Visualization of Dropwise Condensation on a Superhydrophobic Microporous Surface

Condensation behavior on a superhydrophobic microporous surface was visually compared with that on a plain surface in the water saturated at the pressures of 101.3 kPa (Tsat = 100 °C) and 3.2 kPa (Tsat = 25 °C). The microporous surface was formed by sintering copper powders with the average diameter of 50 µm on the bare copper surface, resulting in the coating thickness of approximately 250 µm. The microporous surface was coated with the polytetrafluoroethylene (PTFE) layer by the spray-coating method to change the wettability into superhydrophobic, which was verified from a measured apparent contact angle of above 150°. While dropwise condensation is observed on both bare and hydrophobic plain surfaces at Psat = 101.3 kPa, filmwise condensation is seen on the superhydrophilic plain surface. At the low saturation pressure of 3.2 kPa, condensation behavior on the bare plain surface is changed into the filmwise condensation with improved wettability. In contrast, on the microporous surface, the only superhydrophobic microporous surface shows dropwise condensation behavior, where the surface is considered at the Cassie-state. However, filmwise condensation behaviors are observed on both superhydrophilic and bare microporous surfaces due to the wicking into the pores.