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 LASER lithography, electroforming and hot embossing shows the high quality standard of the used surfaces.

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. Keywords droplet impact, thin film, two-components interaction, splashing treshold. IntroductionSingle drop impact on a thin liquid layer (i.e. wall film) covers a wide range of industrial applications like coating, printing, cooling or combustion. They are of particular interest since they lead to the ejection of secondary droplets, the so-called splashing.The investigations concerning the droplet impact on wetted walls show that for a given Reynolds number (Re), Ohnesorge number (Oh) and dimensionsless wall film thickness (δ, film thickness h over droplet diameter D), the outcome of one-component droplet impact can be determined. Roughness of the wall can be neglected in the case of 0.1 ≤ δ ≤ 1 [9], when it is completely embedded in the liquid wall film. However, drop-film interactions are affected by the use of different fluids for the droplet and the liquid film. Very little is known for this two-component interaction: the available database is still sparse and therefore the validity of proposed correlations for the deposition/splashing limit cannot be yet generalized to any type of fluid combinations. In this study, we consider a droplet impact of Newtonian fluids onto a wetted, cold, horizontal, solid surface. In the following, the denomination fluid1/fluid2 means that a fluid1 droplet impacts onto a fluid2 wall film and fluid1-fluid2 that both permutations were performed. This study focuses on ...

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.

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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.