Bacteria Around an Acoustic Black Hole: Trapping and Frame-Dragging

TL;DR

This paper explores the hydrodynamics of nematic active fluids around acoustic black holes, revealing potential for trapping active matter and using particles as gyroscopes, with implications for acoustic superradiance.

Contribution

It introduces the concept of acoustic black hole spacetimes in nematic active fluids and analyzes their effects on active particle dynamics and superradiance phenomena.

Findings

Active particles show enhanced concentration near the acoustic horizon.

Particles can function as freely-precessing gyroscopes for frame-dragging detection.

Acoustic superradiance is predicted to occur in nematic fluids with varying viscosity.

Abstract

Motivated by the need to conceive freely-precessing gyroscopes for detecting acoustic frame-dragging predicted recently in rotating acoustic analogue black holes, we report an incipient investigation on the hydrodynamics of nematic active fluids. With a specific assumption on barotropicity of a nematic fluid, we discern acoustic analogue black hole spacetimes experienced by linear perturbations of the velocity potential. For vanishingly small diffusivity of the active particles, linear perturbations of the active particle concentration reveal a profile with an enhancement close to the acoustic horizon, hinting towards the possibility of partial trapping of active matter by the acoustic black hole. We further show that, as anticipated, the dynamical nature of the orientation (`polarization') of individual particles indeed opens up the possibility of their use as freely-precessing gyroscopes. In addition, inclusion of diffusivity of active particles in the inviscid solvent is shown to lead to a small effective viscosity. Depending on the sign of the diffusion coefficient, this can either yield superfluid-like behaviour, or enhance the net viscosity, of the nematic system. In either situation, acoustic superradiance, theoretically analyzed and experimentally observed recently for mildly viscous standard fluids, is thus predicted to occur for nematic fluids.