Droplet acoustofluidics: Recent progress and challenges.

Acoustofluidics, offering contact-free and precise manipulation of micro-objects, has emerged as a transformative tool for various biological and medical applications. In recent years, significant advancements have been made in droplet manipulation using acoustic waves. This review provides an in-depth exploration of acoustofluidic techniques for droplet manipulation, presenting a balanced perspective on the role of this versatile platform across diverse applications. The paper begins by introducing the underlying mechanism of acoustic forces acting on the droplets, followed by a comprehensive discussion of acoustofluidic techniques tailored for essential unit operations, such as droplet generation, separation, merging, splitting, steering, trapping, in-droplet sample manipulation, sample control within sessile droplets, and digital acoustofluidics. Finally, the prospects and limitations of acoustofluidics for droplet manipulations are also discussed, suggesting the future direction of droplet acoustofluidics research.