8:00 PM - SB11.03.20
Reduction in Contact Time of Bouncing Droplets on Dense Nanostructured Superhydrophobic Surfaces
Lin Wang1,Tak-Sing Wong1
The Pennsylvania State University, University Park1
Many plants and insects have developed intriguing wetting properties that enable them to thrive in their natural habitats [1, 2]. Lotus leaf, which is one of the most well-known biomimetic examples, has served as the blueprint for designing superhydrophobic surfaces for over two decades. Specifically, the water repellency of lotus leaves mainly stems from the hydrophobic epicuticular wax coating and low fraction of solid surface (Φs) that is in direct contact with water . As shown by the classical Cassie-Baxter equation (1944), superhydrophobicity can be achieved by a surface with solid fraction (Φs) less than 0.05 . However, some insect surfaces exhibit exceptional water repellent characteristics at solid fractions (Φs) much greater than 0.05. For example, superhydrophobic mosquito eyes, springtails, and cicada wings possess solid fractions (Φs) as high as 0.25 – 0.64 [4 – 6]. In addition, the texture size on these insect surfaces is typically on the order of 100 – 300 nm. To understand why both high solid fraction and nanoscale textures are important on these superhydrophobic insect surfaces, we systematically designed and fabricated a series of synthetic textured surfaces with feature size ranging from 100 nm to 30 µm with solid fractions (Φs) of 0.25 and 0.44, and investigated their static and dynamic wetting behaviors. We discover that nanoscopic textures (i.e., ~100 nm) at high solid fraction (i.e., Φs ~ 0.44) enable reduced droplet contact time (the duration that an impacting droplet is in contact with the solid) by as much as ~2.6 ms, which is ~14% of contact time reduction as compared to those of microscopic counterparts. The amount of contact time reduction is significant, as it is comparable to the timescale for a mosquito to escape from a lethal raindrop collision . Detailed analysis of the results will be presented in the meeting. Our discovery may provide a new physical insight to the design of new superhydrophobic materials for highly dynamic environments.
Keywords: nanostructures | insects | superhydrophobic surfaces | drop impact | contact time
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