Flow coupling between active and passive fluids across water-oil interfaces

TL;DR

This study investigates how flow interactions across water-oil interfaces influence circulatory flows in active fluid droplets, revealing millimeter-scale coupling effects and developing devices to control these flows in real time.

Contribution

It uncovers the role of interfacial flow coupling in active droplets and introduces novel millifluidic devices to manipulate circulatory flows dynamically.

Findings

Flow suppression occurs with thinner oil layers.

Flow coupling length is approximately millimeters.

Devices can trigger or suppress flows in real time.

Abstract

Active fluid droplets surrounded by oil can spontaneously develop circulatory flows. However, the dynamics of the surrounding oil and their influence on the active fluid remain poorly understood. To investigate interactions between the active fluid and the passive oil across their interface, kinesin-driven microtubule-based active fluid droplets were immersed in oil and compressed into a cylinder-like shape. The droplet geometry supported intradroplet circulatory flows, but the circulation was suppressed when the thickness of the oil layer surrounding the droplet decreased. Experiments with tracers and network structure analyses and continuum models based on the dynamics of self-elongating rods demonstrated that the flow transition resulted from flow coupling across the interface between active fluid and oil, with a millimeter-scale coupling length. In addition, two novel millifluidic devices were developed that could trigger and suppress intradroplet circulatory flows in real time: one by changing the thickness of the surrounding oil layer and the other by locally deforming the droplet. This work highlights the role of interfacial dynamics in the active fluid droplet system and shows that circulatory flows within droplets can be affected by millimeter-scale flow coupling across the interface between the active fluid and the oil.