(316g) Surface-Mediated Drop Motion in a Bifurcating Microchannel with Different Wettabilities

The motion of droplets in a bifurcating microchannel with different wettablilities is investigated experimentally. The featured channel is fabricated using the PDMS-based soft lithography technique. Utilizing plasma oxidation with the aid of the self-developed surface treatment method, we are able to create a hydrophilic/hydrophobic contrast between the two branches of the channel. A train of oil droplets is generated in a continuous water phase. Through droplet-wall interactions, effects due to the branches' wettablility difference are assessed by observing the behaviours of the droplets approaching the bifurcating channels. If the channel has a uniform surface wettablility (wherein all the surfaces are hydrophilic), the droplets move evenly towards the branches. However, when there is a wettablility difference between the branches, it can re-direct the motion of the droplets, making the droplets in favor of the hydrophilic end. This biased movement is further quantified by the percentage of the droplets choosing the hydrophilic end, depending on the water flow rates. We also find that due to hydrodynamic interactions, there is an optimal flow rate at which the inclination of the droplets toward the hydrophilic end is the greatest. The detailed mechanisms are elucidated with the aid of a scaling argument.