Unusually diverse surface-wettability features found in the wings of butterflies across Lepidoptera order and evaluation of generic and vertical gibbosity-based models

Abstract

Abstract The present work reports on the model evaluation and validation of surface roughness-mediated wettability characteristics of three different butterflies belonging to the Lepidoptera order. They are, namely, White admiral (WA, Limenitis camilla), Large white (LW, Pieris brassicae), and Dark blue tiger (DBT, Tirumala septentrionis). The unique microstructural assembly of scales along with the built-in ridges and cross-ribs were believed to be largely responsible for displaying remarkable surface texture and consequently, dewetting features while static contact angle (CA) varying in the range 100°–124°. Models based on the scale dimensions, interscale separations along with regular triangular protuberances (vertical gibbosities) predict roughness factors within 1.1–1.7, while CA hysteresis tending to vary in the range 17.5°–45.7°. The DBT wing part exhibited the highest CA and minimal surface adhesion feature, as the droplet was seen to roll off the surface when the base was tilted with the horizontal plane, above 50°. Moreover, the WA wing surface wettability obeys closely the Cassie-Wenzel model, while the DBT wing part is in conformity with the Wenzel-Cassie and Vertical-Gibbsoity based models. The significance of the wettability features and coloration aspects of the natural systems is expected to deliberate scopes for generating and mimicking designs through artificial means with intended relevance in soft-matter, bionics, and interface engineering fields along with fundamental interest for evolutionary reasons.