Continuous Droplet Coalescence in a Microchannel Coflow Using Bulk Acoustic Waves

The coalescence of liquid droplets with a liquid stream has profound importance in various emerging applications, such as biochemical assays. Acoustic force-based droplet manipulation, which offers unique advantages, is consequently gaining attention. However, the physics of acoustics-driven coalescence of liquid droplets with a liquid stream is not well understood. Here, we unravel the mechanism of coalescence of aqueous droplets flowing in an immiscible (oil) phase with a coflowing aqueous stream, when the system is exposed to acoustic radiation force due to bulk acoustic waves. Our study reveals that the acoustic coalescence phenomenon is governed by the interplay between two important timescales, acoustic migration timescale (τac) and advection timescale (τadv), that underpin the phenomenon. We find that the phenomenon is also governed by the acoustic capillary number (Cac) and relative widths of the coflowing oil and aqueous streams (i.e., Waq and Woil). Our results show that, if Cac<0.9 and Waq>Woil are satisfied to ensure the stability of the streams and positioning of the acoustic node in the aqueous phase, respectively, continuous coalescence is observed for (τadv/τac)≥0.85. We exploit the phenomenon for the extraction of droplet contents (beads and cells) into an aqueous stream.