Designing Gels to Absorb Impact—How a Thin Gel Can Protect an Egg from Breaking

In recent years, several routes have been devised to make hydrogels that are strong, flexible and bendable, with prominent examples being the double-network hydrogels. This work investigates whether such strong and flexible gels can absorb impact. That is, can such a gel be wrapped around a brittle or fragile object (such as an egg) and protect the object if it is dropped onto a hard surface. We have studied a range of gels, including gels made by either physical cross-linking (e.g., gelatin) or chemical cross-linking (e.g., acrylamide, AAm) or both (double network gels of AAm and alginate). None of these bare gels are able to absorb impact. We also studied the same gels with various particulate additives, including iron-oxide, carbon-black, and graphene: again, none of these particles improve the protective abilities of the bare gels.<br/><br/>The central result from our work is that the addition of starch granules greatly enhances the protective abilities of any gel. This is observed both with gelatin and AAm gels. When a load strikes a gel containing 20% 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 much less on a starch-bearing gel). An egg covered with a starch-bearing gel survives impact on a hard surface from a height of 25 inches. Similarly, a blueberry covered with the gel resists being crushed when a weight is dropped on it.<br/><br/>We correlate the impact-absorbing 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.<br/><br/>References:<br/>[1] Ganesh, Sairam, Sai Nikhil Subraveti, and Srinivasa R. Raghavan. "How a Gel Can Protect an Egg: A Flexible Hydrogel with Embedded Starch Particles Shields Fragile Objects Against Impact." <i>ACS Applied Materials & Interfaces</i>14.17 (2022): 20014-20022.