  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.
