Bacterial turbulence at compressible fluid interfaces

TL;DR

This study investigates active turbulence generated by bacteria at fluid interfaces, revealing unique compressible flow behaviors and a characteristic vortex size influenced by the fluid's thickness, distinct from bulk turbulence.

Contribution

It demonstrates the existence of interfacial bacterial turbulence with a novel length scale governed by coupling between interfacial and bulk flows, supported by experimental and theoretical analysis.

Findings

Vortex size saturates near 100 μm, independent of bacterial length.

Flow exhibits compressible in-plane turbulence at the interface.

Coupling between interfacial and bulk flows determines the emergent length scale.

Abstract

Dense bacterial suspensions at fluid interfaces provide a natural platform to explore active turbulence in a dimensional mismatch: active units are restricted to a two-dimensional surface, while the induced flows extend into the surrounding three-dimensional liquid. Using hydrophobic Serratia marcescens at the air-water interface, we realize interfacial bacterial turbulence as a distinct class of active turbulence. The system exhibits compressible in-plane flows, with vortex size initially increasing with the thickness of the underlying fluid and saturating near 100 $\mu$ m, independent of bacterial length. This behavior contrasts sharply with bulk active turbulence, where correlation length typically scales with system size. Hydrodynamic theory, together with direct measurements of the three-dimensional flow field, shows that the coupling between interfacial and bulk flows sets the emergent length scale. Our results uncover the fundamental physics of interfacial bacterial turbulence and open new strategies for geometric control of collective active flows.