Ultrasound reforms droplets.

Abstract

Size-controlled monodisperse droplets are indispensable in food, cosmetics, and healthcare industries. Although emulsion formation from bulk phases is well-explored, a robust <i>in situ</i> method to continuously reform existing emulsions is unavailable. Remarkably, we introduce a continuous flow acousto-microfluidics technique which enables simultaneous trapping-coalescence-splitting of droplets to reform an existing polydisperse emulsion into size-controlled droplets with improved monodispersity. In contrast to conventional approaches, our platform enables controlling droplet characteristics <i>in situ</i> by regulating acoustic power without altering hydrodynamical parameters thereby improving response time and facilitates continuous nozzle-less clogging-free droplet generation from a liquid plug in a chamber instead of from a liquid stream at a narrow junction. The technique can process polydisperse droplets produced not only due to fluid-source fluctuations or unstable jetting regime but also externally by non-microfluidic or inexpensive setups. Our theoretical scaling suggests that the sum of capillary (Ca) and acousto-capillary (Ca_a) numbers ∼ (1), and predicts the generated droplet size, both agreeing well with the experimental findings. We identify acousto-visco-capillary number, Ca_av = (Ca Ca_a)^1/2, which governs the generated droplet size. We also explore and characterize acoustic streaming- and coalescence-based mixing of samples inside the trapped plug. Distinctively, our platform is amenable to continuous mixing of inhomogeneous droplets, offering monodisperse mixed-sample droplets, and holds the potential to match current throughput standards through suitable design modifications.