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Pattern Formation in Soft Elastic Films and its Role in Adhesion

Manoj K. Chaudhury Department of Chemical Engineering, Lehigh University, Bethlehem, PA 18015

This story is about elastic instability, and its role in adhesive crack propagation in soft confined films. A hydrostatically stressed soft elastic film relieves the imposed constraint by undergoing a morphological instability, the wavelength of which is dictated by the minimization of the surface and the elastic strain energies of the film. The wavelength is entirely dependent on the film thickness provided that the role of the surface tension of the film is negligible in comparison to that of elasticity. When the role of surface tension is significant, the instability exhibits a longer wave feature. With such a film continually subjected to a stress, the morphologicial patterns evolve into cracks, which, in turn provide the length scale needed to relate the fracture stress to the adhesion energy and the elastic modulus. While for a stiffer film, the thickness provides the relevant length scale for fracture leading to the well-known Kendall-Griffith like criterion, the fracture stress is modified non-trivially by an elasto-capillary number for a very compliant film. The elastic interaction that gives rise to instability in thin films can also be used to assemble meso-scale size particles on a thin membrane or a gel. The last part of this presentation will be focused on the role of elasticity in such long range interactions.