EPJ E Colloquium: Adhesion induced pattern formation in constrained soft films

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Published on 31 July 2015

A hydrostatically-stressed soft elastic film responds by developing a morphological instability, the wavelength of which is dictated by minimisation of the surface and elastic strain energies of the film. For a single film, the wavelength of this transition is entirely dependent on the film's thickness, however in the case of two contacting films a co-operative energy minimisation dictates that the wavelength depends on both the elastic moduli and the thicknesses of the two films.

In addition, the wavelength can depend on the material properties of the film if its surface tension has a pronounced effect when compared to its elasticity. When such a confined film is subjected to a continually-increasing normal displacement, the morphological patterns evolve into cracks which, in turn, govern the adhesive fracture behaviour of the interface. While, in general, the film thickness provides the relevant length scale underlying the well-known Griffith-Kendall criterion of debonding of a rigid disc from a confined film, it is modified non-trivially by the elasto-capillary number for an ultra-soft film.

Depending on the degree of confinement and the spatial distribution of the external stress, various analogues of the canonical instability patterns found in liquid systems can be reproduced in confined thin elastic films.