Dropwise condensation on multi-scale metallic surfaces with nano-features

Non-wetting surfaces engineered from intrinsically hydrophilic metallic materials are promising for self-cleaning, anti-icing and/or condensation heat transfer applications where the durability of the coating is an issue. In this work, we fabricate two metallic non-wetting surfaces with varying number and size of the roughness tiers without further hydrophobic coating procedure. The wetting behaviour and the condensation performance is then addressed. On one hand, the surface resembling a rose petal exhibits a sticky non-wetting behaviour as drops wet the microscopic roughness features with the consequent enhanced drop adhesion. In turn, this stickiness leads to filmwise condensation. On the other hand, the surface resembling a lotus leaf provides super-repellent non-wetting behaviour prompting the continuous nucleation, growth and departure of spherical drops in a dropwise condensation fashion. The additional oxidation step, which creates a third nano-scale roughness tier on the surface resembling a lotus leaf, is found to be paramount in prompting the growth of drops in the Cassie state with the benefit of minimal condensate adhesion. The two different condensation behaviours reported are well supported by a drop surface energy analysis, which accounts for the different wetting performance and the surface structure underneath the condensing drops. Further, we coated the above-mentioned surfaces with polydimethylsiloxane surfaces, which resulted in filmwise condensation due to the smoothening of the different roughness tiers. The first, to the best of our knowledge, continuous dropwise condensation on a metallic surface without the need for a conformal hydrophobic coating is hence demonstrated, which offers a novel path for the design and manufacture of non-coated metallic super-repellent surfaces for condensation phase change applications, amongst others.