Odd elasticity in disordered chiral active materials

TL;DR

This paper introduces a minimal model demonstrating how odd elasticity naturally arises in disordered chiral active materials, revealing unique nonlinear and dynamic behaviors in such systems.

Contribution

It extends the understanding of odd elasticity from ordered to disordered systems using a micropolar elasticity model with active torques.

Findings

Odd elasticity emerges as a nonlinear effect of internal rotations.

Disordered odd solids exhibit regions of dynamic instability.

Bulk wave propagation occurs in overdamped active materials.

Abstract

Chiral active materials are abundant in nature, including the cytoskeleton with attached motor proteins, rotary clusters of bacteria flagella, and self-spinning starfish embryos. These materials break both time reversal and mirror-image (parity) symmetries due to injection of torques at the microscale. Recently, it was found that chiral active materials may show a new type of elastic response termed `odd' elasticity. Currently, odd elasticity is understood microscopically only in ordered structures, e.g., lattice designs of metamaterials. It still remains to explore how odd elasticity can emerge in natural or biological systems, which are usually disordered. To address this, we propose a minimal generic model for disordered `odd solids', using micropolar (Cosserat) elasticity in the presence of local active torques. We find that odd elasticity naturally emerges as a nonlinear effect of internal particle rotations. Exploring the viscoelasticity of this solid, when immersed in active self-spinning solvent (`odd fluid'), we discover both dynamically unstable regions and regions in which bulk waves can propagate even in an overdamped solid.