Effect of nanoparticle surfactants on droplet formation in a flow-focusing microchannel

Nanoparticle surfactants, formed at liquid-liquid interfaces by the interactions between functional groups on nanoparticles and polymers having complementary end-functionality, have been recently proposed as an excellent interface stabilizer to cover liquid droplets for applications of substance encapsulation and delivery. However, the effects of nanoparticle surfactants on the production of liquid droplets in a microfluidic channel have not been comprehensively understood yet, which is a key prerequisite for achieving various functions in real applications. In this study, we have performed a systematic investigation on the effects of nanoparticle surfactants on droplet formation in a flow-focusing microchannel by using microfluidic experiments and theoretical analysis. We have found that simultaneously adding carboxylated nanoparticles into the dispersed phase and amino-terminated polymers into the continuous phases significantly decreases the droplet size but increases the production rate. More importantly, we have indicated that the combined effect of nanoparticles and polymers is much greater than the sum of their individual effects, which is mainly attributed to the significant reduction of the oil-water interfacial tension by the formation of nanoparticle surfactants. Besides, via analyzing the competition between hydrodynamic and interfacial forces acting on the droplet, we have established a theoretical criterion for the prediction of the droplet size with considering the effects of nanoparticle surfactants, which shows a good agreement with the experimental data.