Droplet acoustofluidics: From acoustic principles to micro manipulations

Droplet-based microfluidics have drawn much attention in recent years and have been successfully applied in biochemical analysis, material synthesis, and biomedical engineering. Precise and flexible manipulations of droplets are the basis of various applications. Numerous techniques have been introduced to achieve on-demand control of droplets, including electric, magnetic, acoustic, optical, and thermal methods. Among these, the combination of acoustics and microfluidics (termed acoustofluidics) has shown great potential and advantages in droplet manipulation as it is non-invasive, high-precision, low-cost, easily integrated, and biocompatible. Here, we summarize recent works on acoustofluidic manipulations of droplet-based microfluidics. This paper is structured into three main sections. First, the commonly used acoustic devices in acoustofluidics and their working principles are introduced. Such acoustic devices include interdigital transducers, Lamb wave resonators, and bulk acoustic resonators, and generate acoustic waves with frequencies ranging from kilohertz to gigahertz. Second, the forces and effects involved in droplet manipulations using acoustofluidics are analyzed. Third, the manipulation processes of droplet microfluidics using various acoustofluidic techniques are summarized and compared with other methods, including droplet generation, mixing, splitting, fusion, sorting, transportation, and internal particle patterning. Finally, current challenges and future prospects for acoustofluidic manipulation techniques for droplet-based microfluidics are discussed.