Figure 2.

Double-emulsion droplets formation in a capillary microfluidic device. (a) Four types of droplet formation patterns marked with Type I (red circle), Type II (green triangle), Type III (blue inverted triangle), and Type IV (black square), denoting normal monodisperse double-emulsion droplets, double-emulsion droplets with an ultra-thin shell, more than one type of inner droplet encapsulated in one droplet, and one or none inner droplets encapsulated in one droplet, respectively. (b) Diagram of droplet patterns varying with Q_in and Q_mid at constant Q_out = 30 µL/min. (c) Diagram of droplet patterns varying with Q_mid and Q_out flow rates at (Q_mid − Q_in) = 1 µL/min.