rise to shape instabilities controlled by an active Scriven-Love number. We describe examples for spheroids, membranes tubes and helicoids, discussing the relevance and predictions such examples make for a variety of biological systems from the sub-cellular to tissue level.