Most people buy cornstarch to make custard or gravy, but Scott Waitukaitis and Heinrich Jaeger have used it to solve a longstanding physics problem with a substance known to generations of Dr. Seuss readers as "Oobleck," and to scientists as a non-Newtonian liquid.
This substance, a liquid that can instantaneously turn into a solid under the force of a sudden impact, behaves in surprising ways. It consists of a simple mixture of cornstarch and water, and adults can actually run across a vat of this liquid, as has been done many times on television game shows and programs such as MythBusters.
The University of Chicago's Waitukaitis and Jaeger suspect that many similarly constituted suspensions — liquids laden with micron-sized particles — will behave exactly the same way. Scientists and engineers have attempted to explain the underlying physics of this phenomenon since the 1930s, but with incomplete success.
Current explanations predict a thickening of the suspension when it's subjected to the push-pull of shearing forces, but fall far short of accounting for the large forces needed to keep an adult high and dry while running across a pool of the stuff.
Now Waitukaitis and Jaeger report in the July 12 issue of the journal Nature how compressive forces can generate a rapidly growing, solid-like mass in the suspension. The study culminates a long struggle to understand a phenomenon that has elicited a wide range of explanations over the years.
"We found that when you hit the suspension, a solid-like column grows below the impact site," said Waitukaitis, a graduate student in physics. "The way it grows is similar to how a snowplow works. If I push a shovel in loose snow, a big pile of compacted snow grows out in front of the shovel, which makes it harder and harder for me to push." With the suspension, the "snowplow" is caused by individual cornstarch grains piling up in front of the impacting object and becoming temporarily jammed after compression has halted all movement.
Jaeger's group has studied the physics phenomenon of "jamming" in numerous contexts, such as when the creation of a vacuum turns a fluid substance like coffee grounds into a solid.
Handling suspensions is important to a broad range of industries, from construction to biomedicine. Some engineers are even investigating these suspensions as the basis for a new type of body armor.
"It would be liquid, so it would conform to a particular shape, and when it gets hit hard it knows it needs to become hard," Waitukaitis said. It's a smart material, one that increases resistance with the amount of force applied against it.
Cornstarch and water individually behave strikingly differently to the application of force than they do when mixed. With water, a normal liquid, the resistance to intruding objects is hundreds of times smaller. A bucket of dry cornstarch grains, meanwhile, exists in a jammed state courtesy of gravity, and slamming a rod into the bucket unjams the grains. With mixtures of cornstarch and water, the material starts out unjammed and blunt force drives it to jam locally.
The UChicago experiment highlights how complex and often puzzling phenomena emerge from simple ingredients, and how established ways of looking at them need to be revisited with the benefit of modern technology. Historically, most experiments have looked at relatively small volumes of suspensions, and primarily under conditions of continuous shearing.
"To notice a transient phenomenon of the type that we describe, you need a large set up and you need to look very fast," said Jaeger, the William J. Friedman and Alicia Townsend Professor in Physics.
The UChicago experiment did just that with a combination of high- and low-tech instruments, including force sensors, laser sheets, X-rays, high-speed cameras taking images at 10,000 frames a second, and an industrial cement mixer.
"It's an incredibly messy experiment," Waitukaitis said. "I have a blue jumpsuit I wear all day. When I do these experiments, I'm totally covered in cornstarch."
The experiment was the first to investigate direct compression in these suspensions. The experiment shows that driving a rod into the cornstarch and water mixture easily generated stresses 100 times greater than the largest stresses encountered during shear.
The researchers found that their impacting rod initiated a shock-like, moving front that starts directly beneath the impacting object and then grows downward, transforming the initially liquid suspension into a temporarily jammed state. As the front of this jammed region moves forward, it transforms the liquid region directly ahead of it. "It essentially grows its own solid as it propagates," Jaeger said.
The UChicago scientists called this process "impact-activated solidification." Impacts typically are destructive processes but in the suspension they actually lead to the creation of a solid from a liquid, although only temporarily.
Waitukaitis and Jaeger now are extending this work by collaborating with researchers at Leiden University in The Netherlands to model the propagating shock fronts in more detail. The are also working closely with UChicago colleagues Wendy Zhang, associate professor in physics, and Jake Ellowitz, graduate student in physics. Zhang and Ellowitz are developing simulations to test how altering the ingredients of various suspensions affect their behavior under impact.
"The feedback between the particle movements and the liquid flow makes this challenging. It's actually not at all easy to perform simulations on such a system," Jaeger said.
Citation: "Impact-activated solidification of dense suspensions via dynamic jamming fronts," Nature, July 12, 2012.
Funding: National Science Foundation and U.S. Department of Education (Graduate Assistance in Areas of National Need Fellowship).
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