A Wetting Framework for Vapor-Adsorbed Films: Interfacial Energies, Cloaking, Swelling, and Droplet Mobility

Modification of the vapor phase can dynamically alter interfacial energies, wetting, and droplet motion. We investigate the influence of vapor adsorption on the interfacial properties and droplet mobility of polymer brushes, self-assembled layers, and bare silicon. Ellipsometry and quartz crystal microbalance with dissipation experiments show that polydimethylsiloxane (PDMS) brush thickness increases by ∼45% in n-hexane vapor, whereas a negligible increase is observed for all other surfaces, indicating that PDMS brushes swell in n-hexane vapor. Despite this, we find that vapor-induced changes in interfacial energies dominate the wetting response; swelling actually hinders droplet mobility. Across all nonfluorinated surfaces, alkane vapors adsorb on the solid interface to form ultrathin liquid films that also cloak water droplets. Water droplets that are pinned or move at ≤0.2 mm/s when in air reach velocities above 30 mm/s on PDMS brushes and alkylsilane coatings when in n-pentane and n-hexane vapors. In contrast, a continuous lubricating alkane film does not form on fluorinated polymer brushes, and consequently, droplet velocities remain low (<0.1 mm/s) in all vapor environments. These results identify vapor-induced interfacial energy modification, rather than polymer swelling, as the key mechanism enabling tunable droplet mobility on smooth coatings.