  The surface of a thin liquid film with nonconstant curvature is unstable, as
the Laplace pressure drives a flow mediated by viscosity. We present the
results of experiments on one of the simplest variable curvature surfaces: a
stepped polymer film. Height profiles are measured as a function of time for a
variety of molecular weights. The evolution of the profiles is shown to be
self-similar. This self-similarity offers a precise measurement of the
capillary velocity by comparison with numerical solutions of the thin film
equation. We also derive a master expression for the time dependence of the
excess free energy as a function of the material properties and film geometry.
The experiment and theory are in excellent agreement and indicate the
effectiveness of stepped polymer films to elucidate nanoscale rheological
properties.
