Exploring the Design Features of Wettability-Patterned Surfaces for Condensation Heat Transfer

Condensation heat transfer is a key process in various areas such as power generation, seawater desalination, fog harvesting, and thermal management. There are two well-known vapor condensation mechanisms, dropwise and filmwise; in the former, the condensate forms droplets that coalesce with each other and grow, eventually leaving the surface, while in the latter, a liquid film is formed. It is well known [1] that dropwise condensation outperforms filmwise condensation in terms of heat transfer rate. Over the past two decades a variety of surface engineering methods have been used to promote dropwise condensation. The most promising approaches include the development of superhydrophobic surfaces that enhance condensate removal by forming highly mobile droplets, as well as the surface-energy manipulation to pattern surfaces with various superhydrophobic/superhydrophilic characteristics. Wettability patterning featuring superhydrophilic wedge-shape tracks with filmwise-condensing domains that facilitate capillary-driven condensate drainage, laid on a hydrophobic background, can be effectively used in passive heat spreaders, as demonstrated in our previous works [2], [3]. When implementing wettability patterning, factors such as ratio of hydrophilic to hydrophobic area, total length of the contrast line between the two distinct wettabilities, wedge track geometry, etc., play important roles in the overall surface performance of the condenser. In this study, an environmental chamber where the temperature and relative humidity could be maintained at the desired levels was used to create a controlled environment for condensation of vapor in the presence of non-condensable gases on a cooled copper plate equipped with specifically-designed wettability patterns. The effect of the pressure gradient on superhydrophilic wedge-shape tracks is investigated. The study shows that the heat transfer coefficient enhancement on a wettability-patterned surface compared to a uniformly hydrophobic surface could vary on average from 4% to 24%, depending on the wedge-shape track geometry.