CAREER: Ultrathin sheets on curved liquid surfaces: Stress focusing and interfacial assembly

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Nontechnical AbstractA mismatch in curvature between two surfaces is a common problem in nature and industry: flat bandages don't stick as well to curved knuckles or elbows, maps of the earth exaggerate areas near the poles, and automotive metal must be stamped or forged to make a curved fender. The project investigates such "geometric frustration" in a class of extremely bendable materials that are nonetheless hard to stretch, including ultrathin polymer films, textiles, and lightweight inflatable structures. The research investigates how the curvature of a liquid surface can propel thin polymer films, and how it can also wrinkle and crease them. The results will uncover new ways for controlling liquid interfaces by using flexible sheets, going beyond current methods using soap or solid particles. The research is coupled to a set of education and outreach projects that target a wide range of audiences. The project will train two PhD students, who will work with high school students and undergraduates in the lab. High-school teachers will also conduct research internships. A laboratory YouTube channel will be created, and an installation on wrinkling will be developed for the Museum of Science and Technology in downtown Syracuse, NY.Technical AbstractThe project investigates the mechanical and geometrical behaviors of extremely bendable yet nearly inextensible sheets, a class of materials that includes ultrathin polymer films, textiles, and lightweight inflatable structures. First, curvature-driven assembly of thin polymer films on liquid surfaces is investigated. The energy landscape of an ultrathin sheet on a curved topography can be quantified using simulations that harness a newly-developed geometric framework, which allows one to side-step the highly nonlinear sheet equations in favor of a simple geometric minimization. Experiments study the additional role of gravity and probe the dynamics of a sheet that is propelled by a curvature gradient. The results will uncover new methods for controlling and modifying liquid surfaces with thin films, going beyond what can be accomplished with particle and molecular surfactants. Second, stress-focusing transitions are studied for floating polymer films. It is generally unknown how curvature, tension, and confinement conspire to create sharp stress-focusing features out of smooth wrinkles. A "wrinkle-to-crumple" transition is studied for polymer films in several well-controlled setups including: (i) indentation into a liquid bath, (ii) axial compression on a cylindrically-curved meniscus, and (iii) isotropic compression. The work will shed light on pattern formation and symmetry breaking in nonlinear settings.