Controlling flows of an intra-droplet active fluid across droplet interface

Fluid dynamics of conventional passive fluid are known to be affected by boundary condition. For example, flow rates in a pipe depend on slipperiness of pipe surface. Similarly, active fluid, which consumes fuels locally to flow spontaneously, was reported to self-flow along a meter-long tubing with the flow rate depending on tubing geometry. However, how boundary condition influences fluid dynamics in an active fluid system remains poorly understood. Here, we investigated how a fluid boundary influenced self-organization of confined active fluid by establishing a 3D COMSOL-based nemato-hydrodynamic simulation platform where active fluid was confined in a compressed cylindrical water-in-oil droplet. Since the droplet interface was fluid, the fluid dynamics within and outside the droplet were coupled. Our simulation demonstrated that flow behaviors of intra-droplet active fluid were influenced by the amount of oil that surrounded the droplet: Without altering the droplet geometry, expanding the volume of oil could induce a circulatory flow within the droplet, which resembled our experimental observation. Our work suggested the feasibility of controlling the fluid dynamics of a confined active fluid system across a fluid interface.