It’s a familiar cookie conundrum: You take a pristine-looking Oreo from a package of seemingly identical sandwich cookies, but what now? Should you take a bite? Dunk it in milk? Or open it up to eat the creme filling first? You decide on the last option, and you gently twist the cookie apart without breaking the chocolate wafers. As it so often does, the creme sticks to one side only (Fig. 1). Now that you think about it, you realize Oreo creme never seems to end up equally distributed between the two sides. What causes this spontaneous symmetry breaking? Is it determined by the creme or the cookie? Happily, the physics of fluids helped two MIT students solve this delicious mystery, and along the way, they created the new field of Oreology!
But before there was Oreology, there was rheology—the branch of physics investigating how matter flows. You probably don't think of Oreo creme as "flowing" at all, but it belongs to a fun and frisky class of materials that scientists know as non-Newtonian fluids, which really ought to be called "what-the-fluids?!?"—WTFs, if you will. Like other WTFs, Oreo creme is neither a solid nor a liquid. When you start twisting the cookie, the creme first deforms like a solid. If you twist it far enough, however, the creme will fail at a specific yield stress—the satisfying moment when you can pull the cookie apart!—which is the amount of force distributed over the area of the cookie (Fig. 2). This property puts Oreo creme in the same family as other fluids like toothpaste and creamy peanut butter that only move when enough force is applied to them.
To understand exactly how and why this happens, mechanical engineers Crystal Owens and Max Fan used a rheometer, which is a tool used to measure the flow properties of complex fluids (Fig. 3). In a typical experiment, fluid is placed between two parallel plates. As the top plate rotates, the fluid in the middle resists this motion (sound familiar?), creating a rotational force on the upper plate that can be measured precisely. Based on these same principles, Crystal and Max designed an “Oreometer” in which the wafers themselves act as the parallel plates and the creme filling as the resisting fluid.
The team found that the yield stress of Oreo creme is about twice that of peanut butter or cream cheese. They also tested a number of other variables. Nowadays, the grocery aisles are filled with a variety of Oreo flavors, and Crystal and Max found that most of these flavors have the same yield stress. They also found that the amount of “stuffing” (regular, double, mega) didn’t significantly change the amount of twisting needed. Although failure could occur in the middle of the creme, splitting it in half on both sides of the cookie, the predominant failure mechanism was "adhesive failure" at one of the cookie-creme interfaces, leading to all the creme on one of the two chocolate wafers, no matter how much creme was in the cookie.
But why? Crystal and Max uncovered an important clue when they noticed that the creme would typically end up on the same side (left or right) for all the cookies in a given package. Aha! Since cookies are assembled and placed into the package in the same orientation, they realized the asymmetry must come from the manufacturing process (Fig. 4). They hypothesized that the creme was likely to stick better to the wafer on which it was initially deposited (the "bottom" wafer at the time of manufacturing), and less well to the second wafer subsequently placed on top of the creme. You can test this imprinted memory of cookie birth at home, but it might require eating lots of Oreos to get good statistics!
If you're really serious about becoming an Oreologist, Crystal and Max have developed a 3D-printed “Oreometer” so their science can be replicated at a fraction of the cost of an expensive laboratory rheometer (Fig. 5). You can read all about it here, and find the 3D-printable files here!
Crystal and Max also tested the effects of dipping the cookie in milk, using their rheometer to compare the structural properties of an intact cookie, a single wafer with creme, and a single wafer without creme as they absorbed milk. Further research is needed, they suggest, to find the optimum dunking time to saturate the cookie with milk without it falling apart, and to determine how properties of the milk like fat content influence this process. Who knew science could be so delicious?