Bi-Gaussian stratified theory to understand wettability on rough topographies

Surface topography serves as the fingerprint of a machine element. Most researchers understood topographies from a single-stratum perspective, while some have started to concern a bi-Gaussian stratified viewpoint. In this paper, a two-stage abrasive manufacturing process was employed to produce designed bi-Gaussian stratified topographies on hydrophilic steel discs, and a chemical vapor deposition of 1H,1H,2H,2H-perfluorooctyltrichlorosilane was conducted to form a hydrophobic self-assembled monolayer on each disc. By carefully measuring the static contact angles on these hydrophilic and hydrophobic surfaces, for the first time we were able to provide insights into the mechanism by which bi-Gaussian stratified topographies affect the wettability. Wetting behaviors on hydrophilic surfaces can be well explained by the Wenzel model from a bi-Gaussian stratified viewpoint. Rough components and a small proportion of the upper component can contribute to a small contact angle because of the increased surface-area extension. However, wetting behaviors on hydrophobic surfaces cannot be completely answered by the Cassie model, indicating another significant influence factor, i.e., autocorrelation lengths.