Goose‐bumpy materials

Modifying the topography of a surface often leads to valuable properties. For instance, goosebumps appear on our skin when we are cold, which erects hairs and potentially traps heat along our body. Bumps can also deeply modify the way a hydrophobic solid interacts with water, transforming it into a repellent material.1 Both states are indeed different: While a water drop meets a hydrophobic solid with a contact size comparable to that of the drop, this contact nearly vanishes in the repellent state, which makes the liquid amazingly mobile. Drops then (quickly) move under the action of tiny forces, and they bounce like elastic marbles at impact. Textures on a surface can also trigger more sophisticated properties. Supplementary “overstructures”, that is, structures at a scale intermediate between the millimetric size of drops and the micrometric size of super-hydrophobic texture, generate a dynamical “super-super-hydrophobic” behavior.2 Impacting drops then bounce significantly quicker than what they do on flat surface, reinforcing the ability of the material to repel the water—an effect arising from the reconfiguration of drops colliding with the overstructures. In contrast, nanometric structures can preserve repellency down to the micrometric scale of dew drops, that spontaneously jump out of the material as they form (antifogging effect).3 In such a context, it appears important to achieve programmable surfaces, where we can switch from a wetting state to another one, for instance by transforming a flat material into a bumpy one, and vice-versa. The paper by Hu et al. describes such a commutable substrate.4 The authors dispose on a solid network of elastic cells that can be pneumatically inflated from below, either individually or collectively, so that each cell can switch from a flat to a convex shape. The limit described in the paper, where the cells are slightly bigger than the drops, could seem a bit restricted at first glance, but the study succeeds at convincing us that even this limit is relevant and beyond the proof of concept. The whole material is treated to be super-hydrophobic, and the change of morphology, as the cells are inflated in domes, provides new functions generated by the presence of large, programmable overstructures. The authors obtain two valuable results. (1) Impinging drops spend about twice less time to bounce off the surface when the overstructures are activated. As a matter of fact, the deformation of the impacting water on a dome is different from that on a flat surface, with a tendency to form stretched ligaments indeed known to reduce the contact time by a significant amount.5 (2) A drop deposited on the material can be displaced and propelled by serially actuating the overstructures, whose swelling produces a kind of flick that moves drops by typically one cell unit. This original and elegant effect clearly demonstrates why it is relevant to design a material where cells can be activated independently. In my opinion, the paper is even more stimulating by the perspectives it opens. I bet on at least two families of studies. (1) As it is, the goose-bumpy material developed by Hu et al. could be further investigated, owing to its flexibility. For instance, in cases where it is brought to a temperature low enough to make it coated by frost or by ice, it would be interesting to test how the activation of the cells contributes to break or even to evacuate the solid water. Also, a rapid inflation of the domes could kick water initially present at the surface in its flat state and help to remove it. (2) Even if it is a challenging program, we would dream of a much smaller inflatable texture (say, 10–50 µm) that could lead to the achievement of a material of adjustable repellency—simply hydrophobic in the flat state, and repellent when covered by goosebumps at the scale of 10–50 µm. We also know that the inflation of well-prepared elastic structures can generate amazing shapes,6 which might trigger unusual wetting properties. In a word, the use of reversible, programmable texture seems really promising. None. The author declares no conflict of interest. The authors declare no ethical issues.