The research objective of this project is to fabricate self-healing microfibers and investigate the impact of their shape and size on their self-healing performance. These microfibers will be embedded into construction material (e.g., asphalt in the proposed project) to form a self-healing composite. When this composite develops a crack, the microfibers will be ruptured to release healing agents that can automatically restore the integrity and toughness of the composite. Novel microfluidic devices will be developed to form and solidify microfibers with healing agents encapsulated in them. The proposed self-healing microfibers are expected to outperform existing spherical microcapsules even when used in lower concentrations. The research project will provide a fabrication platform with unique controllability and flexibility. Therefore, it will enable the researchers to investigate how the shape and size of the microfibers affect their self-healing performance by employing the proposed micromechanical modeling and experimental techniques. As a result, a guideline will be provided for the design and implementation of future self-healing systems.
Successful completion of this project will contribute to the nation?s goal of employing more durable construction material so as to provide safer and more sustainable transportation infrastructure systems. The proposed research holds the promise to benefit engineering structures ranging from microelectronic devices to spacecraft, bridges, and other large infrastructures. It can potentially contribute to the commercialization and broad applications of self-healing materials by improving performance and reducing costs. Research results will be widely disseminated through publications to inspire further investigation and stimulate future interdisciplinary collaboration on self-healing materials. Education and outreach activities, such as undergraduate enterprise and summer youth programs, are also planned for a diverse group of students, including minority and female students.
Self-healing material provides a promising solution to prolong the lifetime, cut the repair expense, and reduce the failure risk for a variety of engineering structures, ranging from microelectronic devices to space craft to infrastructures. A generic self-healing material concept is to employ microcapsules of certain chemicals (healing agents) as the embedment, which can be ruptured upon micro cracks to release the chemical and repair the micro cracks. However, the existing fabrication methods for the microcapsules typically utilize bulk emulsification, which offer very limited control on the morphology and size distribution. In this project, microfluidics has been employed as a means to produce mono-dispersed self-healing microcapsules and catalytic microfibers as the embedments for self-healing composites intended for infrastructure applications. The microfluidic platforms for such fabrication have been established. First, droplet microfluidics has been employed to perform the emulsification and subsequent encapsulation of dicyclopentadiene (DCPD). The resulting microcapsules are very uniform, with typical coefficients of variation (CV) of diameter in the range 1-3, which is monodispersed as typically defined as CV smaller than 5. When using these microcapsules in a self-healing composite, the undamaged fracture toughness has been increased by ~25% as compared with that of the microcapsules fabricated by conventional method. Then, a microfluidic spinning process has been developed to encapsulate Grubb’s catalyst in a novel fibrous morphology, which provides uniform catalyst loading and advantageous catalyst morphology to significantly enhance the utilization of the catalyst, which is the most expensive component of the particular material system utilized in this project. Theoretical analyses have been performed to understand the impacts of particle size distribution and morphology on healing performance. A theoretical frame work is thus established for the controlled design and optimization of composite healing performance. Undamaged composite properties and healing performance was experimentally investigated using a tapered double cantilever beam (TDCB) specimen. The experimental data and theoretical analysis are found to match well with each other. The geometry dependent parameters used for calculations have been carefully re-calibrated. They fall within the theoretical bounds set forth by previous researchers while the value currently used in self-healing literature falls outside of these bounds. The micromechanical model integrated with experimental tests also provided a tool to study the detailed mechanism for increased fracture toughness and to evaluate the healing level based on recovered fracture strength of tested specimens The broader impact of this project is mainly its contribution to the nation's goal of more durable and safer infrastructure systems. The proposed research holds the promise to benefit a wide range of engineering structures and materials. It can potentially contribute to the commercialization and broad applications of self-healing materials by improving performance and reducing costs. Research results will be widely disseminated through publications to inspire further investigation and stimulate future interdisciplinary collaboration on self-healing materials. This research produced 9 journal papers (7 published/accepted, 2 in preparation), 5 conference presentations (4 presented, 1 accepted). It has provided full or partial support for 3 Ph.D. students and 8 undergraduate students. The students have gained important skills on microfabrication, design and operation of microfluidic fabrication platform, sample preparation, material characterization, data acquisition and analysis and technical communications.
