The unusual fluid and solid-like properties of cornstarch and water
Cornstarch is a common household item that can be used for a variety of purposes, from thickening a culinary creation to removing the stain from your favorite shirt, but high-impact protective gear probably does not come to mind when looking at the jar of cornstarch in your pantry.
You may have witnessed the unusual properties of cornstarch and water in the kitchen. At certain proportions, you can’t get your spoon to run through the seemingly solid mixture, yet the carrot slice you drop disappears inside the opaque-white concoction. There are plenty of videos online showing people running across a small pool filled with cornstarch and water as if it were solid, but when they try to stand still, they slide in, as if they were sinking in quicksand.
At the Yale School of Engineering & Applied Science, Rijan Maharjan and Eric Brown have been studying this substance, and they recently published their findings about the mixture’s unique properties. The concoction has both fluid and solid-like properties depending on the strength and rate of the shear force, which is a force parallel to the surface to which it is applied. This makes cornstarch and water a shear-thickening fluid (STF) outside the realm of a Newtonian fluid, where only temperature can affect the fluid’s viscosity. In a STF, the thickness increases as the shear increases in rate and magnitude. Surprisingly, however, the calculations based on the viscosity of this mixture cannot explain why people can run on mixtures of cornstarch and water, driving research for other explanations of this mystifying feat.
Maharjan and Brown studied some of the aspects that may be necessary to develop better flow models for STFs, which are currently incomplete. They agitated a mixture of cornstarch and water with a shear, and then waited for it to “relax” to rest, which took significantly longer than they had expected. “This long relaxation time is one of the pieces of the puzzle we would need to try to predict other flows of this material,” Brown said.
Knowing these “pieces of the puzzle” is important in the application of shear-thickening fluids. For example, sometimes STFs break mixing blades or are too thick to flow through a pipe. Understanding how these fluids behave will help avoid these problems. Additionally, Brown mentions experiments where a bowling ball hits the surface of the fluid and stops, while a raw egg also stops at the surface and doesn’t break. This explains how STFs can be used to produce protective gear like helmets for athletes, as the STFs would block the passage of an oncoming ball or other athlete through the helmet, preventing trauma to the athlete’s head. However, Brown says there is room for improvement in how STFs can best be utilized in these types of protective applications, and understanding the properties and limits of these somewhat baffling fluids can help us increase their success in such applications.
 Interview with Eric Brown 1/30/18