Controlling Droplet Shedding Size, Departure Time and Droplet Size Density during Dropwise Condensation on Silicone Oil Grafted Surfaces*

Condensation plays a critical role in numerous industrial and engineering applications with dropwise condensation achieving 6-8 times better heat transfer than filmwise condensation. To date, several surface modification methods promoting dropwise condensation have been investigated. One of these methods involve uniformly applying low surface energy coatings with small contact angle hysteresis (CAH) that stimulate faster and smaller sized droplets shedding. Here, we use silicone oil grafting to create hydrophobic coatings on an otherwise hydrophilic silicon substrate. Based on fabrication parameters such as oil viscosity, volume and/or number of layers. the silicone oil grafting technique adopted achieves contact angles of ~108° and CAH ranging between 1° and 20°. Such surfaces are then subjected to condensation phase-change in a custom-built environmental chamber. We then use ImageJ and MATLAB for image processing where dropwise condensation is observed on all these grafted surfaces irrespective of the various fabrication parameters. However, droplet mobility, shedding time and droplet size distributions, can be actually controlled by changing the fabrication parameters. We observe faster shedding of smaller sized droplets on high viscosity oil grafted surfaces following their lower hysteresis when compared to low viscosity grafted ones. Fast droplet shedding on silicone oil grated surfaces creates space for new droplets to nucleate, grow, coalesce and shed, which is characteristic of high condensation heat transfer rates.