Scientists Finally Know Why Oreo Filling Always Sticks to One Side

To Mechanical Engineer Crystal Owens, Oreos defies logic. When separated, each of the two cookies should have an equal amount of cream. At least, that’s what she expects from physics.

“It’s just logical” owen recount Reverse. So she and a team from MIT looked at the fluid dynamics of Oreos, pioneering a new field. Since the physics of fluid dynamics is called rheology, it’s fitting that the study of Oreo fluidics is called oreology. Owens is also a PhD candidate at MIT, and now one of the world’s foremost oreologists.

In a paper published this week in a specialized issue of the journal Fluid Physics, Owens explains the science behind the mechanics of this snack and provides plans for his oreometer so budding physicists can try their hand.

Here’s the background — The purpose of the Oreology experiment was to explain once and for all why cream sticks to a cookie when an Oreo cookie sandwich is split.

Owens’ foray into oreology began with work on his thesis. She was investigating the rheology of carbon nanotube solutions – some of her research in this area unrelated to cookies is available on the arXiv preprint server, which is a combination of preprints for articles submitted to academic journals and others that may not be placed anywhere. A carbon nanotube is a tube a few nanometers wide made up of one or more layers of carbon.

Owens measured the viscosity or thickness of the solution. She loaded the solution between two parallel plates in her rheometer and spun them. By spinning and shearing, or separating, the solution between two plates allows Owens to measure its resistance to separation, that is, its viscosity. “One day we were looking at this and thinking, ‘This is exactly like an Oreo,'” she says.

To uncover the complex mechanics at work, Owens and his team built a machine to rotate Oreos. It’s a rheometer, but for Oreos, so naturally: The Oreometer. Rheometers measure torque, the force used to rotate an object. The Oreometer is a 3D printed tool powered by rubber bands and coins, and its blueprint is available here.

What’s new – Owens expected the Oreo creme to split perfectly in the middle, because that’s what happens with similar materials like thermoplastics. When heated, thermoplastics are spongy and flexible.

“If you have them between parallel plates and you spin too fast, you won’t be able to make a measurement anymore because the fluid will naturally form a seam in the middle,” says Owens. This natural seam is what would make this industrial “Oreo” with thermoplastic filling split perfectly in half. Thermoplastics behave this way because when the plates (which are analogous to chocolate cookies) rotate, something called shear force acts on the filling. Shear force is how the fluid reacts to the forces of being pulled apart.

However, this is not the case with actual Oreos.

“Cream is not strongly related to wafers,” she says. Since there is no strong bond to evenly support the shear surface in the middle during rotation, the infill separates entirely from a wafer. “So you get one wafer with all the cream and one without none.”

The team also concludes that Oreo cream is a fluid. Fluids are not necessarily liquids like water. Especially when dealing with complex fluids, substances of several consistencies can flow. Oreo cream is considered one of those soft solids that can act like a fluid.

“A lot of foods are essentially soft solids when they’re at rest,” says Owens. You must first apply the force to make them sink. “Everything from butter to yogurt to ice cream will act like this.”

Cream is not just any fluid, but specifically a viscoelastic fluid. Toothpaste is another viscoelastic fluid, says Owens. It remains static in the tube but flows under the force. Once on the toothbrush, it will sit on the bristles without sinking, unlike water. Owens confirmed this physical characteristic in Oreo cream because she observed that the faster she spun the cookies, the more stress it took to divide the cream.

The Oreometer tests the mechanics of Oreo cookies.

Dig into the details — Owens is a fan of the original Oreo, and his favorite part of the cookie is, of course, the cream. Her research led her to offer a few tips to Nabisco, the parent company of Oreo, to help evenly distribute the cream between the cookies so no one ends up with a dry cookie.

“If they put texture inside the wafer, … I think the cream could cling to it, so when you open it it would be even.” Currently, Oreos don’t break even because the insides of the slices are so smooth, Owens thinks. She adds that improved nutrition would be good.

Additionally, handling Oreo wrappers can predetermine which side the cream clings to. The storage conditions and the orientation of the cookies can also have an impact on the result. Ultimately, Owens learned to predict how the cream would split.

“If you gave me a box and I was able to open five Oreos as a pretest, I could predict with an 80 percent chance where the creme would be on the next Oreo,” she says.

However, all varieties of Oreo (and there are a lot) behave the same way. Golden Oreos, for example, broke more easily into small pieces in the oreometer. However, although the conditions might influence each box of cookies to behave a little differently, the behavior was still deterministic after a few test cookies.

Why is it important – What’s more important than developing a perfect technique for eating cookies?

“The biggest thing for me was just figuring out the best way to open Oreos,” says Owens. It is also useful to have the measurement of the minimum force to deform the Oreo cream, known as the yield point. Oreo cream may not be a common material in industrial fluidics today, but it’s always good to be prepared.

Perhaps one of the most consequential results of this discovery is to entice non-scientists to learn physics. Oreo cookies are cheap and widely available, and the hydrometer can be duplicated. “I hope other people will build on the oreometer,” says Owens. Science is always more fun when it tastes good, after all.

Owens is considering a follow-up study on another delicious dessert: ice cream. “It also has a very interesting rheology that is temperature dependent,” she says.

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