Topological sound in active-liquid metamaterials

TL;DR

This paper introduces topological active metamaterials made from active liquids that support unidirectional, backscattering-immune sound modes due to broken time-reversal symmetry and active matter properties.

Contribution

It presents a novel design principle for creating topological metamaterials using active liquids, enabling robust sound propagation in overdamped systems.

Findings

Support for topologically protected sound modes

Unidirectional propagation without backscattering

Breaking of time-reversal symmetry in active metamaterials

Abstract

Liquids composed of self-propelled particles have been experimentally realized using molecular, colloidal, or macroscopic constituents. These active liquids can flow spontaneously even in the absence of an external drive. Unlike spontaneous active flow, the propagation of density waves in confined active liquids is not well explored. Here, we exploit a mapping between density waves on top of a chiral flow and electrons in a synthetic gauge field to lay out design principles for artificial structures termed topological active metamaterials. We design metamaterials that break time-reversal symmetry using lattices composed of annular channels filled with a spontaneously flowing active liquid. Such active metamaterials support topologically protected sound modes that propagate unidirectionally, without backscattering, along either sample edges or domain walls and despite overdamped particle dynamics. Our work illustrates how parity-symmetry breaking in metamaterial structure combined with microscopic irreversibility of active matter leads to novel functionalities that cannot be achieved using only passive materials.