Non-Newtonian fluid shatters like glass

New Scientist : Mixing cornstarch and water in the proper proportions creates oobleck, a non-Newtonian fluid named for a substance in a Dr. Seuss book . Non-Newtonian fluids behave as fluids when poured or poked gently with a finger. However, if struck with more force, they behave as a solid. Why oobleck behaves the way it does was explained last year: When subjected to significant pressure, the water in the material moves away from the contact point much more quickly than the starch, leaving a solid material behind. As recently reported in Physical Review Letters, Matthieu Roché, formerly of Princeton University, and his colleagues have now studied how a thin layer of the material behaves when struck by a falling object. They spread a layer of oobleck over a sheet of plexiglass onto which they dropped a 300-g metal rod from varying heights. The researchers expected that the material would tear like a soft metal. Instead, it fractured like glass or plaster, forming pointy-tipped cracks, before flowing back to normal. However, they also found that if the oobleck exceeded a certain thickness, it did not crack but instead cushioned the impact because the material underneath remained fluid. Better understanding of how non-Newtonian fluids react to varying kinds of impacts may provide insights into how they can be used in ballistic vests or vehicle suspension systems.