Fluctuation Spectra and Force Generation in Non-equilibrium Systems

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.

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Cited by in corpus (10)

• Autonomous actuation of zero modes in mechanical networks far from equilibrium
• Tunable critical Casimir forces counteract Casimir-Lifshitz attraction
• The hydrodynamics of slender swimmers near deformable interfaces
• Bidirectional Wave-Propelled Capillary Spinners
• Casimir force in dense confined electrolytes
• Active matter as the underpinning agency for extraordinary sensitivity of biological membranes to electric fields
• Coexisting Ordered States, Local Equilibrium-like Domains, and Broken Ergodicity in a Non-turbulent Rayleigh-Bénard Convection at Steady-state
• Stokes' second problem and reduction of inertia in active fluids
• Probing hydrodynamic fluctuation-induced forces with an oscillating robot
• Singularities in the acoustic Casimir pressure as a function of reflectivities