Dry under water: Comparative morphology and functional aspects of air‐retaining insect surfaces

Abstract

Superhydrophobic surfaces prevent certain body parts of semiaquatic and aquatic insects from getting wet while submerged in water. The air layer on these surfaces can serve the insects as a physical gill. Using scanning electron microscopy, we investigated the morphology of air-retaining surfaces in five insect species with different levels of adaptation to aquatic habitats. We found surfaces with either large and sparse hairs (setae), small and dense hairs (microtrichia), or hierarchically structured surfaces with both types of hairs. The structural parameters and air-film persistence of these surfaces were compared. Air-film persistence varied between 2 days in the beetle Galerucella nymphaea possessing only sparse setae and more than 120 days in the bugs Notonecta glauca and Ilyocoris cimicoides possessing dense microtrichia (up to 6.6 × 10(6) microtrichia per millimeter square). From our results, we conclude that the density of the surface structures is the most important factor that affects the persistence of air films. Combinations of setae and microtrichia are not decisive for the overall persistence of the air film but might provide a thick air store for a short time and a thin but mechanically more stable air film for a long time. Thus, we assume that a dense cover of microtrichia acts as a "backup system" preventing wetting of the body surface in case the air-water interface is pressed toward the surface. Our findings might be beneficial for the development of biomimetic surfaces for long-term air retention and drag reduction under water. In addition, the biological functions of the different air retention capabilities are discussed.

Superwettability of Gas Bubbles and Its Application: From Bioinspiration to Advanced Materials

“…[24] In fact, several insects hiding or hunting underwater exploit the property of superaerophilicity. [25][26][27][28][29] Water spiders…”

“…Reproduced with permission. [29] Copyright 2011, Wiley-VCH. d) The floating water fern salvinia with long-term air-retention properties.…”

“…Mater. 2017, 29,1703053 Figure 2. Reliable manipulation of gas bubbles by regulating interfacial morphologies and chemical components.…”

“…However, the air-film persistence of such an interface is relatively low, probably because it can only withstand low water pressures. [27][28][29] When the floating water ferns Salvinia molesta are submerged in water, they are capable of holding an air layer for several weeks, presumably limited by their lifetime. The eggbeater-shaped structures very efficiently act as pillars supporting the air-water interface (tent-like), preventing it from further approaching the leaf surface.…”

Slippery and never wet

“…To avoid the Wenzel state, we took guidance from aquatic insects such as the water boatman Notonecta glauca, that can breathe underwater thanks to the air retention feature of their superhydrophobic hairy exoskeleton. Barthlott et al observed that while aquatic insects with long hair can support a large volume of air, insects with small hairy structures had supreme lifetime of the air film, arguing that downward progression of the air-water interface is hindered due to the fact that it costs more energy to displace an interface with a smaller radius of curvature [5]. We introduced a dual-level topography by coating a standard, micropillared substrate with nanofilaments (Fig.…”