## Publication 39

*Blister Patterns and Energy Minimization in Compressed Thin Films on Compliant Substrates*

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##### Abstract:

This paper is motivated by the complex blister patterns sometimes seen in thin elastic films on thick, compliant substrates. These patterns are often induced by an elastic mist which compresses the lm. Blistering permits the film to expand locally, reducing the elastic energy of the system. It is therefore natural to ask: what is the minimum elastic energy achievable by blistering on a fixed area fraction of the substrate? This is a variational problem involving both the elastic deformation of the film and substrate and the geometry of the blistered region. It involves three small parameters: the nondimensionalized thickness of the film, the compliance ratio of the film/substrate pair and the mismatch strain. In formulating the problem, we use a small-slope Foppl-von Karman approximation for the elastic energy of the film, and a local approximation for the elastic energy of the substrate.For a 1D version of the problem, we obtain ''matching" upper and lower bounds on the minimum energy, in the sense that both bounds have the same scaling behavior with respect to the small parameters. The upper bound is straightforward and familiar: it is achieved by periodic blistering on a specific length scale. The lower bound is more subtle, since it must be proved without any assumption on the geometry of the blistered region.

For a 2D version of the problem, our results are less complete. Our upper and lower bounds only ''match" in their scaling with respect to the nondimensionalized thickness, not in the dependence on the compliance ratio and the mismatch strain. The lower bound is an easy consequence of our one-dimensional analysis. The upper bound considers a 2D lattice of blisters, and uses ideas from the literature on the folding or ''crumpling" of a confined elastic sheet. Our main 2D result is that in a certain parameter regime, the elastic energy of this lattice is significantly lower than that of a few large blisters.

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