Enhancing the thermal efficiency of a broad range of condenser devices requires means of achieving sustainable dropwise condensation on metallic surfaces, where heat transfer can be further enhanced, by harvesting the advantage of the sweeping action of vapor flow over the surface, facilitating a reduction in the droplet departure diameter. Here, we present a rationally driven, hierarchical texturing process of copper surfaces, guided by fundamental principles of wettability and coalescence, which achieves controlled droplet departure under vapor flow conditions and thus significantly enhances phase change thermal transport. The desired texture is attained by fabricating an array of 3D laser-structured truncated microcones on the surface, covered with papillae-like nanostructures and a hydrolytically stable, low surface energy self-assembled-monolayer coating. Passive droplet departure on this surface is achieved through progressive coalescence of droplets arising from microcavities formed by the microcone array, resulting in depinning and subsequent departure of the depinned condensate drops through vapor shear. The synergistic combination of vapor shear and the sustained dropwise condensation on the hierarchical copper surface results in a nearly 700% increase in heat transfer coefficients as compared to filmwise condensation from identical, standard unstructured surfaces.

In this study, the water drop impact on a copper-based nanotextured superhydrophobic surface inside a uniform electric field is investigated. Because of the wider attention that drop impact draws in the scientific community, this study gives emphasis on the effect of the electric field on the droplet's residence time, a quantity that plays a key role in processes that involve heat and/or mass transport between the surface and impacting droplet. The reduction of the residence time is of vital importance especially for anti-icing and pathogen-transmissioncontainment applications. Shorter residence times enable droplets to rebound at supercooled surfaces before the occurrence of ice nucleation. Moreover, they restrict the likelihood of the deposition of viruses and bacteria for the case of pathogen-laden impacting droplets. Reduction of the residence time is achieved by a twofold strategy. The surface is textured in the nanoscale with the growth of a Cu(OH) 2 nanoneedle cluster so that the nanoroughness topography in combination with the hydrophobic coating imparts to the surface an extreme water-repellent behavior and impalement resistance. Moreover, we introduce an additional external force exerted on the droplet, which originates from an electric field. We focus on the range of the electric Bond number 0 ≤ Bo e ≤ 0.060. In this range, we observe two different interesting behaviors: (a) For 0 ≤ Bo e ≤ 0.020, the contact time reduces with the applied electric field. We also conduct simulations to support our experimental findings concerning the effect of the electric field on the contact time. (b) For 0.025 ≤ Bo e ≤ 0.060, the contact time increases. We demonstrate that this happens because of partial discharges that induce electrowetting, resulting in altering the wetting behavior of the droplet during retraction. Even though limitations exist, the application of electric fields can be considered to be a promising and flexible strategy for reducing the residence time because it can be applied on a wide range of superhydrophobic surfaces.