Cornstarch mixed with a little water is peculiar stuff. At first glance it seems like any other liquid—you can pour it from one bowl to another or dip your hands in it. But give it a squeeze or strike the surface of the fluid with a hard blow, and the cornstarch slurry suddenly firms up—you can roll it into balls, walk on it and even bounce on it.
Vigorously stirring the mixture will also turn it nearly to stone. Yale University physicist Eric Brown is fond of demonstrating the weirdness of cornstarch and water, sometimes called Oobleck, by mixing them together with a metal shaft. Stir forcefully enough, he says, and he can actually break the rod. Stranger still, the transition is reversible: ease up on the stirring, and what seemed solid turns right back to liquid.
Physicists long struggled to fully account for the rapid liquid-to-solid shift, known as shear thickening. Eventually, in 2003, a team of French experimenters saw the first hints that shear thickening is the by-product of friction between the particles.
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More recently, researchers have confirmed that view with detailed simulations of particle interactions. At low starch concentrations, the liquid lubricates the particles, allowing them to move more or less freely, says co-author Jeffrey Morris, a professor of chemical engineering at the City University of New York who co-authored a new study on the phenomenon in Physical Review Letters. Even with more particles, water still “has that nearly perfect lubricating role,” Morris says, until someone starts stirring a little too hard. The extra force slams suspended particles together, and their rough surfaces prevent particles from sliding past one another. Instead they form long, rigid chains held together by friction, which give shear-thickened fluids their near-solid feel, says lead study author Ryohei Seto, also at C.U.N.Y.
“Shear thickening is remarkable,” Morris says, noting that it took countless experiments and theoretical studies to answer “a basic question” in physics. Many more questions remain, Brown says. It is not yet clear, for instance, whether the same microscopic interactions responsible for shear thickening also account for Oobleck's impact resistance.