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Uncover the scientific mechanism of rheology How “twist” can divide the cream of Oreo cookies evenly?

  When we eat Oreo cookies, we always unconsciously think of this ad slogan – “twist, lick, soak”.
  Usually, after we “twist” the Oreo, we find a common phenomenon: the cream center in the biscuit sticks to one of the double biscuit, instead of being divided into two equally.
  Recently, scientists at the Massachusetts Institute of Technology launched a study on this phenomenon.
  On April 19, 2022, a related paper was published under the title of “About Oreo Science, Research on the Fracture and Flow of Oreo Sandwich Biscuits”.
  What is the connection between “twist” and scientific mechanism? It turns out that the principle behind this is similar to the standard test of rheology when we twist the cookie. That is, how does the non-Newtonian fluid flow when it is twisted and squeezed under the action of pressure.

  Fan Rui, an undergraduate student in the Department of Mechanical Engineering at the Massachusetts Institute of Technology, told the media: “There is a very interesting problem, which is how to evenly distribute the cream sandwiched between two cookies, and it turns out that this problem is difficult.”
  So how does the twisting pressure on the cookie work in the cream? In response to this problem, the MIT team conducted standard rheological experiments on biscuits.
  They tested with different flavors and quantities of cookies, and found an interesting phenomenon, no matter how other factors (such as the taste, amount of cream, etc.) changed, the cream in the middle of the Oreo cookie was “twisted”. , almost always sticking to one of the sandwich biscuits, rather than being evenly divided between the two biscuits.
  Of course, there is an instance where, in relatively old cookies, the cream will occasionally stick evenly on two different cookies. In addition, they noted that there was a pattern in the biscuits that the cream preferred to stick, which was related to the original position in the cookie box and presumably related to the environment after the biscuits were made.
  The researchers found through observational studies that most of the cream will stick to the biscuits that are positioned inward. To understand this rule, if the Oreo on the left side of the cookie box is twisted apart, the cream will stick to the cookie on the right; on the contrary, the cookie on the right side of the cookie box will almost stick to the cream. Cookies on the left.

  Manual twisting is such a phenomenon, then using special equipment to twist the Oreo cookies to different degrees in a controlled manner, will the state of the cream be different? To confirm the problem, the team built a device for 3D printing called the “Oreometer.” Using the Oreometer device, the Oreo biscuits can be held well and the pressure to twist the biscuits can be adjusted.
  The team also measured the torque required for Oreos. They found that the torque required to “twist” an Oreo is similar to the torque required to turn a doorknob in everyday life, about 1/10 the torque required to unscrew a bottle cap.
  By looking at how the cream reacts to different pressures, the researchers tended to equate the creamy texture to “mushy”, which is fundamentally different from crumbly, tough, or rubbery. Through various experiments, they speculated that the reason why the filling cream prefers to stick to only one biscuit is the manufacturing process.
  In the process of making cookies, they also made new discoveries. Oreo cookies are made by placing one cookie down, “distributing” cream to that cookie, and placing a second cookie on top of the cream. “Clearly, a little time delay may have allowed the cream to stick better to the first cookie,” said Christo Owens, lead author of the paper and a Ph.D. student in mechanical engineering at MIT.

  Owens’ research interests are in 3D printing and the fluid mechanics and rheology of printable fluid inks. The speculation that Oreos and rheology are “associated” comes from Owens’ related work at Princeton University. As early as 2016, physicists proposed, “In fact, when Oreos are twisted by hand, The cream almost always comes off a cookie.” So she noted that there is a “similarity” to a sandwich cookie.
  ”Scientifically, sandwich biscuits present a paradigmatic model of parallel-plate rheological measurements, in which a fluid sample (cream) is sandwiched between two parallel plates (biscuits). When the biscuit is rotated in the opposite direction, the cream deforms and flows and eventually cracked, causing the biscuit to split into two pieces,” Owens said.
  Of course, Oreo cream and fluid don’t have exactly the same properties, but Oreo cream has its own characteristics, like a “yield stress fluid,” being a soft solid that isn’t disturbed by other factors. However, when the pressure reaches a certain level, flow occurs, which is similar to toothpaste, icing, creamy cosmetics, etc. in daily life.
  ”My 3D printing fluid is in the same class of material as Oreo cream,” Owens said, “so this new understanding can help me better when I try to print flexible electronics from carbon nanotube slurries. Design the inks because they deform in almost exactly the same way.”

  To quantify the fluidity of the cream, the researchers conducted experiments on cream-filled biscuits with different fillings and flavors. They fixed the Oreos in the top and bottom positions of the Oreometer unit, tested them with varying torques and angles, and plugged the measurements into the equation by observing how each biscuit twisted apart. In addition, they also recorded in detail the final state and position of the cream when the cookies were twisted open in each experiment.
  In this experiment, the researcher’s experimental sample is about 20 boxes of Oreo cookies, which are rich in both fillings and biscuits, such as regular, double and super fillings, as well as regular, dark chocolate and “gold” Wafer cookies. But even though these variables were different, the results were consistent: when the Orly cookie was “twisted” open, the cream filling was overwhelmingly stuck to one of the cookies.
  So, what fields does this conclusion inspire in the future? Scientists study the “twist” of Oreo, and through in-depth research on the properties of cream, it may provide new ideas for the design and production of other complex fluid materials in the future.

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