Abstract
Emulsions are thermodynamically unstable and the emulsion droplets coalesce over a certain time depending on conditions. The coalescence of emulsion droplets is important in many industrial operations. The kinetics of emulsion coalescence determines emulsion stability and rheology, which may depend on the distribution of droplet sizes in the system. Because of the difficulty of getting monodisperse emulsion, the coalescence of droplets was only investigated macroscopically, and microscopic observation of droplets coalescence has not been studied. Recently we have developed microchannel (MC) emulsification, which is a novel technique for preparing monodisperse water-in-oil and oil-in-water emulsions. The MC plate has uniform micrometer-sized channels fabricated on a single-crystal silicon substrate using photolithographic and etching processes. Emulsions with a relative standard deviation (RSD) of approximately 10% have been successfully prepared by applying this technique. A new observation system, which monitors coalescence process in dense-packed emulsion droplets in real time, has been developed. In this study, we prepared oil-in-water emulsion with mean droplet size of 35 micrometers and RSD of 10% by MC emulsification. Coalescence processes were observed in the system and recorded through the eyepiece with a videocamera. From the images, effects of temperature and salt concentration on the coalescence rate in the flocculation of emulsion droplets were evaluated.
