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      in Security

      MIT engineers introduce the Oreometer

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      When you twist open an Oreo cookie to get to the creamy center, 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 center 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.

      In pursuit of an answer, the team subjected cookies to standard rheology tests in the lab and found that no matter the flavor or amount of stuffing, the cream at the center of an Oreo almost always sticks to one wafer when twisted open. Only for older boxes of cookies does the cream sometimes separate more evenly between both wafers.

      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.

      So, why does the cookie’s cream glom to one side rather than splitting evenly between both? The manufacturing process may be to blame.

      “Videos of the manufacturing process show that they put the first wafer down, then dispense a ball of cream onto that wafer before putting the second wafer on top,” says Crystal Owens, an MIT mechanical engineering PhD candidate who studies the properties of complex fluids. “Apparently that little time delay may make the cream stick better to the first wafer.”

      The team’s study isn’t simply a sweet diversion from bread-and-butter research; it’s also an opportunity to make the science of rheology accessible to others. To that end, the researchers have designed a 3D-printable “Oreometer” — a simple device that firmly grasps an Oreo cookie and uses pennies and rubber bands to control the twisting force that progressively twists the cookie open. Instructions for the tabletop device can be found here.

      The new study, “On Oreology, the fracture and flow of ‘milk’s favorite cookie,’” appears today in Kitchen Flows, a special issue of the journal Physics of Fluids. It was conceived of early in the Covid-19 pandemic, when many scientists’ labs were closed or difficult to access. In addition to Owens and Fan, co-authors are mechanical engineering professors Gareth McKinley and A. John Hart.

      Confection connection

      A standard test in rheology places a fluid, slurry, or other flowable material onto the base of an instrument known as a rheometer. A parallel plate above the base can be lowered onto the test material. The plate is then twisted as sensors track the applied rotation and torque.

      Owens, who regularly uses a laboratory rheometer to test fluid materials such as 3D-printable inks, couldn’t help noting a similarity with sandwich cookies. As she writes in the new study:

      “Scientifically, sandwich cookies present a paradigmatic model of parallel plate rheometry in which a fluid sample, the cream, is held between two parallel plates, the wafers. When the wafers are counter-rotated, the cream deforms, flows, and ultimately fractures, leading to separation of the cookie into two pieces.”

      While Oreo cream may not appear to possess fluid-like properties, it is considered a “yield stress fluid” — a soft solid when unperturbed that can start to flow under enough stress, the way toothpaste, frosting, certain cosmetics, and concrete do.

      Curious as to whether others had explored the connection between Oreos and rheology, Owens found mention of a 2016 Princeton University study in which physicists first reported that indeed, when twisting Oreos by hand, the cream almost always came off on one wafer.

      “We wanted to build on this to see what actually causes this effect and if we could control it if we mounted the Oreos carefully onto our rheometer,” she says.

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      Cookie twist

      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.

      In all, the team went through about 20 boxes of Oreos, including regular, Double Stuf, and Mega Stuf levels of filling, and regular, dark chocolate, and “golden” wafer flavors. Surprisingly, they found that no matter the amount of cream filling or flavor, the cream almost always separated onto one wafer.

      “We had expected an effect based on size,” Owens says. “If there was more cream between layers, it should be easier to deform. But that’s not actually the case.”

      Curiously, when they mapped each cookie’s result to its original position in the box, they noticed the cream tended to stick to the inward-facing wafer: Cookies on the left side of the box twisted such that the cream ended up on the right wafer, whereas cookies on the right side separated with cream mostly on the left wafer. They suspect this box distribution may be a result of post-manufacturing environmental effects, such as heating or jostling that may cause cream to peel slightly away from the outer wafers, even before twisting.

      The understanding gained from the properties of Oreo cream could potentially be applied to the design of other complex fluid materials.

      “My 3D printing fluids are in the same class of materials as Oreo cream,” she says. “So, this new understanding can help me better design ink when I’m trying to print flexible electronics from a slurry of carbon nanotubes, because they deform in almost exactly the same way.”

      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. More

    • 75 Shares129 Views

      in Energy

      Nurturing human communities and natural ecosystems

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      When she was in 7th grade, Heidi Li and the five other members of the Oyster Gardening Club cultivated hundreds of oysters to help repopulate the Chesapeake Bay. On the day they released the oysters into the bay, the event attracted TV journalists and local officials, including the governor. The attention opened the young Li’s eyes to the ways that a seemingly small effort in her local community could have a real-world impact.

