Rate-dependent creasing of a viscoelastic liquid

The free surface of an elastic material develops sharp cusp-like creases when compressed beyond a critical strain. We show experimentally that a viscoelastic liquid undergoes a similar, but rate-dependent, creasing instability such that the strain required for creasing increases as rate decreases. A model is developed wherein the creasing criterion known previously for neo-Hookean elastic solids is applied to the elastic portion of the deformation of a viscoelastic liquid. Using the upper-convected Maxwell model for viscoelasticity, we derive an analytical criterion for viscoelastic creasing which is in good agreement with experiments. It predicts that the strain for creasing increases with decreasing Weissenberg number, and creasing is not possible below a critical Weissenberg number.