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.
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.
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.
Written by Jennifer Chu, MIT News Office
Paper: “On Oreology, the fracture and flow of ‘milk’s favorite cookie’”
Physics of Fluids
“On Oreology, the fracture and flow of ‘milk’s favorite cookie"
Unapofungua keki ya Oreo ili ufikie katikati ya cream, unaiga mtihani wa kawaida katika rheology - utafiti wa jinsi nyenzo isiyo ya Newton inavyotiririka inapofungwa, kushinikizwa, au kusisitizwa kwa njia nyingine. Wataalamu wa MIT wametumia teknolojia ya kipekee ya kipekee ya kipekee ya kipekee ya kipekee ya kipekee ya kipekee ya kipekee ya kipekee ya kipekee ya kipekee ya kipekee ya kipekee ya kipekee ya kipekee ya kipekee ya kipekee ya kipekee ya kipekee ya kipekee ya kipekee ya kipekee. "Kuna tatizo la kuvutia la kujaribu kupata cream kusambaza sawasawa kati ya wafers mbili, ambayo inageuka kuwa ngumu sana", anasema Max Fan, mwanafunzi wa shahada ya kwanza katika Idara ya Uhandisi wa Mashine ya MIT. Ili kupata jibu, timu hiyo ilifanya kuki zifanyiwe vipimo vya kawaida vya rheology katika maabara na kugundua kwamba bila kujali ladha au kiasi cha kujaza, cream katikati ya Oreo karibu daima huambatana na wafer moja wakati wa kufunguliwa. Ni kwa masanduku ya zamani tu ya biskuti ambayo nyakati nyingine cream hutenganishwa kwa usawa zaidi kati ya vipande vyote viwili. Watafiti pia walipima torque inayohitajika kugeuka Oreo, na kugundua kuwa sawa na torque inayohitajika kugeuka mlango na karibu 1 10th ya kile kinachohitajika kugeuka kufungua cap ya chupa. Cream <unk>s kushindwa mkazo <unk> yaani Kwa mfano, nguvu kwa eneo linalohitajika ili kupata cream kutiririka, au deform <unk> ni mara mbili ya cream jibini na siagi ya karanga, na kuhusu ukubwa sawa na jibini mozzarella. Kwa kuzingatia jinsi krimu hiyo inavyoitikia mkazo, timu hiyo inataja muundo wake kuwa "mwenye umajimaji", badala ya kuwa dhaifu, mgumu, au kama mpira. Kwa hiyo, kwa nini cream ya biskuti hujipinda upande mmoja badala ya kugawanyika kwa usawa kati ya pete zote mbili? Huenda utaratibu wa utengenezaji ukawa na hatia. "Video za mchakato wa utengenezaji zinaonyesha kwamba wao huweka wafer ya kwanza chini, kisha kutoa mpira wa cream juu ya wafer hiyo kabla ya kuweka wafer ya pili juu, ""anasema Crystal Owens, mgombea wa PhD wa uhandisi wa mashine wa MIT ambaye hujifunza mali ya kioevu tata. """ "Inaonekana kwamba kuchelewesha kidogo kunaweza kufanya cream kushikamana vizuri na wafer ya kwanza. ""Utafiti wa timu si tu kupotosha tamu kutoka kwa utafiti wa mkate na siagi; pia ni fursa ya kufanya sayansi ya rheology kupatikana kwa wengine. """ Kwa ajili ya hili, watafiti wamebuni 3D-printable Oreometer, kifaa rahisi ambayo imara grabs Oreo cookie na hutumia senti na rubber bands kudhibiti nguvu twisting kwamba hatua kwa hatua twists cookie wazi. Maagizo kwa ajili ya kifaa tabletop inaweza kupatikana hapa. Utafiti huo mpya, "On Oreology, the fracture and flow of ‘milk favorite cookie," umechapishwa leo katika Kitchen Flows, toleo maalum la jarida la Physics of Fluids. Ilianzishwa mapema katika janga la COVID-19, wakati maabara ya wanasayansi wengi zilikuwa zimefungwa au ngumu kupata. Mbali na Owens na Fan, waandishi ni maprofesa wa uhandisi wa mashine Gareth McKinley na A. John Hart. Mtihani wa kawaida katika rheology huweka kioevu, slurry, au nyenzo nyingine inayoweza kutiririka kwenye msingi wa chombo kinachojulikana kama rheometer. sahani sambamba juu ya msingi inaweza kushuka juu ya nyenzo mtihani. Kisha sahani hiyo huzungushwa huku sensorer zikifuatilia mzunguko na torque inayotumiwa. Owens, ambaye hutumia mara kwa mara rheometer ya maabara kupima vifaa vya kioevu kama vile wino wa kuchapisha 3D, hakuweza kusaidia kugundua kufanana na biskuti