Direct Measurement of the Surface Tension of a Soft Elastic Hydrogel: Exploration of Elasto-Capillary Instability in Adhesion

An adhesively stressed thin film of a soft hydrogel confined between two rigid flat substrates auto-roughens with its dominant wavelength exhibiting pronounced dependence on the film thickness (H). A linear stability analysis confirmed that this long wavelength instability (~7H) is due to an elasto-capillary effect, the implementation of which required direct measurements of the surface tension and the elasticity of the gel. The surface tension of the gel was estimated from the fundamental spherical harmonic of a hemispherical cap of the gel that was excited by an external noise. The shear modulus of the gel was determined from its resonant shear mode in a confined geometry. During the course of this study, it was found that a high density steel ball submerges itself inside the gel by balancing its excess weight with the accumulated strain induced elastic force that allows another estimation of its elastic modulus. The large ratio (1.8 mm) of the surface tension to its elasticity ascertains the role of elasto-capillarity in the adhesion induced pattern formation with such gels. Experimental results are in accord with a linear stability analysis that predicts that the rescaled wavelength is linear with H, which modifies the conventional stress to pull-off a rigid flat object from a very soft film by a multiplicative factor. The analysis also suggests some new results related to the role of the finite dilation of a material in interfacial pattern formation that may have non-trivial consequences in the adhesive delamination of very thin and/or soft elastic films via self-generated cracks.