Using vibration to generate bespoke emulsions*

We investigate emulsion formation due to fragmentation of the interface between two stably stratified layers of immiscible liquids in a horizontally vibrated container. The horizontal oscillation, which drives an oscillatory shear flow between the fluid layers, is redirected into vertical forcing near the vibrating end-walls by harmonic sloshing. We find that the sloshing front is destabilized into subharmonic surface waves (SWs) through a Faraday instability.  The critical forcing accelerations for the onset of these waves match predictions of the Mathieu equation with a modified shear rate evaluated over the Stokes layer thickness. A further increase in forcing acceleration leads to droplets breaking off the tips of the SWs. For a small viscosity ratio between the upper and lower layer (N=9), the droplet formation is irregular. However, for N=45 we can tune the forcing parameters to generate trains of monodisperse droplets moving away from the end wall, where the droplet size increases with acceleration. Our system offers the potential for large volumes of bespoke surfactant-free emulsions, e.g., for solvent extraction, which can rapidly phase-separate upon interruption of the forcing.