The secret to understanding how oobleck — a Dr. Seuss-derived nickname for the combination of corn starch and water — works has finally been unraveled, a new Cornell study reports.
As a non-Newtonian fluid, oobleck has properties of both liquids and solids. When stirred slowly, Oobleck behaves like a liquid, but when squeezed or punched, it suddenly solidifies. This phenomenon of increased fluid resistance, or viscosity, as more pressure is applied is called shear thickening and it occurs in suspensions, or mixtures of particles suspended in a liquid.
Researchers at Cornell and the University of Edinburgh worked together to figure out the underlying mechanism in shear thickening.
According to first author Neil Lin grad, there are two likely explanations for shear thickening. The first and most popular one proposes that when stress is applied, particles are pushed closer together and the fluid between them drains out causing resistance.
“When the shear created by pressure is severe, this resistance can be so large that it locks the particles into clusters,” Lin said. “And these large clusters of particles, due to the hydrodynamic interactions, can inhibit the global flow, which gives you the shear thickening behavior.”
The second hypothesis suggests contact forces, rather than hydrodynamic forces, are behind shear thickening.
“When you deform or shear the sample, the particles come into close contact with each other,” Lin said. Instead of the resistance from fluid drainage, “the frictional contact force between particles gives you this shear thickening behavior.”
Lin’s research group found that this latter hypothesis was correct in explaining shear thickening.
“The first hypothesis has been a more popular explanation in the field for the last few decades,” Lin said. “And we basically proved that this is not the reason. It’s the other way around.”
To test the two hypotheses, researchers performed an experiment in which they stirred a suspension until it created a constant amount of resistance and then measured the resistance just after reversing the direction of stirring. If the hydrodynamic forces hypothesis was correct, the resistance should remain constant after switching the stirring direction, Lin said. The force due to resistance, according to this hypothesis, is the same in the fluid whether the fluid is being drained or flowing back into the space between particles, so it should stay constant when the direction is reversed. If the contact forces hypothesis was correct, the resistance should drop, they reasoned, as soon as the stirring direction is reversed.
Using an MIT-supplied rheometer, a device specifically meant to measure how fluids respond to changes in applied pressure, the researchers stirred a suspension of spherical micro-particles in salt water and measured the resistance in the fluid immediately after the direction of stirring was reversed. They found that the force suddenly drops, which matches the contact force hypothesis. Since the interactions between these micro-particles are simpler than those between corn starch molecules, using this simplified suspension allowed the researchers to unambiguously nail down the origin of thickening as fitting the contact hypothesis.
By beginning to understand the mechanism of thickening, researchers can start to improve its current applications. For example, one of the most popular examples of non-Newtonian fluids in use is body armor. It exploits its strange properties by solidifying under the impact of the bullet, providing protection to the wearer.
Knowledge about thickening can also help to optimize its other applications, potentially improving concussion-preventing helmets, reducing problems in industrial processes, and aiding in the creation of flexible, yet protective spacesuits for astronauts, according to Lin.
The results of this study were published in Physical Review Letters.