Swarming bottom feeders: Flocking at solid-liquid interfaces

TL;DR

This paper develops a hydrodynamic theory for collective flocking behavior at solid-liquid interfaces, predicting stability, long-range order, giant fluctuations, and anomalous diffusion, with implications for experimental observation.

Contribution

It introduces a comprehensive hydrodynamic framework for understanding flocking at solid-liquid interfaces, including stability criteria and novel fluctuation and diffusion predictions.

Findings

Systems are stable with long-range order over a wide parameter range.

Giant number fluctuations grow as the 3/4 power of mean number.

Tagged particle diffusion is anomalously rapid parallel to the interface.

Abstract

We present the hydrodynamic theory of coherent collective motion ("flocking") at a solid-liquid interface, and many of its predictions for experiment. We find that such systems are stable, and have long-range orientational order, over a wide range of parameters. When stable, these systems exhibit "giant number fluctuations", which grow as the 3/4th power of the mean number. Stable systems also exhibit anomalous rapid diffusion of tagged particles suspended in the passive fluid along any directions in a plane parallel to the solid-liquid interface, whereas the diffusivity along the direction perpendicular to the plane is not anomalous. In the remaining parameter space, the system becomes unstable.