Non-equilibrium forces following quenches in active and thermal matter

TL;DR

This paper investigates transient non-equilibrium forces in active and thermal matter following quenches, revealing two types of forces—density-induced and fluctuation-induced—with distinct origins, properties, and potential experimental signatures.

Contribution

It introduces a detailed analysis of transient forces after quenches in active and thermal systems, distinguishing between density-induced and fluctuation-induced forces with analytical and simulation evidence.

Findings

Identification of two distinct transient forces: density-induced and fluctuation-induced.

Analytical and simulation results showing the relaxation dynamics of these forces.

Proposals for experimental observation of the transient forces.

Abstract

Non-equilibrium systems are known to exhibit long-ranged correlations due to conservation of quantities like density or momentum. This, in turn, leads to long-ranged fluctuation-induced (Casimir) forces, predicted to arise in a variety of non-equilibrium settings. Here, we study such forces, which arise transiently between parallel plates or compact inclusions in a gas of particles, following a change ("quench") in temperature or activity of the medium. Analytical calculations, as well as numerical simulations of passive or active Brownian particles, indicate two distinct forces: (i) The immediate effect of the quench is adsorption or desorption of particles of the medium to the immersed objects, which in turn initiates a front of relaxing (mean) density. This leads to time-dependent {\it density-induced forces}. (ii) A long-term effect of the quench is that density fluctuations are modified, manifested as transient (long-ranged) (pair-)correlations that relax diffusively to their (short-ranged) steady-state limit. As a result, transient {\it fluctuation-induced forces} emerge. We discuss the properties of fluctuation-induced and density-induced forces as regards universality, relaxation as a function of time, and scaling with distance between objects. Their distinct signatures allow us to distinguish the two types of forces in simulation data. Finally, we propose several scenarios for their experimental observation.