Thermally Actuated Droplet Motion on Chemically Homogeneous, Striated, and Defected Surfaces

Experiments have demonstrated thermocapillary actuation on uniformly grafted partial-wettable surfaces. Droplet mobilization only occurs above a threshold thermal gradient or threshold droplet radius [1]. We characterized the motion instead in terms of a threshold depinning force, which successfully describes all the liquids tested. Above the depinning transition, the droplet speed, which is controlled by thermocapillary, capillary and viscous forces, increases monotonically with this reduced force parameterization. These results agree well to numerical predictions of a generalized Ford and Nadim model by using two fitting parameters, the slip coefficient and the magnitude of contact angle hysteresis [2–3]. In this follow-up study, we developed a doubly grafted surface, on which alkyltrichlorosilane coated stripes are surrounded by a more hydrophobic coating, perfluoroctyl-trichlorosilane. The quality of alkyltrichlorosilane coated stripes was still good for the thermocapillary droplet actuation, in which droplets were driven on the alkyltrichlorosilane surface and confined by the perfluoroctyl-trichlorosilane. The experimental results are also well described by a derived approximate three-dimensional model equation, which resembles the parameterization. The droplets are driven by thermocapillary force and retarded by contact angle hysteretic force, represented as contact angle hysteresis. This contact angle hysteresis is caused by chemical heterogeneity, surface roughness etc [4]. In the last part of this presentation, we will also present the thermocapillary droplet motion on a designed defected surface, which shows a tiny defect can severely hinge the droplet.