Heat-transfer enhancement and evaporation mechanisms on roughness-controlled wettability-contrast surfaces

Abstract

The problem of efficiently removing high heat fluxes (>100 W/cm²) from integrated circuits is particularly relevant. Its solution requires beyond heat-transfer surfaces with specified functional properties. In this study, the mechanisms of water droplet evaporation on biphilic aluminum heat-transfer surfaces with spatially controlled wetting regions were investigated. For this purpose, a novel approach for creating heat-transfer surfaces was developed based on a combination of laser processing of metal surfaces and hydrophobization by grafting alkyl groups (-CH₂- and -CH₃-) from oil thermolysis products to the textured surface. Experimental data on the influence of wettability, as an independent factor, on the droplet evaporation characteristics in the surface heating temperature range of 80–300 °C were obtained. Heat transfer mechanisms were studied using high-speed optical visualization, particle image velocimetry (PIV), and planar laser-induced fluorescence (PLIF). Direct temperature measurements were also conducted in the near-surface layer of the sample at a depth of 500 μm from the heat transfer surface using microscale thermocouples. The results show that slower evaporation on biphilic surfaces promotes enhanced local cooling. Wettability contrast induces complex internal thermocapillary (Marangoni) flows and significantly increases the length of the three-phase contact line due to the formation of air pockets under the droplet on superhydrophobic regions of the heat-transfer surface. At a temperature of 80 °C, biphilic surfaces with a superhydrophobic region fraction of 30–45 % provide a 79–81 % greater temperature reduction in the near-surface layer of the sample than a polished surface, despite a lower evaporation rate (29–74 % lower). Moreover, optimal wetting contrast shifts the maximum cooling efficiency to higher temperatures (160 °C versus 140 °C for a polished surface), thereby delaying the onset of the Leidenfrost effect. For surfaces combining high roughness (Sdr ≈ 700 %) with hydrophilic/superhydrophilic regions, the cooling efficiency at moderate temperatures increases up to 20 times.