Freezing of an acoustically levitated drop: From liquid disk to ice ring

Drop freezing plays an important role in industries ranging from food and materials to biomedicine, but the contact between drops and substrates is unavoidable. The contact effect has great influence on the nucleation and growth of ice. To eliminate the wall effect on drop freezing, we studied the freezing of liquid drops under container-free conditions by combining the sound field and temperature field. The levitated drops exhibited varied frozen morphologies, such as ice saucer, ice cake, and ice ring, depending on their initial shapes. For a levitated liquid disk, it eventually freezes to form a levitated ice ring, which is caused by the atomization rupture of its central liquid film. In addition, we accomplished the landing–lifting manipulation of the frozen drops by controlling the sound field. Our work highlights the coupling effects between thermodynamically driven phase transitions and mechanically driven drop dynamics involved in acoustic levitation. The investigation of levitated drop freezing contributes to a in-depth understanding of the phase transition process and dynamic behavior of fluids in a container-free environment, and also inspires and expands new methods for the fabrication of ring-shaped materials.