Double droplet formation and flow characteristics in microchannel

Double emulsions, such as water-oil-water, are oil droplets that contain an aqueous droplet, which are dispersed within another aqueous phase. These double emulsions can be formed via encapsulation in microchannels, which provide a controllable and efficient environment for applications in pharmaceuticals and food. However, due to the difficulty in stabilizing double emulsions, the inner droplets can easily coalesce. Addition of surfactants can enhance the stability of double emulsions.

This study aims to investigate experimentally the effects of surfactants, flow rates and other parameters on the encapsulation process to control double emulsion formation. It investigates the formation of the drop-in-drop pattern within double flow-focusing glass microchannels. Glycerol was mixed with water to match the refractive index of the silicon oil, facilitating optical access for high-speed imaging measurements. A cationic (SDS) and a non-ionic (TX-100) surfactant were added in the outer and inner aqueous phases at below and above critical micelle concentration (CMC). A flow pattern map was generated to identify the boundary of the drop-in-drop regime under varying interfacial tension and flow rate conditions. The encapsulation rate and stability of the drop-in-drop regime were then investigated for a range of capillary numbers.

The results improve the understanding of the mechanisms underlying double emulsion formation and suggest strategies for desired outcomes in various applications.