Tailoring surface properties for controlling droplet formation at microsieve membranes

Abstract

The preparation of water-in-oil emulsions with silicon nitride microfluidic nozzle arrays requires a surface, which is not wetted by the continuous water phase. Hence, we deposited several hydrophobic coatings on silicon nitride and studied the wetting behavior of water in pure and emulsifier containing hexadecane. In pure medias we could demonstrate that an octyl-chain terminated is the optimal hydrophobic coating. It has a high affinity to hexadecane due to its similar chemical nature expressed by a very low solid/liquid interfacial tension and a low affinity to water resulting in the highest measured water in hexadecane contact angle of 164°. Unmodified silicon nitride in contrast has the highest affinity to water and the lowest to hexadecane resulting in a contact angle of 97°. Based on the determination of the solid/liquid interfacial tensions in pure liquids we found for several hydrophobic coatings and for unmodified silicon nitride a deviation from Young's equation, which can be related to negative Hamaker constants of the studied systems. With the soybean-based emulsifier BolecMT we observed for several coatings no negative influence of the adsorbed emulsifier on the hydrophobicity. In fact, the water/hexadecane contact angles increased in time. In case of unmodified silicon nitride the surface was rendered into a more hydrophobic state. For low surface energy coatings, we observed a dramatic time-dependent change of the wetting characteristics. After a certain period of time water drops collapsed and wetted the surface. The time scales of this observed wetting behavior is several magnitudes larger than the drop formation process itself. Therefore, its relevance for the drop formation process could be neglected. With the unmodified silicon nitride surface successful drop formation was not possible and the low surface energy coating performed successfully in membrane emulsification experiments.