Figure 3. Three levels of protection - the anti-wetting skin morphology of springtails.

Multiple design principles are combined to protect collembolan skin against wetting: (A) The hairy cover is the first wetting barrier; liquids can be pinned on the bristle tips. If external forces or very low surface tensions enable liquids to conquer this first barrier, a second principle comes into play: (B) Nanoscopic comb structures of interconnected primary granules can still pin liquids by effective retention of entrapped gas nanobubbles within the surface nanocavities. (C) Gas retention is enforced by the previously unknown fact that the overhanging topographies of the structural elements exhibit a negative curvature (with respect to an orthogonal axis to the surface). The result is a forced Cassie state, through which a dramatically reduced solid–liquid contact area leads to increased macroscopic contact angles of drops on the skin surface. As schematically shown in (D), the design principle protects the surface against wetting independent of the surface chemistry and even at very low surface tensions of the liquid and at elevated pressures.