The Effect of Patterned Micro-Structure on the Apparent Contact Angle and Three-Dimensional Contact Line

The measurement of the apparent contact angle on structured surfaces is much more difficult to obtain than on smooth surfaces because the pinning of liquid to the roughness has a tremendous influence on the three phase contact line. The results presented here clearly show an apparent contact angle variation along the three phase contact line. Accordingly, not only one value for the apparent contact angle can be provided, but a contact angle distribution or an interval has to be given to characterize the wetting behavior. For measuring the apparent contact angle distribution on regularly structured surfaces, namely micrometric pillars and grooves, an experimental approach is presented and the results are provided. A short introduction into the manufacturing process of such structured surfaces, which is a combination of Direct LASER Writing (DLW) lithography, electroforming and hot embossing shows the high quality standard of the used surfaces

Generalized analysis of the deposition/splashing limit for one- and two-component droplet impacts upon thin films

[EN] Single drop impacts on thin liquid layers are of particular interest because of the ejection of secondary droplets, the so-called splashing. Only a few studies handle the deposition/splashing limit for two-component interaction, where the liquid properties of the impacting drop and wall film differ significantly. This study aims at identifying a unified approach for one- and two-component interactions to determine the deposition/splashing limit. Therefore, a large database of both interactions is considered, which includes data from literature for one-component interactions plus the following binary combinations: hyspin-hexadecane, diesel-hexadecane and diesel-motor oil. Furthermore, a systematic study of two-component interactions with several silicon oils and hexadecane is performed. To map the outcomes, the Ohnesorge number Oh and the Reynolds number Re calculated with arithmetically averaged fluid properties between droplet and wall film fluid are chosen. The dimensionsless film thickness δ is added to form a 3D plot, where one- and two-component experiments are combined. Existing correlations from the literature are revised regarding both interactions and their consistency is checked. The investigated range of high viscosity fluids allow us to propose an improvement of the correlation for high Oh. Our results show that the arithmetically averaged fluid properties lead to a good repartition of both one- and twocomponents interactions toward the deposition/splashing limit. They also corroborate the previous findings that an increase of δ inhibits splashing but its influence is decreasing with increasing Oh.The authors kindly acknowledge the financial support of this work by the Deutsche Forschungsgemeinschaft (DFG) in the frame of the International Research Training Group "Droplet Interaction Technologies" (DROPIT)Bernard, R.; Foltyn, P.; Geppert, A.; Lamanna, G.; Weigand, B. (2017). Generalized analysis of the deposition/splashing limit for one- and two-component droplet impacts upon thin films. En Ilass Europe. 28th european conference on Liquid Atomization and Spray Systems. Editorial Universitat Politècnica de València. 730-737. https://doi.org/10.4995/ILASS2017.2017.4810OCS73073

Air entrapment and bubble formation during droplet impact onto a single cubic pillar

We study the vertical impact of a droplet onto a cubic pillar of comparable size placed on a flat surface, by means of numerical simulations and experiments. Strikingly, during the impact a large volume of air is trapped around the pillar side faces. Impingement upon different positions of the pillar top surface strongly influences the size and the position of the entrapped air. By comparing the droplet morphological changes during the impact from both computations and experiments, we show that the direct numerical simulations, based on the Volume of Fluid method, provide additional and new insight into the droplet dynamics. We elucidate, with the computational results, the three-dimensional air entrapment process as well as the evolution of the entrapped air into bubbles.Deutsche ForschungsgemeinschaftProjekt DEA

Effect of Surface Wettability on the Droplet Impact Morphologies on Dry Smooth Polycarbonate Surfaces

The 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.Fundação para a Ciência e a Tecnologia (FCT), Deutsche Forschungs-gemeinschaft (DFG)info:eu-repo/semantics/publishedVersio

The Effect of Patterned Micro-Structure on the Apparent Contact Angle and Three-Dimensional Contact Line

The measurement of the apparent contact angle on structured surfaces is much more difficult to obtain than on smooth surfaces because the pinning of liquid to the roughness has a tremendous influence on the three phase contact line. The results presented here clearly show an apparent contact angle variation along the three phase contact line. Accordingly, not only one value for the apparent contact angle can be provided, but a contact angle distribution or an interval has to be given to characterize the wetting behavior. For measuring the apparent contact angle distribution on regularly structured surfaces, namely micrometric pillars and grooves, an experimental approach is presented and the results are provided. A short introduction into the manufacturing process of such structured surfaces, which is a combination of Direct LASER Writing (DLW) lithography, electroforming and hot embossing shows the high quality standard of the used surfaces

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.The authors kindly acknowledge the financial support of this work by the Deutsche Forschungsgemeinschaft (DFG) in the frame of the International Research Training Group “Droplet Interaction Technologies” (GRK 2160: DROPIT) under the Project No. 270852890. Additionally, the authors would like to acknowledge the support of Fundação para a Ciência e a Tecnologia (FCT) through the Ph.D. Scholarship No. SFRH/BD/140009/2018 and the Project No. UIDB/50022/2020. The mold fabrication, the surface replication, and the SEM analysis were carried out with the support of the Karlsruhe Nano Micro Facility (KNMF Proposal No. 2016-017-016369, www.knmf.kit.edu) and a Helmholtz Research Infrastructure at Karlsruhe Institute of Technology (KIT, www.kit.edu).info:eu-repo/semantics/publishedVersio

Influence of wetting behavior on the morphology of droplet impacts onto dry smooth surfaces

The 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.Fundação para a Ciência e a Tecnologia e Deutsche Forschungsgemeinschaft (DFG)info:eu-repo/semantics/publishedVersio

The influence of Wettability on the Droplet Impact onto Micro-Structured Surfaces

The 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.Fundação para a Ciênciae a Tecnologia (FCT) and Deutsche Forschungs-gemeinschaft (DFG)info:eu-repo/semantics/publishedVersio

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 patterns

This 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 authors acknowledge the financial support of this work from the Deutsche Forschungsgemeinschaft (DFG) in the frame of the International Research Training Group “Droplet Interaction Technologies” (GRK 2160: DROPIT) under Project No. 270852890. Additionally, the authors acknowledge the support of Fundação para a Ciência e a Tecnologia (FCT) through Ph.D. Scholarship No. SFRH/BD/140009/2018 and Project No. UIDB/50022/2020. Furthermore, the authors would like to thank Johannes Müller (ITLR) for the very fruitful discussions about occurring optical phenomena during the development of the extended LPSMinfo:eu-repo/semantics/publishedVersio