Nanoscale Surface Texture To Reduce Bouncing Droplet Contact Time

nanoscale surface textures.
Examples of water-repellent insects equipped with high solid fraction nanoscale surface textures.

Nature continues to shed light on the amazing and wide range of mechanisms employed in the world of plants and animals. Nanoscale (1 x 10 -7 or 3.937 x 10-8 inches) textured surfaces help insects to survive in any number of harsh and unfriendly environments. Nanoscale surfaces play diverse roles in nature. They provide mosquitoes’ eyes with anti-fogging properties, moth eyes with anti-reflection properties, and provide cicada with the ability to self-clean.

The relationship between surface texture and water-repellent functionality was noted by Cassie and Baxter in the paper “Large Contact Angles of Plant and Animal Surfaces,” published already in 1945. Adam and Wenzel had previously demonstrated that rough surfaces provide a greater apparent contact angle than the true contact angle of materials with smooth surfaces when the true contact angle is greater to 90 degrees. Cassie and Baxter took this theory further and applied it to porous surfaces as well as surfaces that are so rough that a great deal of air is entrapped where the water and solid meet.

The concept of high solid fraction nanoscale surface textures, providing a basis for designing water-repellent surfaces, has received further scientific investigation. A research team, including scientists in the fields of biomedical engineering, materials science, and mechanical engineering from the Pennsylvania State University, led by Lin Wang demonstrated how high solid fraction surfaces consisting of a surface texture size smaller than 100 nanometers reduced the contact time of bouncing liquid droplets.

The research team discovered that when surfaces with a high solid fraction have a texture size smaller than 100 nanometers, the contact time is reduced by approximately 2.6 milliseconds when compared to surfaces with a texture size greater than 300 nanometers. The research team investigated the relationship between texture size and liquid-solid interactions by creating textured surfaces resembling those found on insects (inspired by biology), and covering the surfaces with a silane monolayer in order to create a hydrophobic, or water-hating, surface. Various experiments examined the relationship between the contact time of bouncing water drops and the nature of the texture size of the surface.

Experiments demonstrated that when a liquid drop makes contact with a textured surface, it spreads out to its maximum diameter before retracting from the surface. The retraction process is analogous with a spring, and therefore the concept of the spring constant helps explain the process. The spring constant of the retracting droplet is strongly affected by the liquid-air interfacial tension, and the liquid-solid interaction has a negligible effect and can be ignored. However, when droplets hit high solid fraction textured surfaces with a solid fraction measuring 0.44 there is additional energy in the retraction process as a result of the three-phase contact lines that form beneath the droplet. Consequently, the liquid-solid interaction can no longer be dismissed. When hitting a high solid fraction with a nanoscale textured surface multiple three-phase contact lines form under the droplet, and the resulting additional energy affects the retraction speed of the droplet.

Compact nanoscale textures reduce contact time of bouncing ...
Comparison of contact time of bouncing water droplets on textured surfaces. (A) Time-lapse images of bouncing water droplets (diameter d0 ~ 2.3 mm, Weber number We ~ 31.6) on surfaces with solid fraction Φs = 0.44. The droplet detached faster from ~100 nm textures than the one from ~300 nm textures. D denotes the texture cap size of each re-entrant pillar, and tc denotes contact time. (B) Identical drop impact experiments on surfaces with solid fraction Φs = 0.25. Droplets detached simultaneously from both surfaces. Insets showing the SEM images of fabricated nanoscale re-entrant textures. Scale bars in all SEM images, 200 nm; scale bar in the optical image, 1mm. Credit: Science Advances, doi: 10.1126/sciadv.abb2307
Droplet Contact Times
Comparison of contact times of bouncing water droplets on surfaces with different texture size at solid fraction

Additional experiments examined the motion, or kinematics, of the droplets as they spread and then retracted when bouncing off a textured surface. Surfaces with a solid fraction of 0.44 allow the droplets to retract without any delay. However, the water droplet retraction time increases on surfaces with solid fractions higher than 0.44. It appears that the minimum possible contact time is reached on textured surfaces with a 0.44 solid fraction.

Further experiments looked at the effect the pressure stability of the textured surface has on the water droplets as they make contact with the surface. Two types of impact pressures were investigated, the water hammer pressure occurring at the surface of the liquid and solid first come into contact, and the dynamic pressure happening as the water droplet spreads after contact. These experiments demonstrated the importance of a high solid fraction to survive the impact pressure of water droplets in order to quickly sluff them off.

The article “Compact Nanoscale Textures Reduce Contact Time of Bouncing Droplets” in Science Advances (https://advances.sciencemag.org/content/6/29/eabb2307) the authors Lin Want, Ruoxi Wang, Jing Wang, and Tak-Sing Wong discuss how their study demonstrates that using nanoscale textures on surfaces with a high solid fraction decreases the water contact angle hysteresis, resulting in a decrease in contact time of bouncing water droplets.

The authors feel that the results of the study can have far-reaching effects in a variety of fields. In the meantime, they plan to further test their theory to see if it will accurately predict the shortest contact time of bouncing water droplets on surfaces with textures less than 100 nanometers and a high solid fraction greater than 0.25.

Further information: Lin Wang et al. Compact nanoscale textures reduce contact time of bouncing droplets, Science Advances (2020). DOI: 10.1126/sciadv.abb2307

A. B. D. CASSIE et al. Large Contact Angles of Plant and Animal Surfaces, Nature (2008). DOI: 10.1038/155021a0

Yahua Liu et al. Pancake bouncing on superhydrophobic surfaces, Nature Physics (2014). DOI: 10.1038/nphys2980

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