      “I got to see firsthand how we can make change at a grassroots level and how that impacts where we are,” she says.

      Growing up in Howard County, Maryland, Li was constantly surrounded by nature. Her family made frequent trips to the Chesapeake Bay, as it reminded them of her parent’s home in Shandong, China. Li worked to bridge the cultural gap between parents, who grew up in China, and their children, who grew up in the U.S., and attended Chinese school every Sunday for 12 years. These experiences instilled in her a community-oriented mindset, which Li brought with her to MIT, where she now majors in materials science and engineering.

      During her first year, Li pursued a microbiology research project through the Undergraduate Research Opportunities Program (UROP) in the Department of Civil and Environmental Engineering. She studied microbes in aquatic environments, analyzing how the cleanliness of water impacted immunity and behavioral changes of the marine bacteria.

      The experience led her to consider the ways environmental policy affected sustainability efforts. She began applying the problem to energy, asking herself questions such as, “How can you take this specific economic principle and apply it to energy? What has energy policy looked like in the past and how can we tailor that to apply to our current energy system?”

      To explore the intersection of policy and energy, Li participated in the Roosevelt Project, through the Center of Energy and Environmental Policy Research, during the summer after her junior year. The project used case studies targeting specific communities in vulnerable areas to propose methods for a more sustainable future. Li focused on Pittsburgh, Pennsylvania, evaluating the efficiency of an energy transition from natural gas and fossil fuels to carbon-capture, which would mean redistributing the carbon dioxide produced by the coal industry. After traveling to Pittsburgh and interviewing stakeholders in the area, Li watched as local community leaders created physical places for citizens to share their ideas and opinions on the energy transition

      “I watched community leaders create a safe space for people from the surrounding town to share their ideas for entrepreneurship. I saw how important community is and how to create change at a grassroots level,” she says.

      In the summer of 2021, Li pursued an internship through the energy consulting firm Wood Mackenzie, where she looked at technologies that could potentially help with the energy transition from fossil fuels to renewable energy. Her job was to make sure the technology could be implemented efficiently and cost-effectively, optimizing the resources available to the surrounding area. The project allowed Li to engage with industry-based efforts to chart and analyze the technological advancements for various decarbonization scenarios. She hopes to continue looking at both the local, community-based, and external, industry-based, inputs on how economic policy would affect stakeholders.

      On campus, Li is the current president of the Sustainable Energy Alliance (SEA), where she aims to make students more conscious about climate change and their impact on the environment. During summer of her sophomore year, Li chaired a sustainability hackathon for over 200 high school students, where she designed and led the “Protecting Climate Refugees” and “Tackling Environmental Injustice” challenges to inspire students to think about humanitarian efforts for protecting frontline communities.

      “The whole goal of this is to empower students to think about solutions for themselves. Empowering students is really important to show them they can make change and inspire hope in themselves and the people around them,” she says.

      Li also hosted and produced “Open SEAcrets,” a podcast designed to engage MIT students with topics surrounding energy sustainability and provide them with the opportunity to share their opinions on the subject. She sees the podcast as a platform to raise awareness about energy, climate change, and environmental policy, while also inspiring a sense of community with listeners.

      When she is not in the classroom or the lab, Li relaxes by playing volleyball. She joined the Volleyball Club during her first year at MIT, though she has been playing since she was 12. The sport allows her to not only relieve stress, but also have conversations with both undergrads and graduate students, who bring different their backgrounds, interests, and experiences to conversations. The sport has also taught Li about teamwork, trust, and the importance of community in ways that her other experience doesn’t.

      Looking ahead, Li is currently working on a UROP project, called Climate Action Through Education (CATE), that designs climate change curriculum for K-12 grades and aims to show how climate change and energy are integral to peoples’ daily lives. Seeing the energy transition as an interdisciplinary problem, she wants to educate students about the problems of climate change and sustainability using perspectives from math, science, history, and psychology to name a few areas.

      But above all, Li wants to empower younger generations to develop solution-minded approaches to environmentalism. She hopes to give local communities a voice in policy implementation, with the end goal of a more sustainable future for all.

      “Finding a community you really thrive in will allow you to push yourself and be the best version of yourself you can be. I want to take this mindset and create spaces for people and establish and instill this sense of community,” she says. More