the fluid physics of twisted Oreo cookies

How do you eat your Oreos?

Maybe you twist the top layer, separating the cookie into two parts, then eat them one at a time. If not, do you dip the biscuit in milk to soften it just enough? Or maybe you just put the whole thing in your mouth, all for efficiency of course.

Munching on an Oreo while testing its mechanical properties in the lab is apparently a legitimate research methodology, according to a team of rheologists – physicists who study complex fluids – from the Massachusetts Institute of Technology in the US.

In a groundbreaking new study, the authors introduced an emerging field called “Oreology”, derived from Nabisco Oreo for the cookie and the Greek rheo logia for “flow study”. It is the study of the flow and fracture of sandwich biscuits and the research has been published in the journal Fluid physics.

Oreo cream is a member of the class of fluid soft solids known as “yield-stressed fluids”, which are fluids that act like soft solids when undisturbed and flow only under a sufficiently large amount of applied stress.

The researchers characterized the flow and fracture of Oreos, finding that the cream – of which they found a “mushy” rheological texture – tends to stick to only one side of the cookie.

“Rheology can be used to measure the texture of food based on tensile stresses and strains,” says first author Crystal Owens, a graduate student in MIT’s Department of Mechanical Engineering. “We were able to characterize Oreo cream as quantitatively pasty.”

Oreo Cream
The team affixed cookies to a lab rheometer and designed a 3D-printed oreometer to study the influences of rotation rate, flavor, amount of cream, and environment on Oreos. Credit: Crystal Owens.

The team used a laboratory rheometer — an instrument that characterizes the flow of a substance in response to forces — to measure the failure mechanics of an Oreo filling. The rheometer clamped one side of the cookie in place and carefully twisted the other until the filling failed and the cookie broke, after which the amount of cream on each wafer could be determined by visual inspection.

“I had in mind that if you twisted the Oreos perfectly, you should divide the cream perfectly in the middle,” says Owens. “But what really happens is that the cream almost always comes off on one side.”

In fact, almost all of the cream (95%) remained on just one of the cookies after the break, and it seems the production process is the likely cause. In the boxes tested, 80% of the cookies had cream-heavy sides oriented evenly in one direction, rather than 50% as one would expect from random chance.

In an in-depth investigation of this phenomenon, rheologists also tested the influence of turnover rate, amount of cream and flavor on post-mortem cream distribution.

After being dunked in milk, the Oreos degraded rapidly, crumbling after about 60 seconds. Flavor and filling seemed to have little effect on the mechanics of the cookies, but the clean separation of the cookies depended on the rate of rotation.

“If you try to twist the Oreos faster, it will actually take more tension and more stress to break them,” Owens advises. “So maybe this is a lesson for people who are stressed and desperate to open their cookies.

“It will be easier if you do it a little slower.”

The team encourages further contributions to this emerging field of study, but acknowledges the fact that a laboratory rheometer is not widely available.

But researchers have found a way to overcome this hurdle, by designing an open-source 3D-printed “oreometer” — a rheometer specifically designed to twist Oreos — for use in higher-accuracy home studies.

Powered by rubber bands and coins, the team hopes to encourage educators and Oreo enthusiasts to continue studying cookies and learning more about rheology.

“One of the main things we can do with the oreometer is to develop an in-home education and self-discovery plan, where you teach people the basic properties of fluids like shear stress and stress,” concludes author Max Fan, an undergraduate student at MIT. .

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