Comprehensive Review on the Droplet Dynamics of an Acoustically Levitated Colloidal Droplet

In the context of clean energy transition, colloidal fuel droplets have attracted significant attention, owing to their high energy density and better combustion performance. Yet, considerable uncertainty remains regarding the dynamics and evaporation of colloidal droplets under the influence of an acoustic field. Among other methods, acoustic levitation provides a contactless platform for investigating droplet dynamics in the absence of solid interfaces. This Review presents a complete exploration of droplet dynamics in acoustically levitated colloidal droplets, from fundamentals to advanced applications. First, the fundamentals of acoustic levitation, including ultrasonic standing-wave generation and acoustic radiation forces that enable the stable, contact-free levitation of droplets, are outlined. The influence of acoustic levitation on the adjacent flow and thermal fields is considered, with an emphasis on acoustic streaming and other transport phenomena that substantially thin boundary layers and promote heat and mass transfers along levitated droplets. Subsequently, the influence of the acoustic field on droplet oscillations, droplet breakup and coalescence, and droplet evaporation behavior is discussed in detail. Across these aspects, recent studies have shown that the dynamics of colloidal droplets differ from those of pure fluids. Finally, current limitations and future research directions that could yield further insights into droplet dynamics are highlighted, as colloidal droplets have wide-ranging interdisciplinary applications.