Controlling Droplet Size Density during Dropwise Condensation on Silicone Oil Grafted Surfaces

Condensation is a critical phenomenon for various industrial applications, with dropwise condensation able to achieve higher heat transfer rates (6-8 times) than filmwise condensation. Various surface modification techniques have been explored to promote dropwise condensation via the application of low surface energy coatings, which create low contact angle hysteresis (CAH) surfaces and stimulate smaller size droplets shedding. In this work, silicone oil grafting is exploited to transform an intrinsically hydrophilic silicon substrate (CA ≈ 60^o) into a hydrophobic surface (CA ≈ 108^o). Different fabrication parameters (5, 20, and 100 cSt) enable different CAH ranging between 1° to 20°, and as such, dropwise condensation with different droplet mobilities and droplet size distributions. High viscosity oil grafted surfaces show the lowest CAH and shedding of very mobile small-sized droplets when compared to low viscosity oil, creating space for new droplets to nucleate, grow, coalesce, and shed. This technique has also been utilized to impose surface patterns for control of the droplet size distribution and mobility during condensation.