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Capillary and Cascading Wrinkling in Thin Polymer F

Thomas P. Russell Polymer Science and Engineering Department, University of Massachusetts

Thin sheets are much more easily bent than stretched by external forces. One might imagine that it is possible to achieve a purely stretched state of a sheet by applying a tension in the plane of the sheet. However, even a sheet subject to purely planar tension will often choose to deform out of plane to form wrinkles. This is an everyday phenomenon that can be seen on our skin as it is stretched by smiling, scars In this work, we report on a study of wrinkling of films under capillary forces, which has thus far remained relatively unexplored. Since thin films are often immersed in fluid environments, both in biological and in synthetic soft materials, the elastic deformation of films under surface tension is a relatively commonplace situation. Thin polymer films form an ideal experimental setting in which to explore wrinkling phenomena : we study films with very high aspect ratios (the ratio of lateral size, D, to thickness h is D/h 5x105) which can be treated accurately in the framework of 2-dimensional elasticity. Our results demonstrate that wrinkling can be used as the basis for a metrology of both the elastic modulus and the thickness of ultrathin films using a very elementary apparatus – no more than a low-magnification microscope and a dish of fluid. Finally, we show that wrinkling patterns can also be used to characterize dynamical relaxation in ultrathin films.