Spatial wettability patterning of glass microchips for water-in-oil-in-water (W/O/W) double emulsion preparation

Double emulsions are of great potential in many applications, including cosmetics, foods, and drug delivery. However, current preparation methods for double emulsions are not able to satisfactorily control emulsion size and structure. Microfluidic techniques open up new opportunities for this field but challenges still remain. This article presents a partially hydrophilic and partially hydrophobic glass microchip for generating water-in-oil-in-water (W/O/W) double emulsions. The device was first hydrophobized by forming an octadecyltrichlorosilane layer through self-assembly on the entire microchannel surface. Then, a hydrophilic surface was obtained by spatially photodegrading the hydrophobic organic layer with a deep UV-light source and a photomask. Thus, a distinctive wettability contrast was achieved on different parts of the microchannel surface. By optimizing the UV irradiation time, W/O/W double emulsions were successfully created on the microchip. Double emulsions prepared by this device were monodisperse and had narrow size distribution with coefficient of variation values less than 8%. Moreover, both the size and the number of inner droplets could be controlled by adjusting the flow rates.