Collaborative Research: Emergent Mechanics of Randomly Packed Elastic Filaments

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When a cardinal builds her iconic cup-nest, she uses her own body as template and molds thin twigs, grass strands, and bark strips into a structure that, despite its softness, reliably holds its shape against various mechanical perturbations. This naturally-selected engineering solution is the result of a subtle interplay between geometry, elasticity, and friction that has not yet been characterized or modeled despite its potential in building, packaging, self-repairing, shock-absorption, and material reusability. The 'bird nest', if defined as a random packing of slender, elastic elements, is an unusual material: it is cohesive without attractive interactions; it is collectively soft and plastic while its elements are hard and elastic. Through coordinated physical and computational experiments, this collaborative project will advance the science of soft granular materials by relating bulk mechanical properties of idealized 'nest systems' with variations in constituents' properties and geometry. Results will generate new knowledge in granular physics, and will appeal to emerging aleatory architecture and engineering paradigms. Indeed, the ability to build through impermanent contacts and design lightweight materials with prescriptive mechanical properties cuts through many areas of high current importance: civil engineering and architectures (reliable, inexpensive, reusable and self-repairing materials), transportation (lightweight composites, shock absorbers), advanced manufacturing. This is in line with the national need of increasing industry competitiveness, which advances the national health, prosperity, and welfare; and secures the national defense. The project also promises to capture the imagination of a broad audience by creating an unusual bridge between relatable protagonists (birds) and often inaccessible fields of physics and engineering. Additionally, STEM outreach activities will be conducted at individual institutions to attract middle school students and female students, respectively, towards science and engineering. Undergraduate students will also be offered positions in either group for exposure to advance engineering research.With increasing aspect ratio, the mechanical behavior of disordered granular packings changes. Where applied stresses distribute in chains of 1D contacts for spheroids, slenderness introduces bending moments and long-range interaction. Impermanent frictional contacts set the system apart from semi-flexible polymer networks and other non-woven materials which derive mechanical response from permanent crosslinks. Experimental evidence from disordered, randomly packed, elastic fibers or filaments based structures, such as the bird nests, suggest that these material systems exhibit frequency-dependent elastoplastic behavior, finite tensile response, and enhanced specific strength. In the absence of a theoretical framework and strictly applicable principles of statistical mechanics, an experimental platform for the benchmarking and physical characterization of these materials will be developed in this project. These will be complemented by a high fidelity computational counterpart to direct a search for novel mechanical states and transitions. The project will provide insights into the relationship between macroscopic and microscopic mechanics of bird nest-like systems, paving the way towards prescriptive design of novel materials. Moreover, it will spur new directions in granular physics theory and explain a functional mechanism from a naturally-selected engineered structure.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.