Due to recent advances in nanofabrication, phasechange condensation heat transfer has seen a renaissance. Compared to conventional heat transfer surfaces, nanostructured surfaces impregnated with chemically matched lubrication films (hereinafter referred to as "nanostructured lubricated surfaces") have been demonstrated to improve vapor-side phase-change condensation heat transfer by facilitating droplet nucleation, growth, and departure. While the presence of nanoscale roughness improves performance longevity by stabilizing the lubrication film via capillary forces, such enhancement is shortlived due to the eventual loss of lubrication oil by the departing droplets. The objective of this study is to characterize oil depletion caused by pendant droplets during condensation. For our study, we nanostructured, chemically functionalized, and lubricated horizontal copper tubes that are widely used in shell-and-tube heat exchangers in power plants and process industries. Using high-speed fluorescence imaging and thermogravimetric analysis, we show that shedding droplets exert a shear force on the oil in the wetting ridge at the water−oil interface. The viscous shear draws the lubrication film from the nanostructured surface onto the upper portion of the droplet and forms a ring-like oil skirt. Through detailed theoretical analysis, we show that the thickness of this oil skirt scales with the classical Landau−Levich−Derjaguin (LLD) theory for dipcoating. Our results reveal that droplets falling from horizontal tubes break unequally and leave behind small satellite droplets that retain the bulk of the oil in the wetting ridge. This observation is in stark contrast with the earlier description of droplets shedding from tilted flat plates where the entire oil-filled wetting ridge is demonstrated to leave the surface upon droplet departure. By selecting lubrication oils of varying viscosity and spreading coefficient, we provide evidence that the contribution of the wrapping layer to the rate of oil depletion is insignificant. Furthermore, we show that due to the nanoscale features on the tubes, nearly half of the lubrication film remains on the surface after 10 h of continuous steam condensation at ambient pressure, 23 °C, and 60% relative humidity, a 2−3-fold improvement over previous results.The insights gained from this work will provide guidelines for the rational design of long-lasting nanostructured lubricated surfaces for phase-change condensation.

Phase-change condensation is commonplace in nature and industry. Since the 1930s, it is well understood that vapor condenses in filmwise mode on clean metallic surfaces whereas it condenses by forming discrete droplets on surfaces coated with a promoter material. In both filmwise and dropwise modes, the condensate is removed when gravity overcomes pinning forces. In this work, we show rapid condensate transport through cracks that formed due to material shrinkage when a copper tube is coated with silica inverse opal structures. Importantly, the high hydraulic conductivity of the cracks promote axial condensate transport that is beneficial for condensation heat transfer. In our experiments, the cracks improved the heat transfer coefficient from ≈ 12 kW/m2 K for laminar filmwise condensation on smooth clean copper tubes to ≈ 80 kW/m2 K for inverse opal coated copper tubes; nearly a sevenfold increase from filmwise condensation and identical enhancement with state-of-the-art dropwise condensation. Furthermore, our results show that impregnating the porous structure with oil further improves the heat transfer coefficient by an additional 30% to ≈ 103 kW/m2 K. Importantly, compared to the fast-degrading dropwise condensation, the inverse opal coated copper tubes maintained high heat transfer rates when the experiments were repeated > 20 times; each experiment lasting 3–4 h. In addition to the new coating approach, the insights gained from this work present a strategy to minimize oil depletion during condensation from lubricated surfaces.

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