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.
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.
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.
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
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.
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.
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.
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.
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.
notice a transient phenomenon of the type that we describe, you need a
large setup and you need to look very fast,” said Jaeger, the William J.
Friedman and Alicia Townsend Professor in Physics.
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.
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.”
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.
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.
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.
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.
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.