Wetting-Induced\nElastocapillary Deformation of Supported\nThin Rubbery Polymer Films

Whereas classical surface chemistry holds that capillarity\ncontrols\nfluid behavior, recent investigations indicate that it also dominates\nthe mechanics of soft solids at scales below the elastocapillary length\n(<i>l</i><sub>e</sub>), which is the ratio of surface tension\nγ of liquid to elastic modulus <i>E</i> of the solid.\nWe used atomic force microscopy to probe elastocapillary deformations\ninduced by droplets of various radii (<i>R</i>) on partially\nwetting rubbery films of entangled polymers possessing thicknesses\ndown to 230 nm. The transition from elasticity to capillarity dominated\ndeformation with decreasing <i>R</i> values is visualized\nwith high spatial resolution. The elasticity-to-capillarity transition\nshifted to lower <i>R</i> values, when the film thickness\n(<i>h</i>) is reduced to a threshold below approximately\n10 times of the bulk <i>l</i><sub>e</sub> (<i>l</i><sub>e,bulk</sub>) values, indicative of a smaller <i>l</i><sub>e</sub> on the thin films. This enabled the identification of\na thickness-dependent elastocapillary length (<i>l</i><sub>e,<i>h</i></sub>) ∼ (<i>h</i><sup>3</sup>γ/<i>E</i>)<sup>1/4</sup> for soft polymer films\non rigid substrates, and, by extension, suggests the scaling <i>l</i><sub>e,<i>h</i></sub> <i>∼ h</i><sup>–n</sup>, where <i>n</i> varies with the contrast\nbetween the moduli of the films and substrates. The results resolve\nthe foundation of the fluid wetting and interactions with thin, substrate-supported\nsoft films.