ISBN 9783843956048

978-3-8439-5604-8, Reihe Thermodynamik

Patrick Palmetshofer
Droplet impact onto dry and wet structured surfaces of varying size, shape and wettability

209 Seiten, Dissertation Universität Stuttgart (2025), Softcover, A5

Zusammenfassung / Abstract

In this thesis, the influence of varying surface structure shapes and sizes on static wetting and dynamic droplet impacts are investigated experimentally. Spreading shapes, splashing limits and air entrapment processes are discussed. Additionally, the effects of surface structures submerged in a liquid film are investigated with a focus on crown development and splashing limits.

For micrometric ramps, pyramids and staggered cubes, the apparent contact angle is increased over the inherent contact angle on a flat surface and azimuthal variations in the apparent contact angles and contact lines emerge.

During droplet impacts onto micrometric and sub-millimetric structures, the maximum spreading distances align with preferred spreading axes within the structures in the inertial phase of the impact. This creates hexagonal shapes for micrometric cubes on a hexagonal grid and diamond (rhomboid) spreading shapes on the sub-millimeter structures.

The structure-size and wettability can affect the splashing limit: Prompt splashing is well predicted by correlations for hydrophilic surfaces up to a structure size of 0.2 mm, while finger splashing must be considered separately from prompt and crown splashing.

Additionally, silicone oil droplet impacts onto sub-millimetric structures and single millimetric pillars submerged in a liquid film are reported. In these cases, the crown splashing behavior is dominated by the film height above the pillar tops more than the pillar size. Crown streaks are observed due to azimuthal variations in the crown thickness. The most notable effects, however, are observed if the pre-existing liquid film only barely reaches the tops of the pillars. In such cases, new morphologies such as symmetric crown-bottom breakdown. A correlation for the maximum crown height is developed to be applicable to barely and fully submerged flat and structured surfaces.