Fabrication and testing of surface ratchets primed with hydrophobic parylene and hexamethyldisilazane for transporting droplets

We demonstrate a way to transport droplets by an arrowed micropillar array surface with hydrophobic parylene. The lightly hydrophilic parylene surface could be changed to hydrophobic one by treating with fluorine-based plasma (CF4 or SF6). The droplet on this hydrophobic parylene surface with arrowed ratchets could be transported by a speaker, and the average measured velocity was 29 mm/s. Moreover, the authors compared driving performance of the parylene surface ratchets with the ones modified by the hexamethyldisilazane (HMDS) vapor. Both the results of using hydrophobic parylene and HMDS were better than the previous work.