New insights into suspended drops: When soft matter meets acoustic levitation

Acoustic levitation has developed into a popular but elegant tool for the study of drops as well as soft matter due to its exceptional levitation capabilities to a variety of liquid samples. The acoustically levitated drops offer opportunities for the investigation of a wide range of fundamental issues related to liquid drops. In this review, the unique physics/chemical processes involved in acoustically levitated drops are dealt with. We first introduce the dynamics of the acoustically levitated drops, including drop oscillation, coalescence, and the associated capillary phenomena. The bubble formation and stability are also discussed. Depending on the inhibition of solid surfaces and the nonlinear effects of ultrasound, the self-assembly of colloidal particles at the air–liquid interface as well as granular matter in air is reviewed. In particular, the exploration of biological drops by using acoustic levitation is also highlighted. In the end, the concept of acoustic-levitation-fluidics and possible potential topics are proposed.