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Engineering, Analysis and Design

Tuesday, November 29, 2022

Oreometer, the Fracture and Flow of OREO

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 centre, 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 centre 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.

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.

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.

FIG. 1. (a) What happens when you twist an Oreo? (b) Eventually it splits into two parts, exposing the creme (c) and (d) We observed that in a typical failure profile for Oreos from newly opened boxes, the creme most often tends to remain on one side, “wafer 1,” with a consistent orientation per box. In this case, wafer 1 faces to the left side of the upright box for a standard size package of regular Oreos. The creme occasionally splits between sides, often due to defects or small fractures in one or both wafers. (e) This is consistent for cookies with different creme levels, with a strong bias toward wafers facing one side of the box rather than the other,
facing left for regular, right for double, and up for mega (where rows are oriented vertically rather than horizontally) in standard size packages.
FIG. 2. (a) A cookie is mounted for testing on parallel plate fixtures of a laboratory rheometer and adhered to the metal plates by a low-temperature thermoplastic adhesive. (b) The sandwich cookie is a
layered composite with two solid wafers enclosing a central cream layer. When one wafer is rotated relative to the other, the creme deforms in torsion. (c) The resulting velocity field within the creme is a function of applied rotation rate, material height, and radius from the center. In this figure, red colors indicate kinematic properties, and blue colors indicate geometric properties in the colored web version of this article
FIG. 3. (a) and (b) Creme failure tests were repeated for different rotation rates, and our tests revealed a rate dependence, with the faster rotation requiring higher stress and strain. At the highest speed only, 10 rad/s (about 370 rotations per minute, as fast as a typical ceiling fan rotates), the wafer tended to break along with the creme, as shown in the inset to (b) of the cookie post-failure but still mounted on the rheometer. The dashed line indicates a line of slope one relating yield stress to yield strain. (c) and (d) The “stuf” level or height of creme was not found to influence mechanics of failure or creme distribution, and (e) the Oreo flavor did not influence stress levels, although (f) flavors seemed to influence how likely a wafer was to break, with images shown for one cookie each after twisting for the Dark Chocolate and Golden flavors. For the Golden variety, this wafer breakage was substantial enough to influence creme distribution onto some top-wafer segments and some bottom-wafer segment

Conclusion

Through a series of experiments with a laboratory rheometer used to hold whole Oreo cookies, they determined that creme distribution upon cookie separation by torsional rotation is not a function of rate of rotation, creme “stuf” (i.e., filling height H) level, or flavor, but was mostly determined by the preexisting level of adhesion between the cookie creme and each wafer. In most cases, creme delaminated from the wafer with a preferential orientation with respect to the package within any one box, allowing prediction of failure direction with 80% accuracy. Despite the consistent failure mode, there was some amount of cookie-to-cookie and box-to-box variation in failure stress and strain. Apparent reflow of creme due to unknown causes had the most significant effect in altering failure type, allowing for improved creme–wafer bonds and subsequent cohesive failure of the creme.

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.

Original article

https://aip.scitation.org/doi/10.1063/5.0085362

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