Formation and dynamics of partially wetting droplets in square microchannels

We experimentally study the production and evolution of partially wetting droplets as a function of the dynamic advancing contact angle and the viscosity of the external phase in microchannels. The natural spreading properties of immersed droplets are measured using high-speed goniometry and their forced spreading behaviors are probed in confined microgeometries. Low- and high-viscosity microfluidic segmented flows are generated by focusing water in a continuous phase of silicone oil using square microchannels. The shape and stability of the lubricating film between droplets and channel walls permit the classification of typical flow regimes, including wetting, thin film, thick film, and constant film thickness. Hysteretic partially wetting systems are shown to exhibit two modes of droplets formation, namely dripping and rivulet. Small-scale multiphase flows are investigated as a function of capillary number, droplet size, concentration, and velocity. We also discuss the occurrence of dynamic wetting transitions and stick-and-slip motion of microfluidic droplets. This study shows the possibility to control droplet dynamic wetting behavior with flow rates of injection in microfluidic platforms.