This was at least partially funded - for some reason - by the Defense Department
Introducing the MIT Oreometer
By JENNIFER CHU, MASSACHUSETTS INSTITUTE OF TECHNOLOGY
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| MIT researchers have designed a 3D-printable “Oreometer” to put an Oreo’s cream filling through a battery of tests to understand what happens when two wafers are twisted apart. |
Mechanical engineers put an Oreo’s cream filling through a battery of tests to understand what happens when two wafers are twisted apart.
When you twist an Oreo
cookie open to get to the creamy center, you’re mimicking a basic rheological
test. (Rheology is the study of how a non-Newtonian material flows when
twisted, pressed, or otherwise strained.) MIT engineers have
now subjected the sandwich cookie to rigorous materials testing in order to
answer a vexing question: why does the cookie’s cream stick to only one wafer
when twisted apart?
“There’s the fascinating
problem of trying to get the cream to distribute evenly between the two wafers,
which turns out to be really hard,” says Max Fan, an undergraduate in MIT’s
Department of Mechanical Engineering.
Why does the cookie’s cream stick to just one wafer when twisted apart? MIT engineers pursue the answer.
In search of an answer,
the team exposed cookies to normal rheology experiments in the lab and
discovered that, regardless of flavor or amount of stuffing, the cream in the
center of an Oreo almost always adheres to one wafer when twisted open. Only in
older boxes of cookies does the cream sometimes divide more equally between the
two wafers.
The researchers also measured the torque required to twist open an Oreo, and found it to be similar to the torque required to turn a doorknob and about 1/10th what’s needed to twist open a bottlecap. The cream’s failure stress — i.e. the force per area required to get the cream to flow, or deform — is twice that of cream cheese and peanut butter, and about the same magnitude as mozzarella cheese. Judging from the cream’s response to stress, the team classifies its texture as “mushy,” rather than brittle, tough, or rubbery.
When you twist open an
Oreo cookie to get to the creamy center, you’re mimicking a standard test in
rheology — the study of how a non-Newtonian material flows when twisted,
pressed, or otherwise stressed.
So, why does the
cookie’s cream glom to one side rather than splitting evenly between both? The
manufacturing process may be to blame.
“Videos of the
manufacturing process show that they put the first wafer down, then dispense a
ball of cream onto that wafer before putting the second wafer on top,” says
Crystal Owens, an MIT mechanical engineering PhD candidate who studies the
properties of complex fluids. “Apparently that little time delay may make the
cream stick better to the first wafer.”
The team’s study isn’t
simply a sweet diversion from bread-and-butter research; it’s also an
opportunity to make the science of rheology accessible to others. To that end,
the researchers have designed a 3D-printable “Oreometer” — a simple device that
firmly grasps an Oreo cookie and uses pennies and rubber bands to control the
twisting force that progressively twists the cookie open. Instructions for the
tabletop device can be found here.
The new study, “On
Oreology, the fracture and flow of ‘milk’s favorite cookie,’” appears today
in Kitchen Flows, a special issue of the journal Physics of Fluids. It was conceived of early in the
Covid-19 pandemic, when many scientists’ labs were closed or difficult to
access. In addition to Owens and Fan, co-authors are mechanical engineering
professors Gareth McKinley and A. John Hart.
Confection connection
A standard test in
rheology places a fluid, slurry, or other flowable material onto the base of an
instrument known as a rheometer. A parallel plate above the base can be lowered
onto the test material. The plate is then twisted as sensors track the applied
rotation and torque.
Owens, who regularly
uses a laboratory rheometer to test fluid materials such as 3D-printable inks,
couldn’t help noting a similarity with sandwich cookies. As she writes in the
new study:
“Scientifically,
sandwich cookies present a paradigmatic model of parallel plate rheometry in
which a fluid sample, the cream, is held between two parallel plates, the
wafers. When the wafers are counter-rotated, the cream deforms, flows, and
ultimately fractures, leading to separation of the cookie into two pieces.”
While Oreo cream may not
appear to possess fluid-like properties, it is considered a “yield stress
fluid” — a soft solid when unperturbed that can start to flow under enough
stress, the way toothpaste, frosting, certain cosmetics, and concrete do.
Curious as to whether
others had explored the connection between Oreos and rheology, Owens found
mention of a 2016 Princeton University study in
which physicists first reported that indeed, when twisting Oreos by hand, the
cream almost always came off on one wafer.
“We wanted to build on
this to see what actually causes this effect and if we could control it if we
mounted the Oreos carefully onto our rheometer,” she says.
Cookie twist
In an experiment that
they would repeat for multiple cookies of various fillings and flavors, the
researchers glued an Oreo to both the top and bottom plates of a rheometer and
applied varying degrees of torque and angular rotation, noting the values that
successfully twisted each cookie apart. They plugged the measurements into
equations to calculate the cream’s viscoelasticity, or flowability. For each
experiment, they also noted the cream’s “post-mortem distribution,” or where
the cream ended up after twisting open.
In all, the team went
through about 20 boxes of Oreos, including regular, Double Stuf, and Mega Stuf
levels of filling, and regular, dark chocolate, and “golden” wafer flavors.
Surprisingly, they found that no matter the amount of cream filling or flavor,
the cream almost always separated onto one wafer.
“We had expected an effect based on size,” Owens says. “If there was more cream between layers, it should be easier to deform. But that’s not actually the case.”
Curiously, when they
mapped each cookie’s result to its original position in the box, they noticed
the cream tended to stick to the inward-facing wafer: Cookies on the left side
of the box twisted such that the cream ended up on the right wafer, whereas
cookies on the right side separated with cream mostly on the left wafer. They suspect
this box distribution may be a result of post-manufacturing environmental
effects, such as heating or jostling that may cause cream to peel slightly away
from the outer wafers, even before twisting.
The understanding gained
from the properties of Oreo cream could potentially be applied to the design of
other complex fluid materials.
“My 3D printing fluids
are in the same class of materials as Oreo cream,” she says. “So, this new
understanding can help me better design ink when I’m trying to print flexible
electronics from a slurry of carbon nanotubes, because they deform in almost
exactly the same way.”
As for the cookie
itself, she suggests that if the inside of Oreo wafers were more textured, the
cream might grip better onto both sides and split more evenly when twisted.
“As they are now, we
found there’s no trick to twisting that would split the cream evenly,” Owens
concludes.
Reference: “On Oreology,
the fracture and flow of “milk’s favorite cookie®”” by Crystal E. Owens, Max R.
Fan, A. John Hart and Gareth H. McKinley, 19 April 2022, Physics of Fluids.
DOI:
10.1063/5.0085362
