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- Influence of wetting behavior on the morphology of droplet impacts onto dry smooth surfacesPublication . Foltyn, Patrick; Ribeiro, Daniela; Silva, André; Lamanna, Grazia; Weigand, BernhardThe influence of wettability on the morphology of droplet impacts onto dry surfaces is often neglected in the literature, despite its significant effect on the resulting morphology. In this work, the role of wettability is investigated systematically by considering droplet impact processes on smooth dry surfaces of two different materials. The wetting behavior is varied not only by employing two different fluids, but most importantly by varying the surface properties by plasma activation and polymerization. Overall, this leads to four different wetting behaviors for each surface. The changes in impact morphology are visualized by means of a three-perspective experimental facility. In particular, the bottom view employs a total internal reflection-configuration for visualizing the exact droplet contact area and contact time. This enables us to characterize the main features of the different wetting behaviors. Overall, we found that surface wettability mainly influences the receding phase, resulting in higher receding rates with decreasing wettability but also the maximum spreading diameter.
- Measurement of the lamella thickness during droplet impact onto differently wettable smooth surfaces using an extension of the LASER Pattern Shift Method with naturally occurring patternsPublication . Foltyn, Patrick; Rihm, Lynn Kristin; Ribeiro, Daniela; Silva, André; Weigand, BernhardThis study shows that the LASER Pattern Shift Method (LPSM) is a powerful measurement technique for film thickness measurements. In this paper, the approach of the LPSM is extended, which is now able to measure the lamella thickness during droplet impacts on smooth surfaces using the naturally occurring air bubble shadows. With the help of this rather new measurement technique, the influence of different experimental parameters on the lamella thickness could be systematically assessed, e.g., the influence of impact velocity, liquid properties, and surface wettability. Upon comparing the obtained results to an analytical correlation in the literature, good agreement could be found for its validity range.
- The influence of Wettability on the Droplet Impact onto Micro-Structured SurfacesPublication . Ribeiro, Daniela; Foltyn, Patrick; Silva, André; Lamanna, Grazia; Weigand, BernhardThe flourishing of applications in need of self-cleaning mechanisms increased the search forwater repellent hydrophobic surfaces with induced roughness. Disclosing the small-scale inter-face phenomena on the wetting behavior is essential to design efficient hydrophobic materialswith defined topography. On the other hand, the spreading behavior concerning the forma-tion of thin films on a surface is required to assure the quality of spray cooling and coatings.The contact angle undoubtedly plays an important role in the droplet impact, providing differ-ent outcomes. Moreover, an open question is, how surface topography can affect the impactprocess. Therefore, to evaluate these matters, different surface patterns were manufacturedto assess the surface topography influence on the impact dynamic behavior. Additionally, thewettability of the micro-structured surfaces was flexibly influenced through plasma activationand plasma polymerization. The impact of distilled water and isopropanol droplets on the dif-ferent surface patterns was captured from three perspectives providing high-quality images ofthe phenomena. Different surface morphologies can be obtained depending on the surfacemicro-structures and wettability, affecting spreading shape and evolution. The fluid penetrationwithin the micro-structures is a key feature influencing not only the structures of the outcomesbut also the transition between regimes.
- Effect of Surface Wettability on the Droplet Impact Morphologies on Dry Smooth Polycarbonate SurfacesPublication . Foltyn, Patrick; Ribeiro, Daniela; Silva, André; Lamanna, Grazia; Weigand, BernhardThe surface wettability has a significant influence on the morphology and spreading behaviorduring droplet impacts on dry smooth walls. On the way for predicting spreading diameters independency of the wetting behavior, the experimental database was extended by an experi-mental study in which distilled water and isopropanol droplets have impacted onto dry smoothLexan®(Polycarbonate) surfaces at four different impact velocities. The range of Reynoldsnumbers was set between1,135and12,240and for the Weber number between80and1,165.The surface material, and thus also the characteristic surface roughness, were kept constant,while the wetting behavior was modified using plasma activation and plasma polymerizationprocesses. Different contact angles have been investigated in a range from full wetting to non-wetting for water and from full wetting to partial wetting for isopropanol. The experiments havebeen conducted on a newly designed test rig. High-speed diffuse backlight images at20 kHzfrom the top and the lateral perspective are acquired on a shared CMOS-sensor simultane-ously. A bottom perspective in a total internal reflection configuration is acquiring the footprintof the droplet impact. This enables to better define the maximum spreading diameter and todistinguish between wetted and non-wetted areas.
- Influence of wetting behavior on the morphology of droplet impacts onto dry-patterned micro-structured surfacesPublication . Foltyn, Patrick; Ribeiro, Daniela; Silva, André; Lamanna, Grazia; Weigand, BernhardThe 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.