Nonuniform Receding Dynamics of Thin Viscous Oil Films on Roughened Surfaces

Liquids in contact with chemically and physically heterogeneous surfaces are ubiquitously encountered, yet little is understood about how those surface heterogeneities affect the dewetting dynamics of thin liquid films. Variations in the local contact line mobility cause nonuniform dewetting, and our working hypothesis was to decouple those heterogeneities to explain the role of each on the local contact line mobility of a viscous liquid. Thin liquid film recession was measured on roughened surfaces that were either clean or contaminated by clustered macromolecules. Atomic force microscopy was used to characterize the surface features (peaks and troughs) before and after droplet dewetting. Surfaces were initially wetted by the highly viscous liquid in air before flooding with water. On roughened uncontaminated surfaces, a pattern of smaller micron-sized droplets dispersed around the receded primary droplet remained on the surface. On chemically contaminated surfaces those micron-sized droplets were only seen on the roughest surfaces, and on smoother surfaces, secondary daughter droplets were formed but were much larger and less in number. The larger droplets formed due to the chemical heterogeneity. The smaller micron-sized droplets formed due to variations in the local contact line velocity which occurred at surface pits due to the changing apparent contact angle. Hence, the size of daughter droplets corresponded to the size of the surface pits. For roughened surfaces, the effect of chemical heterogeneity was less significant than the physical roughness which had more of an influence on modulating the local mobility of the receding contact line.