Scientists finally know why Oreo filling always sticks to one side

To mechanical engineer Crystal Owens, Oreos defy logic. When split apart, each of the two cookies ought to have an equal amount of creme. At least, that’s what she expects from the physics.

“It’s just logical,” Owens tells Inverse. So, she and a team at MIT looked into the fluid dynamics of Oreos, pioneering a new field. Since the physics of fluid dynamics is called rheology, it’s only apt that the study of Oreo fluidics be called Oreology. Owens is also a PhD candidate at MIT, and now the world’s pre-eminent Oreologist.

In a paper published this week in a specialty issue of the journal Physics of Fluids, Owens explains the science behind this snack’s mechanics, and provides the blueprints for her Oreometer so that budding physicists can take a crack at it.

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

Owens’ foray into Oreology branched off from work on her dissertation. She was investigating rheology of carbon nanotube solutions — some of her research in this non-cookie field is available on the pre-print server arXiv, which is a combination of pre-prints for papers submitted to academic journals and others that may not end up placed anywhere. A carbon nanotube is a tube a few nanometers wide made from one or more layers of carbon.

Owens was measuring the solution’s viscosity, or thickness. She loaded the solution between two parallel plates in her rheometer and rotated them. Rotating and shearing, or separating, the solution between two plates allows Owens to measure its resistance to separating, which is its viscosity. “One day, we were looking at that, and we were thinking, ‘This is exactly like an Oreo,’” she says.

To find out the complex mechanics at work, Owens and her team built a machine to rotate Oreos open. 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 pennies, and its blueprint is available here.

What’s new — Owens expected Oreo creme to split perfectly down the middle because that’s what happens with similar materials like thermoplastics. When heated, thermoplastics are squishy and pliable.

“If you have them between parallel plates and you rotate too fast, then you’ll stop being able to make a measurement because the fluid will naturally form a seam at the middle,” Owens says. This natural seam is what would cause this industrial “Oreo” with thermoplastic filling to split perfectly in half. The thermoplastics behave this way because as the plates (which are analogous to the chocolate cookies) rotate, something called shear force acts on the filling. Shear force is how the fluid responds to the forces of being pulled apart.

However, this is not so with actual Oreos.

“The creme is not strongly bonded to the wafers,” she says. Since there’s no strong bond to evenly bear the shear surface to the middle during rotation, the filling separates entirely from one wafer. “So you get one wafer with all the creme and one with none.”

The team also concludes that Oreo creme is a fluid. Fluids aren’t necessarily liquids like water. Especially when it comes to complex fluids, substances of many consistencies can flow. Oreo creme qualifies as one of these soft solids that can act as a fluid.

“A lot of foods are essentially soft solids when they're at rest,” Owens says. One must first apply force to make them flow. “Anything from butter to yogurt to ice cream is going to act like that.”

The creme isn’t just any fluid, but specifically viscoelastic fluid. Toothpaste is another viscoelastic fluid, Owens explains. It remains static in the tube but flows out under force. Once on the toothbrush, it will sit atop the bristles without sinking down, unlike water. Owens confirmed this physical characteristic in Oreo creme because she observed the faster she rotated the cookies, the more stress it takes to split the creme.

Digging into the details — Owens is a fan of the original Oreo, and her favorite part of the cookie is, of course, the creme. Her research has led her to offer some advice to Nabisco, Oreo’s parent company, to help evenly disperse creme between the cookies so nobody is left with a dry biscuit.

“If they put texture on the inside of the wafer, ... I think the creme would be able to grab onto that, so when you twist it open, it would be uniform.” Oreos currently don’t break even because the inside of the wafers are so smooth, Owens thinks. She adds that improved nutrition would be nice.

Additionally, handling of Oreo packages may predetermine to which side the creme clings. Storage conditions and cookie orientation can impact the outcome, too. Ultimately, Owens learned how to predict how the creme would split.

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

However, not all Oreo varieties (and there are a lot) behave the same. Golden Oreos, for instance, broke into smaller pieces more easily in the Oreometer. However, while conditions might influence each box of cookies to behave a little differently, the behavior was always deterministic after a few trial cookies.

Why it matters — What could matter more than developing a perfect cookie-eating technique?

“The most important thing to me was just figuring out how to open Oreos the best,” Owens says. It’s also useful to have the measurement of minimum force to make Oreo creme deform, known as yield stress. Oreo creme might not be a common material in industrial fluidics now, but it’s always good to be prepared.

Perhaps one of the more consequential results from this finding is in enticing non-scientists to learn physics. Oreo cookies are cheap and widely available, and the Oreometer can be duplicated. “I hope that other people build on the Oreometer,” Owens says. Science is always more fun when it tastes good, after all.

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