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VIRTUE OF NANO TEXTURES ON INSECTS SUPERHYDROPHOBIC SURFACES
Restricted (Penn State Only)
Materials Science and Engineering
Master of Science
Date of Defense:
November 05, 2018
Tak Sing Wong, Thesis Advisor
Carlo G. Pantano, Committee Member
Pak Kin Wong, Committee Member
Many plants and insects have developed intriguing wetting properties that enable them to thrive in their natural habitats. Lotus leaf, which is one of the most intriguing examples, has served as the blueprint for the design of superhydrophobic surfaces for over two decades. It is found that the water repellency of lotus leaves stems from the micro/nanoscale-hierarchical structures and the hydrophobic epicuticular wax coating. As shown by the classical Cassie-Baxter equation (1944), one essential requirement to achieving superhydrophobicity is that the surface has to have low solid fraction, Φ_s (i.e., area percentage of the solid surface that is in direct contact with water). Typically, solid fraction of less than 0.05 is required to achieve superhydrophobicity. However, some insect surfaces exhibit exceptional water repellent characteristics which are contradicting to the prediction by the Cassie-Baxter equation. For example, superhydrophobic mosquito eyes possess solid fraction as high as 0.44. The solid fractions on springtails and cicada wings are as high as 0.25 – 0.64. It is also found that the texture size on these insect surfaces (100 – 300 nm) are much smaller than those on lotus leaves (5 – 10 μm). In order to understand the fundamental reasons why high solid fraction and nanoscale textures are utilized on these insect superhydrophobic surfaces, we systematically designed and fabricated a series of textured surfaces with well-controlled geometries and investigated their static and dynamic wetting behaviors. It is found that at low solid fraction (i.e. Φ_s < 0.25) the static wettability is insensitive to texture size. However, at high solid fraction (i.e. Φ_s ~ 0.44) we discovered that the contact time (the duration that a bouncing droplet is in contact with the solid) is significantly reduced when the surface textures are in nanoscale regime. Our discovery may provide a new physical insight as to why nanoscopic surface textures with high solid fraction are essential for superhydrophobic insects to thrive in their dynamic environments.
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