Fluctuation Spectra and Force Generation in Non-equilibrium Systems

TL;DR

This paper demonstrates that the fluctuation spectrum of an active medium governs the non-equilibrium forces on inclusions, revealing how spectral properties influence force behavior such as attraction, repulsion, and oscillations.

Contribution

It establishes a link between the fluctuation spectrum and force phenomenology in non-equilibrium systems, providing a unified framework for understanding active forces.

Findings

Force oscillates between attraction and repulsion depending on spectrum peak

Force depends on the width and position of the spectral peak

Active Brownian particles and Casimir effect are explained through spectral analysis

Abstract

Many biological systems are appropriately viewed as passive inclusions immersed in an active bath: from proteins on active membranes to microscopic swimmers confined by boundaries. The non-equilibrium forces exerted by the active bath on the inclusions or boundaries often regulate function, and such forces may also be exploited in artificial active materials. Nonetheless, the general phenomenology of these active forces remains elusive. We show that the fluctuation spectrum of the active medium, the partitioning of energy as a function of wavenumber, controls the phenomenology of force generation. We find that for a narrow, unimodal spectrum, the force exerted by a non-equilibrium system on two embedded walls depends on the width and the position of the peak in the fluctuation spectrum, and oscillates between repulsion and attraction as a function of wall separation. We examine two apparently disparate examples: the Maritime Casimir effect and recent simulations of active Brownian particles. A key implication of our work is that important non-equilibrium interactions are encoded within the fluctuation spectrum. In this sense the noise becomes the signal.