Collaborative Research: Wrinkling and Folding of Thin Films on Viscoelastic Substrates by Experiments and Modeling

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This award supports fundamental research to develop controllable surface wrinkles and folds on a variety of polymeric materials. Surface properties of a material are important for many applications. The capability to design and control surface texture would open opportunities to realize material surface properties that are otherwise unavailable, such as smart surfaces with tunable adhesion and self-cleaning properties. One viable approach to controllable surface texture is to create surface wrinkles, which can be accomplished easily by mechanically compressing a thin film coating attached to a relatively compliant substrate material. Current understanding in this field is well-developed for situations when both the thin film and the substrate material are elastic. However, many practical applications involve polymeric materials that are not elastic, but either viscous or viscoelastic, with time-dependent mechanical properties. Furthermore, complex surface textures with localized folds may be achieved on polymer surfaces by increasing compression. This research will enable potential applications using wrinkle-textured surfaces for controlling wettability, friction and adhesion, as well as stimulus responsive smart surfaces in energy, healthcare, aerospace, and automotive industries. Therefore, results from this research will benefit the U.S. economy and society. This research involves a collaboration between two institutions and across disciplines including mechanics, material science, and chemical engineering. The interdisciplinary collaboration will help broaden participation in research from diverse background and positively impact engineering education.Past research on wrinkling has focused predominantly on thin films attached to soft elastic substrates. On viscoelastic substrates, however, wrinkling is kinetically-limited, and the wrinkle patterns intrinsically depend on the rate of compression. The primary hypothesis of this research is that the entire spectrum of the wrinkling behavior between the elastic and viscous limits can be exploited by using well-selected model polymeric materials. The research team will perform both computational and experimental studies. A simple and elegant method will be developed to impose well-controlled large compressive deformation on polymer-supported thin films, which allows the effects of strain and strain rate to be examined independently. Numerical simulations based on finite element method will be conducted in conjunction with experiments to understand the underlying mechanisms and to predict formation of wrinkles and folding structures. Beyond the immediate focus on viscoelastic substrates, this project will broadly elucidate the role of dissipative effects in a wide range of wrinkling and folding phenomena observed in nature.