Sub-millimeter air-filled toroidal bubbles featuring easily reversible and rapid shape change.*

Researchers have experimentally observed and generated liquid-filled vesicles in the toroidal topology, but small, air-filled toroids have not been reported. Here we report a facile method to generate air-filled toroidal bubbles coated by a particularly rugged amphipathic protein membrane made from filamentous fungi. The "air donuts" fall into the sub-millimeter size and are believed to be the world's first air-filled toroidal bubbles in this size scale. The major and minor ring radii ratio of the "air donuts" has a value of
√2, which is in an agreement with the liquid-filled vesicle literature. During the formation of "air donuts" under certain easy-to-meet conditions, the bubbles' shapes transform from cylinders to spheres, and ultimately to toroids. The sphere-to-toroid transition is reversible, which is surprising from the topology point of view. The air-filled toroidal vesicles could provide opportunities in the field of aerosol (flavor and fragrance) delivery applications.