Topography versus chemistry - How can we control surface wetting?

Abstract

Hypothesis: Wetting characterization and the production of engineered surfaces showing distinct contact angles or spreading behavior is of major importance for many industrial and scientific applications. As chemical composition plays a major role in the wetting behavior of flat samples, wettability, capillary forces and resulting droplet spreading on anisotropic surface patterns are expected to be highly dependent on surface chemistry as well.

Experiments: To gain understanding of the fundamental principles of the interplay between surface topography and surface chemistry regarding water wettability, anisotropic line patterns were produced on steel samples in a direct laser writing process. Homogeneous surface coatings allowed for a chemical masking of the laser patterns and therewith the identification of the influence of surface chemistry on static contact angles and wetting anisotropy.

Findings: While a carbon coating leads to pronounced wettability and spreading along the topographic anisotropy, an inert gold-palladium coating can fully suppress anisotropic droplet spreading. Model calculations show that an amorphous carbon coating leads to Wenzel wetting while the gold-palladium coating causes air inclusions between the water and the surface in the Cassie-Baxter wetting state. Only in combination with the right chemical composition of the surface, directional patterns show their potential of anisotropic wetting behavior.

Keywords: Cassie-Baxter wetting; Direct laser writing; PVD-coating; Static contact angle; Wenzel wetting; Wetting; Wetting anisotropy.