Containerless emulsification of acoustically levitated composite drop

Emulsions are inherently thermodynamically unstable dispersions that are widely involved in food processing, cosmetic preparation, and drug delivery. The existing ultrasonic emulsification techniques mainly rely on the direct contact between the sonicator probe and liquids, which causes localized high temperature and pressure within the liquid and influences the final properties of the obtained emulsion. In this work, a containerless emulsification approach has been realized by using ultrasonic levitation. The emulsification of water‒oil system can be promoted by adjusting the emitter‒reflector distance to alter the acoustic radiation pressure on the surface of the levitated drop. The dynamic behaviors of the emulsification process were monitored by using a high-speed camera, and the sound field was analyzed via numerical simulation. The experimental results showed that atomization of droplets driven by sound field was the main driving force for emulsification. This method can be used to prepare emulsions in which the average diameter of the droplets was about 2–3 µm. The work provided a new method for containerless emulsification, thus shedding light on the preparation of contamination-free pharmaceuticals.