Investigation of droplet coalescence in nanoparticle suspensions by a microfluidic collision experiment.

Understanding the phenomenon of droplet coalescence in nanoparticle suspensions is extremely important for the preparation of Pickering emulsions. A microfluidic platform, which can provide compulsive droplet collisions, was developed to imitate the droplet coalescence process in the early stages of emulsification. Microscope videos showed the variations in the droplet coalescence percentage, droplet contact time, and liquid film drainage time in different working systems containing 158-306 nm polystyrene (PS) particles in the continuous oil phase. The intersections of the half and total droplet contact times as well as the liquid film drainage time indicated the transitions of coalescence percentage. The additional hydrodynamic resistance in the liquid film between the approaching interfaces caused by the embedded hydrophobic nanoparticles was understood to be the main reason for reduced droplet coalescence, whereas hydrophilic particles were found to promote coalescence. As a novel method, the microfluidic collision experiment provided accurate and quantitative data for analyzing the formation of Pickering emulsions.