Fingering Instability During Debonding: From a Viscous Liquid to a Soft Elastic Solid

We investigate the fingering instability during debonding of a confined viscoelastic layer in a circular lifted Hele-Shaw cell.\footnote{J. Nase, A. Lindner, C. Creton, PRL \textbf{101}, 074503 (2008)} We use PDMS with different degrees of crosslinking, ensuring a continuous transition from a viscous liquid to a soft elastic solid. During debonding, a fingering instability with characteristic initial wavelength $\lambda$ evolves. When going from a liquid to a solid, we observe a transition from bulk to interfacial mechanisms. We predict this transition from linear viscoelastic and surface properties. We show that for the interfacial mechanism, $\lambda$ depends solely on the film thickness, whereas for the bulk mechanism, $\lambda$ depends on the material parameters. $lambda$ is in both cases in quantitative agreement with linear stability analysis. For a Newtonian oil, we discuss in detail the coarsening of the pattern during debonding. Adapting a recent 3D technique, we visualize for the first time in situ the contact line between viscoelastic material and air in three dimensions, providing direct access to the boundary conditions.