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Wrinkling Of Floating Thin Polymer Films

This thesis presents an extensive study of wrinkling of thin polystyrene films, tens of nanometers in thickness, floating on the surface of water or water modified with surfactant. First, we study the wrinkling of floating thin polystyrene films under a capillary force exerted by a drop of water placed on its surface. The wrinkling pattern is characterized by the number and length of wrinkles. A metrology for measuring the elasticity and thickness of ultrathin films is constructed by combining the scaling relations that are developed for the length of the wrinkles with those for the number of wrinkles. This metrology is validated on polymer films modified by plasticizer. While the polystyrene films are modified with a large mass fraction of plasticizer, the relaxation of the wrinkles is observed and characterized, which affords a simple method to study the viscoelastic response of ultrathin films. Casting air bubbles beneath the films instead of placing water drops on the films, we observe the film inside the contact line is slightly deformed out of plane and there are hierarchical wrinkling patterns around both sides of the contact line. Second, we construct a metrology to measure the strength of the interaction between two localized wrinkle patterns induced by placing two drops of water on a floating thin polymer film. Third, we study the wrinkling of a floating thin polymer film due to a point force exerted on its surface. Wrinkling occurs in the film only when the pushing depth reaches a critical value. The threshold is measured and is consistent with theoretical prediction. Finally, we study the behavior of incompressible, rectangular films floating on liquid and pushed inwards along two opposite edges. Far from the uncompressed edges the membranes buckle along the force direction, developing a periodic pattern of wrinkles. Approaching the uncompressed edges, the coarse pattern in the bulk is matched to fine structures by a smooth evolution to higher wave numbers. We show how the observed multi-scale morphology is controlled by a dimensionless parameter that quantifies the relative strength of the edge forces and the rigidity of the bulk patterns.
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