Wetting dynamics on multilevel structures

Abstract

Natural surfaces such as lotus leaves show a behavior known as water repellency. It happens due to their micro- and nanoscale surface roughness. Inspired by these natural designs, a model surface of pillars can be fabricated. These designs are studied to understand how surface geometry affects wetting and droplet dynamics. Although pillar surfaces with uniform heights have been studied, there are few studies on the combined effects of pillar spacing and height differences on drop impact. Uniform-height pillar structures have been studied well because it was convenient to fabricate and allowed a basic understanding of the fundamental physical mechanisms. However, the natural surfaces are not uniform as an array of pillars. In this work, the surface design is made complex in a step-by-step manner. This approach builds understanding of physics established by the previous work in the literature. This master’s thesis investigated drop behaviour on samples with two different pillar heights within a single sample. The surfaces were fabricated from polydimethylsiloxane (PDMS) with two different coatings. Two sets of samples were studied representing microscale and nano-microscale roughness. High-speed imaging was used to capture the impact events. The phenomena observed from these events were lateral spread, jet formation and complete bounce. These events were observed at different impact heights between 5mm and 40mm. The experimentation showed that the drop impact behavior depends on pillar spacing and the number of short pillars. For microscale roughness, the impact produced jets formed by rapid air cavity collapse. The jet peak position and jet speed increased with an increase in impact height and reaching up to about 6 –7 m/s. Smaller pillar spacing (50 μm) showed nearly constant jet behavior across all samples, whereas larger spacing (150 μm) showed that adding short pillars enhanced air cavity collapse and produced taller jets. For nano-microscale roughness, three impact regimes were observed: complete bouncing, partial bouncing, and impalement. Energy dissipation increased with impact height in all samples. However, surfaces with short pillars dissipated less energy. The lateral spread increased with impact height but was not affected by short pillars at small spacing. Overall, increasing the number of short pillars improved jet formation and reduced energy loss.