Probing the Ability of Flexible Hydrogels to Protect Fragile Objects

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Hydrogels are networks of polymer chains that are swollen in water. In recent years, several routes have been devised to make hydrogels that are flexible and bendable. This thesis investigates whether such flexible gels can be wrapped around brittle or fragile objects (such as an egg or a fruit) and protect against impact. We study gels made by either physical crosslinking (e.g., gelatin) or chemical crosslinking (e.g., acrylamide) and also the same gels with various particulate additives. None of the bare gels are protective, and nanoparticles like iron oxide or silica do not help. However, the addition of starch granules to the above gels greatly enhances their protective abilities. When a load strikes a gelatin gel with 20 wt% starch, the peak impact force is reduced by 25% when compared to a bare gel without the starch. Correspondingly, the coefficient of restitution (COR) is also lowered by the presence of starch (i.e., a ball bounces less on a starch-bearing gel). We correlate the protective effects of starch granules to their ability to shear-thicken water. When starch granules are gelatinized by heat, they no longer give rise to shear-thickening, and in turn their protective ability in a gel is also eliminated. Our research can guide the rational design of protective coatings or armor for fragile objects, which could be applied in the sports, defense, and consumer sectors.