Biphilic Surfaces with Optimum Hydrophobic Islands on a Superhydrophobic Background for Dropwise Flow Condensation

Sustaining dropwise condensation is of great importance in many applications, especially in confined spaces. In this regard, superhydrophobic surfaces enhance condensation heat transfer performance due to the discrete droplet formation and rapid removal. On the other hand, droplets tend to nucleate easier and faster on hydrophobic surfaces compared to superhydrophobic ones. To take advantage of the mixed wettability, we fabricated biphilic surfaces and integrated them to small channels to assess their effect on thermal performance in flow condensation in small channels. Hydrophobic islands in the range of 100-900 μm diameter were fabricated using a combination of wet etching, surface functionalization, and physical vapor deposition (PVD) techniques. Condensation experiments were performed in a minichannel with a length, width, and height of 37, 10, and 1 mm, respectively. Here, we report optimum island diameters for the hydrophobic islands in terms of the maximum thermal performance. We show that considering the optimum point for each steam mass flux corresponding to the best heat transfer performance, the condensation heat transfer coefficient is increased by 51, 48, 42, 40, and 36% compared to the plain reference hydrophobic surface for steam mass fluxes of 10, 20, 30, 40, and 50 kg/m² s, respectively. The optimum island diameters are obtained as 200, 300, 400, 400, and 500 μm, with the ratios of hydrophobic to superhydrophobic surface areas (A* = A_hydrophobic/A_{superhydrophobic}) of 3.2, 7.6, 14.4, 14.4, and 24.4%, for steam mass fluxes of 10, 20, 30, 40, and 50 kg/m² s, respectively. The liquid film forming on the liquid-vapor interface acts as an insulation layer and generates thermal resistance, and bridges appear on the patterned areas and deteriorate the thermal performance. Therefore, it is crucial to characterize the role of droplet mobility on biphilic surfaces to avoid the occurrence of bridging. Through visualization, we demonstrate that the optimum conditions correspond to enhanced droplet nucleation and rapid sweeping regions, where droplet pinning and bridging do not occur. The trends in condensation heat transfer with surface mixed wettability can be divided into three regions: enhanced droplet nucleation and rapid sweeping, highly pinned droplet, and bridging droplet segments. We reveal that the interfacial heat transfer augmentation in the enhanced droplet nucleation and rapid sweeping region is due to both spatial control of droplet nucleation and an increase in the sweeping period. Furthermore, by fitting the experimental data, a correlation for predicting the optimum island diameter for biphilic surfaces is proposed for condensation heat transfer in confined channels, which will be a valuable guideline for engineers and researchers working on the design and development of thermal systems.