Abstract: Double emulsions have huge potential applications in many fields, but their wide use is limited by the instability. We have established a plane experimental observation method based on the microscope observation on the instability of double emulsion clusters to understand the key factors that affect the instability of droplets. A glass capillary device was used to produce the uniform monodisperse double emulsion droplets, and the dynamic information of the double emulsions was recorded through a microscope. We found that the instability of the double emulsions was caused together by diffusion and coalescence. And through the light curing experiment, the coalescence phenomenon of the internal droplets and the outer phase became visible. The experimental results show that the thicker the oil film thickness of the W/O/W droplets, the more difficult the coalescence of the inner droplets and the outer phase, and the more stable the double emulsions. The effect of glycerol on the stability of double emulsions was studied by changing the concentration of glycerol in the outer phase. We found that the stability time of the double emulsions increased with the decrease of the glycerol concentration, but when the glycerol concentration was reduced to 10 wt%, the stability of the double emulsions became worse. When the glycerol concentration was not less than 40 wt%, the instability of the double emulsions was more caused by the conversion of W/O/W droplets to O/W droplets. When the glycerol concentration was not more than 30 wt%, the instability of the double emulsions was more caused by the conversion of W/O/W droplets to W/W droplets. In addition, by using different surfactants, it was found that the double emulsions formulated with polyvinyl alcohol (PVA) had higher yield and a longer stable time.

Key words: double emulsions, coalescence, glycerol, surfactant, stability