Rapid wetting - influence of surface geometry and fluid properties

Wetting of a liquid on a solid surface is ubiquitous in everyday life, such as morning dew on a plant leaf and raindrops hitting on a glass window. However, this phenomenon is far away from being completely understood. Its dynamics are complicated due to its physics which involves multi-length scales from molecular motions to macroscopic fluid motions as well as numerous parameters such as viscosity, surface tension, contact angles, and so on. Moreover, the situation becomes more complicated in rapid situations - initial rapid wetting and droplet impact. Not only the understanding of contact line physics and wetting on complete surfaces from a theoretical point of view, but surface properties are also of importance in most practical situations. In real life, non-smooth surfaces are found more often than smooth surfaces. Natural organisms take advantage of surface structures to manipulate the wetting behavior. Numerous examples such as lotus leaves, the pitcher plant, water strider, ryegrass and bamboo leaves, and butterfly wings have attracted attention and their properties are replicated by man-made materials and utilized in technical applications such as anti-icing, self-cleaning, drug delivery, water-oil separation, microreactors, and transport in microfluidic channels. However in previous studies on such applications, the detailed mechanisms of how the surface details such as the orientation of the structures and their spacing determine the wetting are scarcely understood. To design surface structures better and optimize for different applications, the understanding of spreading/wetting mechanisms is fundamental. This work intends to elucidate the underlying mechanisms of droplet motion on structured surfaces. The surfaces studied are arrays of inclined microridges and micropillars. The rapid wetting and droplet impact situations are studied through droplet spreading experiments using a high-speed camera and supplemental numerical simulations of comparable droplets. This thesis first reveals the spreading mechanisms on asymmetric microstructures combining experiments and numerical simulations, and presents a theoretical model to predict the influence of the surface details to answer the question - how much is the contact line speed influenced? Together with the microstructures, the influence of additives on the droplet spreading is studied to investigate the influence of complex fluid properties.