za sandwich. "Kama anaandika katika utafiti mpya: ""Kuna sayansi, biskuti sandwich kuwasilisha mfano paradigmatic ya sambamba sahani rheometry katika ambayo sampuli ya kioevu, cream, ni uliofanyika kati ya sahani mbili sambamba, wafers." Wakati waffles ni counter-rotated, cream deforms, mtiririko, na hatimaye fractures, na kusababisha kutenganishwa kwa cookie katika vipande viwili. wakati Oreo cream inaweza kuonekana kuwa na mali kama kioevu, ni kuchukuliwa "kuzalisha mvutano kioevu" - laini imara wakati unperturbed ambayo inaweza kuanza mtiririko chini ya mkazo wa kutosha, njia toothpaste, frosting, baadhi ya vipodozi, na saruji kufanya. Owens, ambaye pia ni mtaalamu wa Oreo, alishangaa kuona ikiwa kuna uhusiano kati ya Oreo na rheology, na alipata kumbukumbu ya utafiti wa Chuo Kikuu cha Princeton mnamo 2016 ambapo wataalamu wa fizikia waliripoti kwa mara ya kwanza kwamba wakati wa kugeuza Oreo kwa mkono, cream karibu kila wakati iliondoka kwenye wafer moja. "Tulitaka kujenga juu ya hii kuona nini kweli husababisha athari hii na kama tunaweza kudhibiti kama sisi imewekwa Oreos kwa uangalifu juu ya rheometer yetu", anasema. Katika jaribio ambalo watafiti walirudia kwa kuki nyingi za kujaza na ladha tofauti, walishikilia Oreo kwenye sahani za juu na za chini za rheometer na kutumia viwango tofauti vya torque na mzunguko wa pembe, wakigundua maadili ambayo yalifanikiwa kugeuza kila keki. Waliingiza vipimo hivyo katika equations ili kuhesabu viscoelasticity ya cream, au kuweza kutiririka. Kwa kila jaribio, pia waliona usambazaji wa cream baada ya kifo, au ambapo cream ilimalizika baada ya kugeuka wazi. Kwa jumla, timu hiyo ilichunguza masanduku 20 ya Oreo, ikiwa ni pamoja na viwango vya kawaida, Double Stuff, na Mega Stuff vya kujaza, na ladha za kawaida, chokoleti nyeusi, na waffle za dhahabu. Kwa kushangaza, walipata kwamba haijalishi kiasi cha cream kujaza au ladha, cream karibu daima kutengwa juu ya wafer moja. "Tulikuwa tunatarajia athari kulingana na ukubwa", Owens anasema. <unk>Ikiwa kulikuwa na cream zaidi kati ya tabaka, inapaswa kuwa rahisi kugeuka. Lakini hiyo si kweli kesi. curiously, wakati wao ramani kila cookie matokeo ya nafasi yake ya awali katika sanduku, waligundua cream alikuwa na mwelekeo wa kushikamana na ndani-kuangalia wafer: Cookies upande wa kushoto wa sanduku twisted hivyo kwamba cream kumalizika juu ya wafer haki, wakati cookies upande wa kulia kutenganishwa na cream hasa juu ya wafer kushoto. Wanasema kwamba kuenea kwa sanduku hili kunaweza kuwa matokeo ya athari za mazingira baada ya utengenezaji, kama vile joto au kushinikiza, ambayo inaweza kusababisha cream kuondoa kidogo kutoka kwa wafers za nje, hata kabla ya kugeuka. Uelewa uliopatikana kutoka kwa mali ya cream ya Oreo unaweza kutumika kwa muundo wa vifaa vingine vya kioevu tata. "Mvuke wangu wa uchapishaji wa 3D uko katika darasa moja la vifaa kama cream ya Oreo", anasema. "Kwa hivyo, uelewa huu mpya unaweza kunisaidia kubuni wino bora wakati ninajaribu kuchapisha vifaa vya elektroniki vya kubadilika kutoka kwa mchanganyiko wa nanotubes za kaboni, kwa sababu zinabadilika kwa njia sawa kabisa. ""Kuhusu cookie yenyewe, anapendekeza kwamba ikiwa ndani ya wafers za Oreo zilikuwa na muundo zaidi, cream inaweza kugawanyika vizuri pande zote mbili na kukamata sawa wakati wa kupotoshwa." "Kama ilivyo sasa, tuligundua hakuna ujanja wa kugeuza ambayo ingegawanya cream kwa usawa", Owens anamalizia. Utafiti huu uliungwa mkono, kwa sehemu, na programu ya MIT UROP na na Programu ya Ushirikiano wa Chuo Kikuu cha Sayansi na Uhandisi wa Ulinzi wa Kitaifa. Imeandikwa na Jennifer Chu, MIT News Office Karatasi: <unk>On Oreology, kuvunjika na mtiririko wa <unk>maziwa<unk> favorite cookie <unk> Physics of Fluids <unk>On Oreology, kuvunjika na mtiririko wa <unk>maziwa<unk> favorite cookie <unk>