Collaborative Research: Emergent Mechanics of Randomly Packed Elastic Filaments

当一只红衣雀建造她标志性的杯窝时，她用自己的身体作为模板，将细树枝、草丝和树皮条塑造成一个结构，尽管它很柔软，但却能可靠地保持其形状，抵抗各种机械扰动。这种自然选择的工程解决方案是几何、弹性和摩擦之间微妙相互作用的结果，尽管它在建筑、包装、自我修复、减震和材料可重复使用方面具有潜力，但尚未被表征或建模。“鸟巢”，如果被定义为细长的弹性元素的随机包装，是一种不寻常的材料：它是有凝聚力的，没有吸引力的相互作用；它的整体是柔软的和可塑的，而它的元素是坚硬的和有弹性的。通过协调物理和计算实验，该合作项目将通过将理想化“巢系统”的整体机械性能与成分特性和几何形状的变化联系起来，推进软颗粒材料的科学。结果将产生颗粒物理的新知识，并将吸引新兴的实验室建筑和工程范式。事实上，通过非永久接触建造和设计具有规定机械性能的轻质材料的能力，贯穿了许多当前高度重要的领域：土木工程和建筑（可靠、廉价、可重复使用和自我修复的材料）、运输（轻质复合材料、减震器）、先进制造业。这符合提高产业竞争力的国家需要，从而促进国家的健康、繁荣和福利；巩固国防。该项目还承诺通过在相关的主角（鸟类）和通常难以接近的物理和工程领域之间建立一座不同寻常的桥梁，来吸引广大观众的想象力。此外，还将在个别院校开展STEM外展活动，分别吸引中学生和女生学习理工科。本科生也将在这两个小组中获得职位，以接触先进的工程研究。随着长径比的增大，无序颗粒填料的力学行为发生变化。当施加的应力分布在球体的一维接触链上时，长细会引入弯矩和远距离相互作用。非永久摩擦接触使该系统有别于半柔性聚合物网络和其他非织造材料，后者从永久交联中获得机械响应。来自无序、随机堆积的弹性纤维或长丝结构（如鸟巢）的实验证据表明，这些材料系统表现出频率依赖的弹塑性行为、有限的拉伸响应和增强的比强度。在缺乏理论框架和严格适用的统计力学原理的情况下，本项目将开发一个对这些材料进行基准测试和物理表征的实验平台。这些将辅以高保真的计算对应物，以指导寻找新的机械状态和过渡。该项目将为类似鸟巢系统的宏观和微观力学之间的关系提供见解，为新材料的规定性设计铺平道路。此外，它将刺激颗粒物理理论的新方向，并从自然选择的工程结构中解释功能机制。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.