If you mix cornstarch and water in the right proportions, you get something that seems not-quite liquid but also not-quite solid. Oobleck flows and settles like a liquid when untouched, but stiffens when you try to pick it up or stir it with a spoon. The properties of oobleck and other non-Newtonian fluids, including silly putty, quicksand, paint, and yogurt, change under stress or pressure, and scientists have long struggled to prove exactly why.
Now, researchers in the University of Chicago’s Physical Sciences Division (PSD) and Pritzker School of Molecular Engineering (PME) have used piezoelectric nanoparticles, which themselves change in response to pressure, to investigate the fundamental physics of non-Newtonian fluids. The team discovered a key role for friction between particles in causing the materials to flip from a fluid to a more solid structure.