Wetting of geometrical structures in the order of the capillary length

Abstract

Wetting phenomena are often analyzed on the basis of single droplets or microscopic liquid flow phenomena. At industry scale such processes are rare and more often larger objects like wide rolls of material or whole car bodies are coated/wetted. At these scales macroscopic flow phenomena have a major impact on the wetting process. However, to simulate such processes it is necessary to reduce the complexity of the object surface in order to reduce the necessary computational resources. Therefore, the effects of geometrical features in the order of the capillary length on wetting have to be known in order to apply models instead of resolving them numerically. In this thesis the capillary rise in macroscopic right-angled corners is analyzed experimentally in order to capture the main physically relevant effects for large scale wetting. At the beginning of the thesis the experimental equipment and post-processing techniques utilized for the experiments are introduced. This experimental setup allows dipping experiments with objects of a size up to two orders of magnitude above the capillary length to be performed at speeds of 0.01 mm/s to 500 mm/s. These experiments are measured with an optical resolution of approximately 20 μm. After the introduction of the test rig, the new insights into capillary corner rise (also called rivulet rise) are presented. The interaction between concave and convex corners is analyzed by varying the inter-corner distance. This reveals the different length scales at which the concave corner alters the wetting at the convex corner and vice versa. Two models for the static shape of a rivulet are derived. One model combines different existing models from literature and the second model can be applied more generally and with fewer simplifications. The dynamics of rivulet rise are analyzed for three different situations. The change in rivulet rise emerging from concave and convex corners in close proximity to each other is analyzed and described empirically. Since industrial surfaces are produced with macroscopic fabrication methods, the corners are not mathematically sharp, but are microscopically round and the effect of this roundness on the rivulet rise is investigated and theoretically modeled. Since many industrial processes involve moving parts and forced wetting, the influence of forced immersion of a corner on the rivulet rise is also analyzed and described with two complementary models. The thesis concludes with first observations about the influence of roughness on wetting of flat walls. For this topic no models have been developed but different fabrication and measurement techniques have been tested and their characteristics are reported.