Hydrophobicity in clearwing butterflies and moths: impact of scale micro and nanostructure, and trade-off with optical transparency

Abstract

AbstractIn opaque butterflies and moths, scales ensure vital functions like camouflage, thermoregulation, and hydrophobicity. Wing transparency in some species – achieved via modified or absent scales – raises the question of whether hydrophobicity can be maintained and of it dependence on scale microstructural (scale presence, morphology, insertion angle, and coloration) and nanostructural (ridge spacing and width) features. To address these questions, we assessed hydrophobicity in 23 clearwing species differing in scale micro and nanofeatures by measuring static contact angle (CA) of water droplets in the opaque and transparent patches of the same individuals at different stages of evaporation. We related these measures to wing structures (macro, micro, and nano) and compared them to predictions from Cassie-Baxter and Wenzel models. We found that overall, transparency is costly for hydrophobicity and this cost depends on scale microstructural features: transparent patches are less hydrophobic and lose more hydrophobicity with water evaporation than opaque patches. This loss is attenuated for higher scale densities, coloured scales (for erect scales), and when combining two types of scales (piliform and lamellar). Nude membranes show lowest hydrophobicity. Best models are Cassie-Baxter models that include scale microstructures for erect scales, and scale micro and nanostructures for flat scales. All findings are consistent with the physics of hydrophobicity, especially on multiscale roughness. Finally, wing hydrophobicity negatively relates to optical transparency. Moreover, tropical species have more hydrophobic transparent patches but similarly hydrophobic opaque patches compared to temperate species. Overall, diverse microstructures are likely functional compromises between multiple requirements.Significance StatementWater repellency is vital for terrestrial organisms. Yet, how microstructural diversity may impact hydrophobicity is unknown. Bridging the gap between biology and physics, we exploit the microstructural diversity found in clearwing butterflies and moths to assess its impact on hydrophobicity, and its ecological relevance. Within a physical framework, we bring experimental and modelling evidence for a major role of microstructures (scale morphology, insertion angle, coloration) and multiscale roughness in determining wing hydrophobicity, with a role of nanostructures restricted to flat scales and nude membrane. For the first time, we evidence some costs for transparency, and a trade-off between optics and hydrophobicity. Beyond novel biological results, this study gives new sources of bioinspiration for applied research on transparent materials in physics.