Entropons as collective excitations in active solids

TL;DR

This paper introduces 'entropons', a new type of vibrational collective excitation in active solids caused by activity-induced entropy production, which coexist with phonons and dominate at high self-propulsion.

Contribution

It proposes the concept of entropons as a novel vibrational mode in non-equilibrium active solids, linking entropy production to collective excitations.

Findings

Entropons are generated by activity and coexist with phonons.

Entropons dominate over phonons at high self-propulsion.

The concept can be experimentally verified in active matter systems.

Abstract

The vibrational dynamics of solids is described by phonons constituting basic collective excitations in equilibrium crystals. Here we consider an active crystal composed of self-propelled particles which bring the system into a non-equilibrium steady-state governed by entropy production. Calculating the entropy production spectrum, we put forward the picture of "entropons", which are vibrational collective excitations responsible for entropy production. Entropons are purely generated by activity and coexist with phonons but dominate over them for large self-propulsion strength. The existence of entropons can be verified in experiments on dense self-propelled colloidal Janus-particles and granular active matter, as well as in living systems such as dense cell monolayers.