MIT engineers introduce the Oreometer
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 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. MIT engineers have now subjected the sandwich cookie to rigorous materials tests to get to the center of a tantalizing question: Why does the cookie’s cream stick to just 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.
In pursuit of an answer, the team subjected cookies to standard rheology tests in the lab and found that no matter the flavor or amount of stuffing, the cream at the center of an Oreo almost always sticks to one wafer when twisted open. Only for older boxes of cookies does the cream sometimes separate more evenly between both 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.
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.
This research was supported, in part, by the MIT UROP program and by the National Defense Science and Engineering Graduate Fellowship Program.
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Written by Jennifer Chu, MIT News Office
Paper: “On Oreology, the fracture and flow of ‘milk’s favorite cookie’”
https://aip.scitation.org/doi/10.1063/5.0085362
JOURNAL
Physics of Fluids
ARTICLE TITLE
“On Oreology, the fracture and flow of ‘milk’s favorite cookie"
Twisting Oreos shows creme filling sticks to one side
Chewing on cookies, creme characteristics with Oreology
Peer-Reviewed PublicationIMAGE: THE TEAM AFFIXED COOKIES TO A LABORATORY RHEOMETER AND DESIGNED A 3D-PRINTED OREOMETER TO STUDY THE INFLUENCES OF ROTATION RATE, FLAVOR, AMOUNT OF CREME, AND ENVIRONMENT ON OREOS. view more
CREDIT: CRYSTAL OWENS
WASHINGTON, April 19, 2022 – Imagine the perfect way to eat an Oreo. Perhaps you twist the top layer, separating the cookie into two parts, and then eat them one by one. Alternatively, you could dunk the treat into milk to soften it just the right amount. Or maybe, if you're a rheologist who studies complex fluids, you snack on the cookie while you test its mechanical properties in your lab.
In Physics of Fluids, by AIP Publishing, researchers from Massachusetts Institute of Technology characterized the flow and fracture of Oreos, finding the creme, which is officially "mushy" in rheological texture, tends to stick to one side of the cookie.
"Rheology can be used to measure the texture of food depending on the failure stresses and strains," said author Crystal Owens. "We were able to characterize Oreo creme as quantitatively mushy."
They placed Oreos in a rheometer, a laboratory instrument they used to measure torque as it fixed one side of the cookie in place and carefully twisted the other. After the filling failed and the cookie broke apart, they quantified the amount of creme on each wafer by visual inspection.
"I had in my mind that if you twist the Oreos perfectly, you should split the creme perfectly in the middle," said Owens. "But what actually happens is the creme almost always comes off of one side."
The authors investigated the influence of milk, cookie flavor, amount of filling, and rotation rate on the final creme distribution. After being dipped in milk, the cookies degraded quickly, crumbling after about 60 seconds. Flavor and filling seemed to have little effect on cookie mechanics, but breaking the cookies apart cleanly did depend on the rotation rate.
"If you try to twist the Oreos faster, it will actually take more strain and more stress to break them," said Owens. "So, maybe this is a lesson for people who are stressed and desperate to open their cookies. It'll be easier if you do it a little bit slower."
The creme may stick consistently to one side because of the way the cookies are manufactured and then oriented during packaging. Cookies from the same box often followed the same trends and varied from box to box, possibly due to different storage conditions.
By also designing an open-source, 3D-printed "Oreometer" powered by rubber bands and coins, the team hopes to encourage educators and Oreo enthusiasts to continue studying the cookies and learning about rheology.
"One of the main things we can do with the Oreometer is develop an at-home education and self-discovery plan, where you teach people about basic fluid properties like shear strain and stress," said author Max Fan.
See a video of the Oreometer at https://www.youtube.com/watch?v=V_gaJ4po_Nw.
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The article, "On Oreology, the fracture and flow of 'milk's favorite cookie®'" is authored by Crystal E. Owens, Max R. Fan (范瑞), A. John Hart, and Gareth H. McKinley. The article will appear in Physics of Fluids on April 19, 2022 (DOI: 10.1063/5.0085362). After that date, it can be accessed at https://aip.scitation.org/doi/full/10.1063/5.0085362.
ABOUT THE JOURNAL
Physics of Fluids is devoted to the publication of original theoretical, computational, and experimental contributions to the dynamics of gases, liquids, and complex fluids. See https://aip.scitation.org/journal/phf.
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JOURNAL
Physics of Fluids
ARTICLE TITLE
On Oreology, the fracture and flow of 'milk's favorite cookie®'
ARTICLE PUBLICATION DATE
19-Apr-2022
ITS BEEN DONE BEFORE
Which Side Of The Oreo Gets The Icing
When You Twist, Proven By Science
The answer is actually pretty surprising.
by MEGAN FRIEDMAN
GETTY IMAGES
There are many ways to eat an Oreo, from just taking a bite to my preferred method, dunking it in milk until it's deliciously soggy. But for many, the time-tested tradition is to twist the cookie in half and see which side got all the icing. And now, engineers with a little too much time on their hands have cracked the code of how to predict which side gets the icing.
Quartz reports that back in 2014, three Princeton graduate students studying mechanical and aerospace engineering were chatting about the "Oreo twist-off game," which would help them settle disputes as kids, wishbone-style. But one of them remembered having one friend as a kid who always won the game. How did he do it? They decided to experiment until they figured it out.
They put cookies into complicated machinery to test exactly what happens with different separation techniques, from twisting to pulling apart. They also tested on their family and friends, using bulk boxes of hundreds of Oreos. But it turns out that there's no complicated physics to the twist-off game, just manufacturing techniques.
If you want to predict which side will get the icing, just test one cookie from the box. Every other cookie in your entire package will turn out the same way. According to the Princeton scientists, just position your Oreo box so the text faces the right way, then take the cookie out from the upper left hand corner. If the icing is on the left side, it'll be on the left side for every other cookie, and the same goes if the icing is on the right.
Oreo won't give away its secrets, but the Princeton team thinks that during manufacturing, a machine pumps the icing onto one wafer and then puts the second wafer on top. The icing just sticks better to the first wafer, kind of like hot glue, and then the cookies are loaded into the packages all in the same way. It's unclear whether the same method goes for the Double Stuf and other varieties, so you'll just have to undergo your own delicious experiments to find out.
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