Limiting hydrophobic behavior and reflectance response of dragonfly and damselfly wings

作者：Aideo Swai Nawami; Mohanta Dambarudhar^*

来源：Applied Surface Science, 2016, 387: 609-616.

DOI：10.1016/j.apsusc.2016.06.049

• Surface roughness
• Contact angle
• Hydrophobic
• Dewetting
• Chitin

In this work, through water contact angle (CA) measurements, we explore hydrophobic behavior of different parts of the hind wings of a dragonfly, Gynacantha Dravida and of a damselfly, Pseudagrion Microcephalum. As we move from the basal to distal region, the contact angle (theta) was found to vary in the range of 120-136 degrees for both the species. Moreover, the wing of the dragonfly was seen to be more hydrophobic than that of the damselfly one. An attempt has also been made to link roughness factor (r(phi)) and solid-water fraction (phi) through the simplified Wenzel and Cassie-Baxter models. We noticed that, r(phi) and (phi) tend to follow a linear relation that gives r(phi) = 1.47 in the limit, Delta theta < 10.1 degrees, latter being recognized as the difference in angle between the measured CA over a surface to that of the CA (similar to 105 degrees) known for a smooth surface. Our experimental data, however, revealed empirical relations which predicted higher r(phi) values, particularly when Delta theta is large. While the overall reflectance response of the distal segment was believed to be stronger than that of the basal part, the edge parts of the dragonfly and damselfly wings exhibited exponential associated growing trends with increasing wavelength. The relative reflectance response, corresponding to similar to 494 nm and 370 nm peaks, gets nearly doubled for the edge specimen as compared to the distal and basal parts. The edge- specimen, which comprises of rectangular shaped, periodic microstructures, displayed carotenoid based two broad peak maxima at similar to 422 nm and similar to 494 nm. The surface roughness which arises through the distribution of oblate-shaped nano-fibrils is believed to be the basis of sub-surface volume scattering. Interrelating nanostructure surface roughness based wettability and reflectance characteristics would provide new insights on structure-property relationship in naturally available soft matter systems including templates of biological origin.

• 出版日期2016-11-30