by Kevin Schofield

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This weekend’s read is a research paper from the journal Physics of Fluids — and before your eyes completely glaze over and your finger reaches for the “delete” key, let me assure you that you will want to keep reading, because this paper is not at all what you are envisioning it to be. For while it is indeed a paper on fluid mechanics with the required complicated mathematical formulas and squiggly-line Greek letters, it probes one of the deepest, darkest problems of the universe: predicting what happens to the creme filling when you twist apart an Oreo. 

In fact, the authors, two MIT researchers, coined a new term for the study of this phenomenon: “Oreology.” This is a clever twist (no pun intended) on “rheology,” the official term for the study of the flow of materials with complex viscosity, such as Oreo creme. This isn’t the first time rheologists have applied their craft to food; for many decades, food scientists have studied how to thicken sauces, sweeten chocolate, tune the flow of fondue, and give ketchup and mayonnaise just the right texture. But twisting apart an Oreo goes one step further, into “parallel plate rheology,” or what happens when a thickened fluid is sandwiched between two moving parallel plates (e.g., chocolate wafers). 

In their quest to understand how Oreos twist apart, the researchers began by collecting experimental data: They bought several packages of Oreos and twisted them apart. They were careful to be consistent in their experiments in maintaining the orientation of individual cookies based upon how they were placed in the original package, since there is a method to how they are assembled and packaged up: A wafer is laid horizontally, a layer of creme is deposited onto it, and then another wafer is placed on top. That means the creme has a bit more time (and the benefit of gravity) to attach itself to the bottom wafer.

They found that in the vast majority of cases, the entire creme layer stays attached to one or the other of the chocolate wafers, with more sticking to the right wafer than the left one. But they also documented several “split” cases: an even split; mostly on the left; mostly on the right; and a “cup and cone,” where the middle goes with one side and an outside ring goes with the other. And it turns out the outcome of twisting apart an Oreo is very dependent upon the cookie’s position inside the package: Those on the left end tended to leave the creme attached to the right wafer, and vice versa. 

These outcomes didn’t vary by the flavor of the creme, the rotation rate, or by the amount of “stuf” in the creme layer. What did seem to matter, though, was the age of the Oreos and how they had been treated over the course of their short-but-oh-so-tasty lives. The researchers noticed that in some Oreos, the creme had creeped out to the edges of the cookie, as one might expect over time or if the package was stored in a hot room: The creme is still a liquid, albeit a very thick one, and will still spread under the right conditions. Those cookies that showed signs of the creme spreading out were more likely to split the creme between the two wafers. 

The researchers then proceeded to the next critically important topic: what happens when an Oreo is dunked in milk. One of their immediate observations is that the factors at play are reversed in the dunking scenario when compared with the twisting scenario: Normally, the wafers are holding the creme together, but when submerged in milk, the creme actually holds the wafers together since the wafers are absorbing the milk. The researchers dunked Oreos for five seconds and then suspended them in air by one wafer, and found that after 30 seconds, they fell apart under their own weight. Your mileage may vary, depending upon what kind of milk you use.

To conduct these experiments, the MIT researchers constructed (and 3D printed) their own rig for holding and twisting Oreos: a machine they call the “Oreometer.” It uses rubber bands to hold the cookies in place, and stacks of coins to precisely set the amount of twisting force to be applied. Including the coins and rubber bands (but not the Oreos), the contraption cost about $6. 

Based upon their experimental findings, the MIT researchers believe they can predict the results of twisting apart an Oreo 80% of the time. In their paper, they suggest future directions for their research, including testing whether squeezing a cookie before twisting, or refrigerating the cookie, makes a difference to how it twists apart. They also note that similar experiments could be conducted on Nutter Butters, and they wildly speculate on the kinds of testing that might be conducted with Fig Newtons.

I would be remiss if I did not point out that this paper wins the award for Best Bibliography Ever. It lists previous research published on the rheology of other foods, including chocolate, fondue, peanut butter, cheese, and coffee spilling. It additionally cites work on lava, powders, fried rice, and fire ants — all of which apparently also exhibit behavior that can be modeled as liquids. The bibliography also points to documents describing how to build a microscope out of Legos or origami. It concludes by pointing to an analysis of the physical properties of cornflakes in a bowl of milk.

On Oreology, the fracture and flow of “milk’s favorite cookie”

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📸 Featured Image: Original photo by YingtuArt/Shutterstock.com; image editing by Emerald team.

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