Droplet dynamics passing through obstructions in confined microchannel flow

Motivated by the previous studies (Lee et al., Lab Chip 10:1160-1166, 2010; Link et al., Phys Rev Lett 92:054503-1-054503-4, 2004), the droplet dynamics passing through obstructions in confined microchannel was explored both numerically and experimentally. The effects of obstruction shape (cylinder and square), droplet size, and capillary number (Ca) on droplet dynamics were investigated. For the size control, due to an obstruction-induced droplet breakup, the cylinder obstruction was found to be advantageous over square type for practical purposes. The thread breakup was attributed to both normal and shear components of velocity gradients near the obstruction, in particular, near the corners of the square. As a result, the square obstruction was considered to generate more non-trivial satellite droplets. The droplet size showed little influence on the droplet dynamics. Considering the wetting process on the cylinder surface, we explored the droplet dynamics passing through two successive cylinder obstructions, where more complicated dynamics was observed depending on Ca (capillary number similar to viscous force / interface tension), cylinder interval, and droplet size. Here, we propose two requirements for independent wetting on each cylinder: (i) low Ca droplet should be manipulated, and (ii) cylinder interval should be larger than channel width. That is, low Ca droplet could intrude the region between two cylinders if the cylinder interval was far enough, while the droplet could not intrude due to geometric hindrance for close obstructions. In the numerical viewpoint, the proposed requirements were also valid for multi-cylinder obstructions up to 6. In addition, we propose a novel design of array structure of cylinders for a selective wetting, which might be useful to fabricate Janus particles. We hereby prove by both simulation and experiments that the wetting on the obstruction is controllable by changing Ca and cylinder design in the multilayer deposition process.