Exploring the Role of Habitat on the Wettability of Cicada Wings

Abstract

Evolutionary pressure has pushed many extant species to develop micro/nanostructures that can significantly affect wettability and enable functionalities such as droplet jumping, self-cleaning, antifogging, antimicrobial, and antireflectivity. In particular, significant effort is underway to understand the insect wing surface structure to establish rational design tools for the development of novel engineered materials. Most studies, however, have focused on superhydrophobic wings obtained from a single insect species, in particular, the Psaltoda claripennis cicada. Here, we investigate the relationship between the spatially dependent wing wettability, topology, and droplet jumping behavior of multiple cicada species and their habitat, lifecycle, and interspecies relatedness. We focus on cicada wings of four different species: Neotibicen pruinosus, N. tibicen, Megatibicen dorsatus, and Magicicada septendecim and take a comparative approach. Using spatially resolved microgoniometry, scanning electron microscopy, atomic force microscopy, and high speed optical microscopy, we show that within cicada species, the wettability of wings is spatially homogeneous across wing cells. All four species were shown to have truncated conical pillars with widely varying length scales ranging from 50 to 400 nm in height. Comparison of the wettability revealed three cicada species with wings that are superhydrophobic (>150°) with low contact angle hysteresis (<5°), resulting in stable droplet jumping behavior. The fourth, more distantly related species (Ma. septendecim) showed only moderate hydrophobic behavior, eliminating some of the beneficial surface functional aspects for this cicada. Correlation between cicada habitat and wing wettability yielded little connection as wetter, swampy environments do not necessarily equate to higher measured wing hydrophobicity. The results, however, do point to species relatedness and reproductive strategy as a closer proxy for predicting wettability and surface structure and resultant enhanced wing surface functionality. This work not only elucidates the differences between inter- and intraspecies cicada wing topology, wettability, and water shedding behavior but also enables the development of rational design tools for the manufacture of artificial surfaces for energy and water applications.

Molecular and Topographical Organization: Influence on Cicada Wing Wettability and Bactericidal Properties

“…Surface roughness can either increase or decrease water contact angles depending on a combination of the intrinsic wettability of the surface [ 49,50 ] as well as on the ability of structural features to trap air beneath the water droplet to enhance its nonwetting properties. [ 49,50 ] A recent report observed very little variation in wettability across the surface of the wing; [ 1 ] however, to mitigate possible extraction differences, contact angles were measured in three different wing locations (mc: proximal; a3: distal; u3: middle cells; see Figure 1) for each MAE time. Advancing contact angles over extraction time are shown in Figure , with colored regions denoting general hydrophobicity.…”

“…The contact angle was measured four times on each replicate sample ( n = 3) for a total of 12 measurements per cicada species and MAE time. Care was taken to ensure the three replicate samples were from three locations (mc: proximal; a3: distal; u3: middle cells) [ 1 ] (Figure 1). The data was analyzed using a circle fitting method on interFAce Measurement and Analysis processing software (FAMAS).…”

“…Insect wings are examples of surfaces that have evolved to exhibit a range of properties, including superhydrophobicity, [ 1–3 ] self‐cleaning, [ 4–6 ] droplet jumping, [ 1,4,7,8 ] optical reflectivity, [ 7,9 ] antifogging, [ 6 ] and antimicrobial activity. [ 10,11 ] These properties have been observed on the wings of cicadas, [ 1,2,7,12,13 ] butterflies, [ 14 ] dragonflies, [ 5,15 ] and damselflies. [ 5 ] Topographical analysis of insect wing surfaces has shown a variety of micro/nanofeatures, ranging from highly ordered, homogeneous pillars (cicadas) to amorphous features (dragonflies) to intricate “Christmas‐tree” like structures ( Morpho butterflies), with each playing a key role in the remarkable surface properties.…”

“…A recent study reported that enhanced functionalities have evolved in cicada species as a result of taxonomic influences and life‐history characteristics such as longer adult lifespans. [ 1 ] Revealing the critical evolutionary pressures that drive the development of certain surface properties and defining the wing surface design and composition is a useful exercise in locating the next generation of natural templates.…”

“…N. pruinosus is an annual cicada (emerges every year) found in urban and forested areas that can be found aboveground as a winged adult for a relatively long time (2–4 months). Its wings have been shown to be superhydrophobic [ 1 ] and are expected to be self‐cleaning and bactericidal based on similar species of cicada. [ 10,12,19 ] M. cassinii , however, is a gregarious periodical cicada that emerges every 17 years in large numbers and lives as a winged adult for less than one month.…”