Static and Dynamic Wetting on Bio-inspired Compact Nano-Textured Surfaces

Open Access
- Author:
- Wang, Lin
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- September 28, 2020
- Committee Members:
- Tak Sing Wong, Dissertation Advisor/Co-Advisor
Tak Sing Wong, Committee Chair/Co-Chair
Clive A Randall, Committee Member
Long-Qing Chen, Committee Member
Pak Kin Wong, Outside Member
John C Mauro, Program Head/Chair - Keywords:
- hydrophobic surface
bio-inspired
water-repellent insects
high solid fraction
contact time
bouncing droplets
reentrant pillars
hoodoo pillars
pressure stability - Abstract:
- Many natural surfaces are capable of shedding water droplets rapidly, which has been attributed to the presence of extremely low solid fraction (Φs ~ 0.01) according to the classical wetting theories. However, recent high-resolution microscopic observations revealed the presence of unusual high solid fraction nanoscale textures on water-repellent insect surfaces. For example, superhydrophobic mosquito eyes, springtails, and cicada wings possess solid fractions Φs as high as 0.25 – 0.64. In addition, the texture size on these insect surfaces is typically on the order of 100 – 300 nm. The objectives of this dissertation are to understand why both high solid fraction and nanoscale textures are important for these superhydrophobic insect surfaces. To achieve these objectives, we systematically designed and fabricated a series of textured surfaces with texture size varying from 100 nm to 30 µm at solid fractions of 0.25 and 0.44, and investigated their static and dynamic wetting behaviors through the use of contact angle goniometry and high-speed optical imaging. Here we show that the contact time of bouncing droplets on high solid fraction surfaces can be reduced by reducing the texture size to nanometer scale. Specifically, we discovered that high solid fraction surfaces (Φs ~ 0.44) with texture size of ~100 nm could reduce the contact time by ~2.6 ms compared to that with texture size of >300 nm. The amount of time reduction compensates about 36.6% of the increased contact time caused by the increased solid fraction from ~0.01 to ~0.44. This texture-size dependent contact time reduction on solid surfaces has not been observed previously, and cannot be explained by existing surface wetting theories. We showed theoretically that the reduction in droplet contact time could be attributed to the dominance of three-phase contact line tension on compact nanoscale textures. Through pressure stability analysis and experiments, we have further shown that high solid fraction (Φs > 0.25) is an important requirement for insects to withstand high-speed impacting raindrops. Our results suggest that the compact and nanoscale textures on water repellent insect surfaces may work synergistically to repel and shed impacting raindrops rapidly, which could be an essential survival strategy for flying insects. Technologically, the ability of compact nanoscale textured materials to repel high-speed impact of liquid droplets with reduced contact time may find use in a range of applications, including fouling-resistant personal protective equipment (PPE), insect-sized flying robots, and miniaturized drones.