Wetting-Induced Elastocapillary Deformation of Supported Thin Rubbery Polymer Films

Whereas classical surface chemistry holds that capillarity controls fluid behavior, recent investigations indicate that it also dominates the mechanics of soft solids at scales below the elastocapillary length (l_e), which is the ratio of surface tension γ of liquid to elastic modulus E of the solid. We used atomic force microscopy to probe elastocapillary deformations induced by droplets of various radii (R) on partially wetting rubbery films of entangled polymers possessing thicknesses down to 230 nm. The transition from elasticity to capillarity dominated deformation with decreasing R values is visualized with high spatial resolution. The elasticity-to-capillarity transition shifted to lower R values, when the film thickness (h) is reduced to a threshold below approximately 10 times of the bulk l_e (l_e,bulk) values, indicative of a smaller l_e on the thin films. This enabled the identification of a thickness-dependent elastocapillary length (l_e,h) ∼ (h³γ/E)^1/4 for soft polymer films on rigid substrates, and, by extension, suggests the scaling l_e,h ∼ h^–n, where n varies with the contrast between the moduli of the films and substrates. The results resolve the foundation of the fluid wetting and interactions with thin, substrate-supported soft films.