Combination of active behaviors and passive structures contributes to the cleanliness of housefly wing surfaces: A new insight for the design of cleaning materials

Abstract

Evolutionary pressure has pushed many extant plants and animals to develop micro/nanostructures on their surfaces to keep them clean. These structures have become ideal models for bio-inspired design. Although microstructures on biological surfaces have been widely studied, little attention has been paid to the combined role of microstructures and animal’s active cleaning behaviors in keeping their surfaces clean. In this study, we explored the relationship between these micro/nanostructures and wettability as well as the role of the housefly cleaning behaviors in keeping their wings clean. Hierarchical structures consisting of microscale macrotrichias with nanoscale grooves on the wings were observed under scanning electron microscope. The wings were hydrophobic (CA = 133.3°) but with high adhesion to water (CAH = 87.5°), indicating that they were non-self-cleaning surfaces. Macroscale droplets standing on the wings could be best described as being in a transitional wetting state between Wenzel and Cassie-Baxter states due to the presence of the nanoscale grooves, which increased the resistance to water penetration. The hydrophobicity decreased (CA = 109.9°) when the nanostructures were removed by coating the wings with a thick layer of polydimethylsiloxane (PDMS). The houseflies could highly efficiently remove the microscale droplets atop the macrotrichias, and reduce bacterial contamination on their wings through grooming and flutter activities. These active cleaning behaviors could offset the absence of self-cleaning properties and play a key role in keeping the wings clean. The results indicate that housefly wings could be used as a template for the design of special functional surfaces. The present findings not only improve our understanding of the wettability and cleaning properties of natural surfaces, but also provide important insights into the design of bio-inspired materials.