Oil-Free Passive Generation of Aqueous Two-Phase System Micro Droplets

The high efficacy of aqueous two-phase system (ATPS) droplets has been proven in a wide variety of biological and medical applications over the last decade due to its excellent biocompatibility. However, due to ultra-low interfacial tension, droplet generation in ATPS is extremely difficult when compared with the conventional water-in-oil (W/O) system. Consequently, without involvement of any external means, current manufacturing methods are either unable to produce droplets or limited to generate them in an extremely narrow range of flow conditions. In this work, we passively produced ATPS droplets with a wide range of droplet size at high generation frequency without employing an oil phase or external forces. We successfully generated ATPS droplets with a radius as small as 7 μm at the maximum frequency up to 300 Hz, which has not been achieved in previous studies. We found that the droplet size is correlated to the pressure and flow rate ratios with the power-law exponents of 0.8 and 0.2, respectively. In addition, this study is the first to report the information of flow rate and Capillary (Ca) number. It was found that the range of Ca in ATPS is 0.3–1.7, as compared to less than 0.1 in the W/O system, indicating the nature of ultra-low interfacial tension in ATPS.