Influence of wetting behavior on the morphology of droplet impacts onto dry-patterned micro-structured surfaces

The influence of surface roughness, especially regularly patterned micro-structures on the physical outcomes of droplet impacts, is far from fully understood. In order to get a deeper insight into the physics of the impact phenomena, a systematic experimental study of the morphology on regularly patterned micro-structured surfaces has been carried out. The used structures with different dimensions were grooves and pillars with a square cross section. With the help of plasma activation and plasma polymerization processes, the surface wettability was modified independently from the surface structure and material. Two different test fluids were used, namely, distilled water and isopropanol, impacting with various impact energies onto the patterned surface samples. For a better characterization of the impact process, high-speed images from three different perspectives have been acquired synchronously. Due to the transparent surface material, the bottom perspective using a total internal reflection configuration was able to visualize air entrapment inside the surface structure. To the authors' knowledge, such images are not available in the literature, yet. The outcomes have been qualitatively investigated, summarized, and compared. A dependency of the outcomes on the impact energy, the surface wettability, and the structure dimensions could be clearly shown. In general, increasing impact energy will promote the tendency of splashing. However, roughness features cannot only trigger splashing, but can also inhibit it, for example, crown splashing. Moreover, reproducible arrangements of air entrapment inside the structure could be found, which was addressed by the authors as “cookie” and “button” due to their appearance.