BACKGROUND: Engineers have designed a way to make police officers and soldiers safer with better body armor. The secret is a new "shear-thickening" fluid. When fabric has been saturated in this new fluid, it becomes strong enough to stop a bullet, but strong enough to wear comfortably.
HOW IT WORKS: When a bullet strikes the fabric, it immediately becomes rigid and keeps the object from penetrating. Once the assault has stopped, the fabric returns to being flexible and lightweight. Material that has been soaked in the new fluid is slightly oily to the touch but otherwise feels and drapes like normal fabric.
ABOUT SMART MATERIALS: The shear-thickening fluid is an example of a "smart material." Smart materials are a class of materials that can sense and respond to changes in the environment, either through the application of electricity or magnetism, or to changes in temperature. In the case of the new body armor, the material responds to a change in pressure. Under normal conditions, the fluid's molecules are weakly bonded and can move around with ease; that's why the material is so flexible. But the shock of an impact causes those chemical bonds to strengthen so the molecules lock into place; once the force from the impact dissipates, the bonds weaken again. That's why the fabric becomes rigid instantly when a bullet strikes, preventing that bullet from penetrating, and reverts to its more flexible state once that force has ceased.
WHAT IS ELASTICITY? Different materials can withstand different amounts of deformation, a property known as elasticity. Most materials are elastic to some degree: when they are deformed or bent by an infusion of incoming energy, they will bounce back to their original shape. But elastic materials all have their limits. Metal springs and rubber bands are very elastic. Plaster and glass are not very elastic; instead, they are brittle and snap with even a small deformation. Energy, like momentum, is conserved, but in a collision, it can turn into different forms of energy, such as heat or noise. How much of the energy is converted depends in part on both the relative toughness and elasticity of the materials involved in the impact. There is no such thing as a perfectly elastic collision, but if there were, all of the energy would be transferred to the target with nothing lost to heat or noise, for example.
The Materials Research Society contributed to the information contained in the TV portion of this report.