Figure 5
From: Flow coupling between active and passive fluids across water–oil interfaces
Active fluid droplets induced chaotic flows in the surrounding oil. (a) Micrograph of an active droplet (\(r \approx\) 2.4 mm, \(h =\) 1 mm) immersed in oil (\(\Delta \approx\) 2.4 mm). To track the flows, both the active fluid and oil were doped with tracers. The gray curve represents the water–oil interface. (b) Time-averaged normalized velocity field and vorticity map revealed circulatory flow in the droplet but no net flow in the oil. The velocity field and vorticity map were plotted as in Fig. 1c. (c) Midplane flow profiles of azimuthal velocities of two droplets with the same shape (\(r \approx\) 2.4 mm, \(h =\) 1 mm) but different oil layer thicknesses (\(\Delta \approx ~\) 2.4 mm, blue curve;\(~\Delta \approx\) 1.1 mm, red curve) show that the thicker oil layer supported intradroplet circulatory flows, whereas the thinner layer did not. The dashed vertical line indicates the water–oil interface. Regardless of how the active fluids flowed within the droplets, the oil developed no net flow. Inset: Schematic of horizontal axis of the plot, \(\rho\). \(\rho > 0\) represents the oil region; \(\rho < 0\) represents the droplet region. (d) The flow speed profiles for both droplets extended across the water–oil interface into the oil region (\(\rho > 0\)). This shows that the active fluid flows near the interface drove flow in the nearby oil. Inset: Close-up of the profiles near the water–oil interface.
