Ultrasound reforms droplets†

Size-controlled monodisperse droplets are indispensable in food, cosmetics, and healthcare industries. Although emulsion formation from bulk phases is well-explored, a robust in situ 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 in situ 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.