Cavity growth in soft adhesives.

The growth process of cavities nucleated at the interface between a rigid surface and a soft adhesive layer has been investigated with a probe method. A tensile stress was applied to the highly confined layer resulting in a negative hydrostatic pressure in the layer. The statistics of appearance and rate of growth of cavities as a function of applied negative stress were monitored with a CCD camera. If large germs of cavities were initially present, most of the cavities became optically visible above a critical level of stress independent of layer thickness. Cavities grew simultaneously and at the same expansion rate as a function of applied stress. In the absence of large germs, cavities became optically visible one after another, reaching a limiting size controlled by the thickness of the layer independently and very rapidly. Although, for each sample, we observed a statistical distribution of critical stress levels where a cavity expanded, the mean cavitation stress depended both on surface topography and more surprisingly on layer thickness. We believe that this new and somewhat surprising result can be interpreted with a model for the growth of small germs in finite size layers (J. Dollhofer, A. Chiche, V. Muralidharan et al., Int. J. Solids Struct. 41, 6111 (2004)). This model is mainly based on the dual notion of an energy activated transition from an unexpanded metastable state to an expanded stable state and to the proportionality of the activation energy with the elastic energy stored in the adhesive layer.