Trapping, coalescence, and splitting of drops in an ultrasound-actuated microcavity

Droplets are sophisticated micro-compartments crucial for lab-on-a-chip and bio-medical applications. There have been significant efforts towards generating monodisperse droplets as the uniformity of droplets affects performance. Recently, we introduced a new method to continuously reform existing emulsions improving monodispersity in an ultrasound-actuated microcavity via a trap-coalesce-split mechanism. Here, we study the kinematics and dynamics of the phenomena to uncover the underlying physics and discuss the operating regime. We experimentally study the kinematical behavior by considering the time evolution of the drop-plug system and the effect of ultrasound power on these variations. We study the dynamics of the system via numerical simulations by considering the primary and secondary acoustic radiation forces, viscous drag force, and interfacial tension force. Remarkably, our study reveals that a combination of richly interconnected trapping, coalescence, and splitting phenomena is crucial for an improved understanding of acousto-microfluidic droplet generation. Our study will find relevance in advancing droplet handling and downstream integration of droplet-based operations in continuous-flow microfluidic systems.