Extracting oil from an oil and water mixture by using their different wetting properties via the Acoustowetting phenomenon*

In our experiments, we place drops of nano-emulsions that are stabilized by surfactants––SDS or Tween 20––atop a solid substrate; thin, micron-thick, oil films appear within minutes atop the sessile drops. Otherwise, the emulsions remain stable for over a year in their container. The large surface area of a small––10 μl––drop containing a kinetically stable oil in water nano-emulsion therein catalyzes the formation of a thermodynamically favorable low energy oil film at the drop boundary and especially at its free surface far before the emulsion destabilizes in the bulk of the mixture. We extract the micron-thick oil film appearing at the emulsion drops’ boundaries using a MHz-frequency Rayleigh type surface acoustic wave (SAW) in the solid substrate. This is the Acoustowetting phenomenon: Lower energy liquids––Oil––are attracted by the SAW to dynamically wet the substrate, leaving the emulsion drop behind, while higher energy liquids––Water––remain still.

 

Moreover, a balance between SAW induced acoustic radiation pressure and emulsion pressure within the drop pushes the emulsion away from the source of the SAW, leading to a non-intuitive extraction of oil from the drop surface normal to the SAW path. During oil extraction, the initially white-shaded emulsion drops become increasingly transparent owing to the reduction in oil content therein down to approximately a third of their initial 10-50% concentration.