Date of Award
Doctor of Philosophy (PhD)
Crumples, Surface Evolver, Thin films, Wrinkles
Thin films can undergo large amplitude nonlinear deformation even under a small applied force which makes predicting their behavior rather challenging. This dissertation focuses on two phenomena in thin films: the morphological transition from sharp to smooth microstructures in geometrically-confined sheets, and a novel locomotion behavior of a thin floating film placed on an interface with a curvature gradient.
In the first portion of the thesis, we use inflated membranes as model system to understand morphological transitions in confined films. We have developed methods to make air-tight membranes out of sheets of materials with varying Young's modulus and thickness. We have observed that increasing the internal pressure in the membranes causes some sharp-edged diamond-like structures called crumples to form. On further increase in pressure, these crumples transition to smooth periodic wrinkles. We have measured this transition pressure across a wide range of materials and geometries. We collect our data, as well as data from other experiments on interfacial polymer films, in an empirical phase diagram for the transition from wrinkles to crumples. We further study the topography of the crumpling pattern on the surface of the membrane and its relation to d-cones.
Small-scale structures like wrinkles and crumples can enable macroscopic shape change of an entire sheet. In the second part of this thesis, we explore how shape changes of a film (through small-scale wrinkling) can enable a film to feel body forces when placed on a liquid with a non-uniform curvature. This study is motivated by examples from nature: arthropod species that exploit the surface tension of water for aquatic locomotion. Current literature addresses stiff or finitely bendable materials that retain their shape on liquid meniscus where the fluid adjusts its interface to accommodate them via an interplay of surface tension and buoyancy or gravity. We introduce an unexplored setting where thin monotonous film that can easily deform by capillary forces is placed on a positively curved liquid meniscus. A thin film can conform to a curved meniscus by forming small-amplitude wrinkles. Once released, it sets in motion towards the flat center while returning to its symmetrical state. We are able to identify the complex mechanics of propulsion experienced by thin film through numerical simulations using Surface Evolver. Our results give a closer look at the velocity of thin film and energetics of the system and its dependence on the position of the sheet on the interface. Our finding curiously shows that an unstretchable film progresses 10 times faster than the same film with finite stretching indicating the importance of elasticity in such systems.
De, Raj, "Geometric confinement of thin films: From crumple formation to curvature-induced propulsion" (2023). Dissertations - ALL. 1678.