Wrinkling of viscous bubble films*

Mechanical instabilities have recently seen a resurgence in their application towards the design of new complex materials. Even though these instabilities were traditionally considered a route towards failure, they are now being utilized to design structures with advanced functionality. The wrinkling pattern adopted by a thin elastic sheet when subjected to compression, is such a mechanical instability that has been extensively studied and used to design flexible electronics and surfaces with tunable patterns. The wrinkling instability can also be observed in thin viscous sheets. A bubble resting at the surface of a very viscous liquid develops a wrinkling pattern during its collapse. Here we show that the underlying wrinkling dynamics of viscous films are similar to those in elastic sheets. We combine experiments and analytical modeling to show that the number of wrinkles follows a scaling arising from the simultaneous balance of compression, bending and inertia.