Air Donuts: Toroidal Bubbles Stabilized by Hydrophobin Protein Surfactant

Hydrophobins are surface-active proteins made by fungi. Whereas typical surfactants such as sodium dodecyl sulfate exhibit a great deal of molecular flexibility, hydrophobin protein surfactants behave as globular solids to create strong, thin biofilms at air-water interfaces. It has been known for a long time that hydrophobin surfactants can stabilize bubbles in unusual shapes, including rods of striking aspect ratio. Under appropriate conditions, these structures can be reformed into air-filled toroids. These ``air donuts'' are stable for hours or even days and feature high surface area.