  When a bubble of air rises to the top of a highly viscous liquid, it forms a
dome-shaped protuberance on the free surface. Unlike a soap bubble, it bursts
so slowly as to collapse under its own weight simultaneously, and folds into a
striking wavy structure. This rippling effect occurs in fact for both elastic
and viscous sheets, and a theory for its onset is formulated. The growth of the
corrugation is governed by the competition between gravitational and bending
(shearing) forces and is exhibited for a range of densities, stiffnesses
(viscosities), and sizes -- a result which arises less from dynamics than from
geometry, suggesting a wide validity. A quantitative expression for the number
of ripples is presented, together with experimental results which are in
agreement with the theoretical predictions.
