Multiple-dimensional micro/nano structural models for hydrophobicity of butterfly wing surfaces and coupling mechanism

The microstructure, wettability and chemical composition of the butterfly wing surfaces were investigated by a scanning electron microscope, a contact angle meter and a Fourier transform infrared spectrometer. The micro/nano structural models for hydrophobicity of the butterfly wing surfaces were established on the basis of the Cassie equation. The hydrophobicity mechanisms were discussed from the perspective of biological coupling. The butterfly wing surfaces are composed of naturally hydrophobic material and possess micro/nano hierarchical structures, including primary structure (micrometric scales), secondary structure (nano longitudinal ridges and lateral bridges) and tertiary structure (nano stripes). The wing surfaces exhibit high hydrophobicity (contact angle 138 degrees-157 degrees) and low adhesion (sliding angle 1 degrees-3 degrees). The micromorphology and self-cleaning performance of the wing surfaces demonstrate remarkable anisotropism. The special complex wettability ascribes to a coupling effect of the material element and the structure element. In micro-dimension, the smaller the width and the bigger the spacing of the scale, the stronger the hydrophobicity of the wing surfaces. In nano-dimension, the smaller the height and the smaller the width and the bigger the spacing of the longitudinal ridge, the stronger the hydrophobicity of the wing surfaces. This work promotes our understanding of the hydrophobicity mechanism of bio-surfaces and may bring inspiration for biomimetic design and preparation of smart interfacial materials.