Thermally mediated double emulsion droplets formation in a six-way junction microfluidic device

The reliable generation of micron-sized droplets is an important process for various applications in droplet-based microfluidics. Planar flow-focusing is a novel microdroplet generation technique emerging in recent years. This technology can be used to generate double emulsion droplets, which have the advantages of high monodispersity, controllable size, and good robustness. However, the existing preparation processes based on this technique are often implemented at ambient temperature, and the droplet generation and size adjustment are realized mainly through regulating the flow rate. In this study, a temperature regulation system of microfluidic chip based on a customized thermoelectric cooler was developed to realize the accurate temperature control of the droplets preparation. The influences of temperature on the generation stability, size, and frequency of double emulsion droplets were investigated. With dipalmitoyl-phosphatidylcholine as the shell material, results showed that the temperature interval for the stable generation of double emulsion droplets was 0-40 degrees C. With the increasing temperature, the generation frequency of the droplets increased, while the inner and outer diameters show the opposite trend. As a result, the encapsulation efficiency of inner aqueous phase firstly increased and then decreased, indicating that the encapsulation productivity could not be improved simply through temperature elevating. When the temperature is reduced to around - 5 degrees C, the inner fluid phase could not be wrapped by the middle fluid phase, failing the formation of double emulsion droplets. When the temperature exceeds 40 degrees C, the generation of double emulsion droplets became unstable. The findings of this study can help for better understanding of double emulsion droplets generation in droplet microfluidic systems operating with different temperatures, which is expected to be practically used in such fields as cryobiology, cell biomechanics, and pharmaceutics.