Mechanical engineers put an Oreo’s cream filling via a battery of assessments to grasp what occurs when two wafers are twisted aside.
When you twist an Oreo cookie open to get to the creamy heart, you’re mimicking a fundamental rheological take a look at. (Rheology is the examine of how a non-Newtonian materials flows when twisted, pressed, or in any other case strained.) MIT engineers have now subjected the sandwich cookie to rigorous supplies testing as a way to reply a vexing query: why does the cookie’s cream stick with just one wafer when twisted aside?
“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 search of a solution, the crew uncovered cookies to regular rheology experiments within the lab and found that, no matter taste or quantity of stuffing, the cream within the heart of an Oreo nearly at all times adheres to 1 wafer when twisted open. Only in older bins of cookies does the cream generally divide extra equally between the 2 wafers.
The researchers additionally measured the torque required to twist open an Oreo, and located it to be just like the torque required to show a doorknob and about 1/tenth what’s wanted to twist open a bottlecap. The cream’s failure stress — i.e. the drive per space required to get the cream to circulation, or deform — is twice that of cream cheese and peanut butter, and about the identical magnitude as mozzarella cheese. Judging from the cream’s response to emphasize, the crew classifies its texture as “mushy,” reasonably than brittle, powerful, or rubbery.
So, why does the cookie’s cream glom to 1 aspect reasonably than splitting evenly between each? The manufacturing course of could also be guilty.
“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 research the properties of complicated fluids. “Apparently that little time delay may make the cream stick better to the first wafer.”
The crew’s examine isn’t merely a candy diversion from bread-and-butter analysis; it’s additionally a chance to make the science of rheology accessible to others. To that finish, the researchers have designed a 3D-printable “Oreometer” — a easy system that firmly grasps an Oreo cookie and makes use of pennies and rubber bands to regulate the twisting drive that progressively twists the cookie open. Instructions for the tabletop system might be discovered right here.
The new examine, “On Oreology, the fracture and flow of ‘milk’s favorite cookie,’” seems as we speak in Kitchen Flows, a particular problem of the journal Physics of Fluids. It was conceived of early within the Covid-19 pandemic, when many scientists’ labs had been closed or tough to entry. In addition to Owens and Fan, co-authors are mechanical engineering professors Gareth McKinley and A. John Hart.
A normal take a look at in rheology locations a fluid, slurry, or different flowable materials onto the bottom of an instrument referred to as a rheometer. A parallel plate above the bottom might be lowered onto the take a look at materials. The plate is then twisted as sensors observe the utilized rotation and torque.
Owens, who frequently makes use of a laboratory rheometer to check fluid supplies similar to 3D-printable inks, couldn’t assist noting a similarity with sandwich cookies. As she writes within the new examine:
“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 might not seem to own fluid-like properties, it’s thought of a “yield stress fluid” — a delicate stable when unperturbed that may begin to circulation below sufficient stress, the best way toothpaste, frosting, sure cosmetics, and concrete do.
Curious as as to whether others had explored the connection between Oreos and rheology, Owens discovered point out of a 2016 Princeton University examine wherein physicists first reported that certainly, when twisting Oreos by hand, the cream nearly at all times got here 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 might repeat for a number of cookies of assorted fillings and flavors, the researchers glued an Oreo to each the highest and backside plates of a rheometer and utilized various levels of torque and angular rotation, noting the values that efficiently twisted every cookie aside. They plugged the measurements into equations to calculate the cream’s viscoelasticity, or flowability. For every experiment, additionally they famous the cream’s “post-mortem distribution,” or the place the cream ended up after twisting open.
In all, the crew went via about 20 bins of Oreos, together with common, Double Stuf, and Mega Stuf ranges of filling, and common, darkish chocolate, and “golden” wafer flavors. Surprisingly, they discovered that irrespective of the quantity of cream filling or taste, the cream nearly at all times 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, once they mapped every cookie’s consequence to its unique place within the field, they seen the cream tended to stay to the inward-facing wafer: Cookies on the left aspect of the field twisted such that the cream ended up on the appropriate wafer, whereas cookies on the appropriate aspect separated with cream totally on the left wafer. They suspect this field distribution could also be a results of post-manufacturing environmental results, similar to heating or jostling that will trigger cream to peel barely away from the outer wafers, even earlier than twisting.
The understanding gained from the properties of Oreo cream might doubtlessly be utilized to the design of different complicated fluid supplies.
“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 means that if the within of Oreo wafers had been extra textured, the cream may grip higher onto either side and break up extra evenly when twisted.
“As they are now, we found there’s no trick to twisting that would split the cream evenly,” Owens concludes.
Reference: “On Oreology, the fracture and flow of “milk’s favorite cookie®”” by Crystal E. Owens, Max R. Fan, A. John Hart and Gareth H. McKinley, 19 April 2022, Physics of Fluids.
This analysis was supported, partially, by the MIT UROP program and by the National Defense Science and Engineering Graduate Fellowship Program.