Boundary Effects on Thin Film Wrinkling Patterns

Wrinkle formation is a phenomenon seen universally in nature and can be examined using a simple bilayer thin film geometry. In our work, wrinkling is initiated by a thermally induced compressive stress in a rigid capping film placed atop a thin liquid film. The thickness of both films affect the wrinkling wavelength, amplitude and shape. Previous experiments have characterized the wrinkling wavelength and amplitude, but the wrinkling patterning has garnered less attention. We focus on the effect of boundaries on the wrinkling patterns. Of specific interest is the effect of a step in the liquid film thickness on the wrinkling morphologies. Since a variation in the thickness of the underlying liquid film affects the wavelength and amplitude of the capping film, the stepped liquid film causes a mismatch in the wavelength at the boundary. Optical and atomic force microscopy are used to analyze two sample geometries: linear and circular stepped boundary conditions.