Energy absorption materials and structures have been extensively used for protecting personnel and infrastructure against collisions, explosions, and impacts. Current existing energy-absorbing materials and structures, such as foams, honeycombs, composites, and thin-walled structures, possess attractive physical and mechanical properties including extreme lightweight, large deformability and easy fabrication. They play a crucial role in enhancing safety measures across various sectors, including transportation, construction, and defense. However, the performance of these materials and structures is susceptible to structural defects, which act as the weakest chain of the system and weaken the overall efficiency. Multiple materials and structures, such as the foam-filled thin-walled tubes, have been combined to further enhance the energy absorption performance. Nevertheless, the bonding strength between dissimilar materials often limits this improvement, as debonding occurs between filler and tube wall under large deformation. Moreover, current solid-based materials lack biocompatibility, preventing direct application for protecting biological tissues. To address above issues, compressible fluids, specifically liquid nanofoam (LN) and suspension hollow glass microsphere (SHGM) have been developed. The LN system, comprising a liquid phase and hydrophobic nanoporous particles, utilizes pressurized liquid flow in nano-channels as its energy absorption mechanism. The SHGM system, consisting of a liquid phase and hollow glass microspheres, dissipates substantial energy as particles collapse within the pressurized liquid surrounding them. The central hypothesis of this study is that not only the high energy absorption efficiency of compressible fluids, but also the strong interaction between these fluids and the surrounding structures can significantly enhance the energy absorption performance. To test this hypothesis, this dissertation presents a systematic experimental study on the interaction between compressible fluids and three different types of structures, specifically structures without imperfections, structures with imperfections, and biostructures. The interaction between compressible fluids and perfect thin-walled tubes has been quantified by an extraordinary strengthening coefficient of 4.3, surpassing achievements by existing solid foam fillers. In addition, compressible fluids are able to mitigate the negative effects of structural imperfections, as the energy absorption capacity of thin-walled tubes filled with compressible fluids is inert from the presence of structural defects. Furthermore, it has been demonstrated that compressible fluids, specifically LN, exhibit excellent biocompatibility and energy absorption efficiency, allowing them to directly interact with biotissues and effectively protect cells and organs from mechanical abuse. In conclusion, compressible fluids are effective fluid energy absorbers not only due to their high energy absorption efficiency but also because of the strong interactions they form with surrounding structures. Their large deformability and excellent biocompatibility enable direct interact with engineering and biological structures with a wide range of mechanical properties. The scientific findings of this study represent a paradigm shift in designing next generation of advanced energy absorption materials and structures.
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