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An in situ observation method to understand thermal Marangoni convection in sessile droplets is reported in this paper. Infrared (IR) thermal imaging is applied in the experiments to visualize the transient temperature distribution on the surface of the droplet. Additionally, micro particle image velocimetry (micro-PIV) sys-tem illustrates the fluid field inside the droplet. Two kinds of droplets are considered in this paper: sessile droplets heated with a hotplate and droplets heated by an electrode at the apex. Convection in both droplets is observed in situ, and the experimental results are in accord with the theory of fluid mechanics. [ABSTRACT FROM AUTHOR]

An analysis based on the energy conservation is presented for the self-propelled droplet during coalescence of two droplets of the same size over a superhydrophobic rough surface. The self-propelled behavior occurs only for the coalescence of droplets with a certain range of radius. An analytical relation is established among the coalescence-induced velocity, surface energy, viscous dissipation, and droplet size if gravity is negligible. The coalescence-induced velocity increases with increasing droplet size to a maximum and then decreases with the size, which is in good accord with the experimental observation reported in the literature. [ABSTRACT FROM AUTHOR]

Understanding the role of pinning force in droplet dynamic wetting is of critical importance for surface science studies. Generally, the pinning force is only related to the surface tension and the change of contact angle. However, there is an obvious correlation between the pinning force and the surface geometry. In this paper, the relation between the surface geometry and contact line pinning was studied with systematical experiments and theoretical analysis. We compared the samples with different edge angles and carried out plenty experiments with different liquids. Meanwhile, the theoretical analysis and molecular simulation were carried out. The results show that the sharp edge has a strong pinning effect on the contact line and can significantly change the contact angle and wetting state of droplets. The maximum contact angle of droplet has a linear relation with the edge angle of substrate. The formula of pinning force was revised to consider the impact of surface topography. According to the relationship between surface defect and contact line, we proposed a model to classify the cases of contact line pinning for the first time. Our research will deepen the understanding of contact line pinning and provide help for potentially industrial production designs. [ABSTRACT FROM AUTHOR]

Interface evolution of a droplet on a dissolvable substrate is controlled by coupling of wetting and dissolution, which makes dissolutive wetting more complicated than wetting on inert substrate. In our experiments, xylene and polystyrene are used as solvent and solute, respectively. We record the continuous evolution of interfacial shape and measure change of geometrical parameters. Then, a model is built based on the Noyes-Whitney equation. The evolution mechanisms of interface and solute concentration under the combined actions of wetting and dissolution are analyzed. There are two stages in dissolutive wetting. The two stages are identified by the speed ratio of wetting and dissolution, which reflects the coupling between wetting and dissolution. The model can recover the evolution of the interfacial shape observed in the experiment. Our work may help to predict change of interfacial shape in dissolutive wetting, and the results provide a better understanding of dissolutive wetting which is a process that occurs in many important application areas. [ABSTRACT FROM AUTHOR]