Analysis of fluid film behaviour using dynamic wetting at a smooth and roughened surface

Abstract

The dynamic wetting technique is described and used to create and aid in analysis of ultrathin (1–5 nm) films on vertically aligned, planar silver substrates of varying microscopic roughness (RMS roughness between 1 and 7 nm). Chlorobenzene and 1,2-dichlorobenzene fluids are applied to hexanethiol monolayer modified Ag substrates. The dynamic wetting approach allows direct investigation of the fluid–solid interface and provides a platform for investigating possible deviations from the hydrodynamic no-slip boundary condition. Surface analysis is carried out by contact angle measurements, vibrational spectroscopy, and ellipsometry. Data describe effects of the surface roughness, surface chemistry, fluid viscosity, and dynamic wetting velocity on the properties of the wetting film. Results indicate that fluid films are not present on very smooth (better than 5 nm RMS) or intermediate roughened substrates (RMS roughness around 7 nm), despite varying surface chemistry and varying wetting velocities. These results provide evidence to support the possibility of molecular slip at solid surfaces.