Ultrasonic modification of hydrophobin bubbles and droplets for multiphase systems*

Cerato Ulmin(CU), a fungal protein classified as Hydrophobin, is an amphipathic surfactant which self-assembles at interfaces. CU can encapsulate air or oil in aqueous solutions to form non-spherical bubbles or droplets, similar to those formed by interfacial composite materials with solid-like surfaces. A sufficient dispersion of CU in water can create concentrated oil droplet/air bubble structures resembling complex multi-phase systems. These structures, which are observed to be stable for days, could have a variety of applications in emulsion engineering, reaction catalysis, and constructing multi-scale metastructures, among many others. Ultrasonic agitation was used to modify these structures, and it was discovered that the frequency of the ultrasound corresponded to the length scale of air-filled structures, by mechanism of Minnaert Resonance. With this method it was possible to select the size distribution of the air-filled structures. Oil-filled structures behaved differently, yet with sufficient ultrasound amplitude, it was possible to produce CU-oil nano-emulsions, likely by the mechanism of cavitation. The size and longevity of these nano-emulsions were studied by dynamic light scattering.