Collaborative Research: Revealing Strengthening and Toughening Mechanisms in Coconut Endocarp through Integrated Multiscale Modeling and Characterization

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Non-Technical Summary:The hard shell of the coconut, called endocarp, is a lightweight material with impressive strength, toughness, and hardness. As with many biological materials, this outstanding behavior is due to a highly complex structure. When studied at increasing magnifications, the endocarp reveals different structures at each magnification level. At the largest level, a porous network can be seen, consisting of bundles of hollow channels. Larger magnifications reveal a graded cellular structure, where larger cells are found toward the inside of the coconut, and smaller cells toward the outside. The cells themselves feature walls consisting of many layers, and each of these layers consists of tiny fibrils. Understanding comprehensively how all of these elements work together to make the coconut so strong and tough is a significant challenge, especially because of their disparity in size. This project will develop novel computer simulation techniques with the capability of treating these different elements simultaneously at the relevant sizes. This project will also develop new experimental techniques to measure and visualize directly how the different elements inside the coconut endocarp interact, to test and calibrate the computer models. This integrated computational and experimental approach will provide unprecedented insights into how the coconut’s structure gives rise to its outstanding performance. These insights and methods can then be used to engineer coconut-inspired lightweight applications that are strong and tough, for instance to improve helmets. This project will provide research opportunities to undergraduate students. For instance, computational and experimental training series will be offered to undergraduate students during the summer. Underrepresented students including female and minority students will participate in this research project. This project will also provide opportunities to students with disabilities to work on computational modeling remotely. Presentations and seminar talks will be offered to middle and high school students to attract them to participate into biomaterial research. Technical Summary:Coconut endocarp is substantially stronger and stiffer than wood, despite sharing the same major ingredients: cellulose, hemi-cellulose, and lignin. The key to this impressive mechanical performance is a sophisticated structure with many levels of structural hierarchy between the molecular scale and the macroscale. This project’s goal is to develop a rigorous understanding of the endocarp’s structure/property relationships by means of a multi-scale effort integrating novel computational and experimental techniques. A concurrent atomic-continuum (CAC) computational tool will be developed to span all relevant length scales of this materials system naturally. This approach will overcome limitations of current computational approaches, where different length scales are treated with conceptually different models that need to be interfaced. The CAC approach will be used on hierarchical materials for the first time, representing a game changing development for the materials sciences. The experimental efforts will mirror the computational work and provide characterization across all length scales. Scanning probe techniques will play a crucial role in characterizing not only the structure and mechanical properties of nano- and microscale constituents of the endocarp, but also their interfacial interactions. The outcome will be a powerful model that is calibrated and verified across multiple length scales. This model can serve as a basis to establish guidelines for bottom-up hierarchical design of synthetic cellular lightweight materials with outstanding mechanical performance, inspired by the coconut endocarp.This project is jointly funded by the Biomaterials progam (BMAT) in the division of materials research (DMR) and the Established Program to Stimulate Competitive Research (EPSCoR).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.