6. Flow Fields and Heat Transfer Associated with an Acoustically Levitated Droplet

Considering the potential applications, a better understanding of the flow fields in an acoustically levitated droplet is of great significance in scientific fields. The flow generated by a nonlinear acoustic field is known as acoustic streaming. Using acoustic levitation, multi-scale acoustic streaming can be induced both inside and outside the droplet. In the internal flow field, the streaming configuration is affected by the physical properties of the droplet, i.e., the droplet diameter and rotation. The external flow field can be characterized by the applied sound pressure, physical properties of the droplet, and surrounding gas. These flow fields play an important role in the heat and mass transfer of the levitated droplet. This chapter provides a comprehensive review of the flow fields, the general theory of acoustic streaming, and an understanding of the heat transfer/mixing enhancement.