Don’t Try to Board a Plane With Peanut Butter
Sticky and solid as it may seem, the spread is technically a liquid.
This article was originally published by The Conversation.
These TSA requirements are drilled into every frequent flier’s head: Liquids may be carried on only in containers that are 3.4 ounces (100 milliliters) in volume or less.
But when the TSA recently confiscated a jar of Jif under this rule, peanut-butter lovers were up in arms. Some skeptics of security may suspect that hungry officers just wanted to make their own peanut-butter-and-jelly sandwiches. The TSA, however, contends that peanut butter is a liquid—and a full-size jar of Jif is over the 3.4-ounce limit.
Just like Americans’ beloved legume-based sandwich ingredient, the story—and the outrage it inspired—spread. However, I’m a mechanical engineer who studies fluid flows, and the TSA action made sense to me. By the scientific definition, peanut butter is indeed a liquid.
To define a liquid, we must first define a fluid. Any material that flows continuously when a shearing force is applied is a fluid. Think of a shearing force as a cutting action through a substance that causes it to flow continuously. For example, moving your arm causes the surrounding air to change shape—or deform, to use the physics term—and flow out of the way. The same thing happens to water when your arm takes a swim stroke.
There are many kinds of fluids. Some act very predictably and move smoothly, as air and water do. These are called Newtonian fluids, named after Sir Isaac Newton. Scientifically, a Newtonian fluid is one in which the shear force varies in direct proportion with the stress it puts on the material, known as the shearing strain. A Newtonian fluid’s resistance to fluid flow—that is, its viscosity—is constant at a given temperature.
Other types of fluids do not move quite as smoothly and easily. Some, like peanut butter, might require a minimum shearing or cutting force in order to start flowing, and this shearing force may vary nonlinearly with shearing strain. Imagine you’re stirring a jar of peanut butter. If you stir really fast, with more shearing force, the PB gets runnier; if you stir slowly, the PB remains stiff. These types of fluids are called non-Newtonian fluids. Peanut butter may stick more than flow—you could consider this movement more chunky-style.
Peanut butter is actually a great example of a non-Newtonian fluid: It doesn’t flow as easily as air or water, but it will flow if sufficient force is applied, such as when a knife spreads it on bread. How easily it flows will also depend on temperature—you may have experienced peanut-butter drips on warm toast.
Our everyday lives—but not our airplane carry-ons—are filled with substances that are unexpected fluids. In general, if it can flow, it’s a fluid. And it will eventually take the shape of its container.
Some surprising fluids are peanut butter’s kitchen neighbors, such as whipped cream, mayonnaise, and cookie batter. You’ll find others in the bathroom—toothpaste, for instance. The natural world is home to many strange fluids, including lava, mud, snow in an avalanche, and quicksand.
Gravel can be considered fluidlike. The individual particles are solids, but a collection of gravel particles can be poured and fill a container—it’s what’s called a “granular fluid,” because it has fluidlike properties. The same can be said of cereal poured out of a box or sugar into a bowl.
You could even consider a cat lying in the sun, flattened out and filling its containerlike skin, to be a fluid. Sleepy, relaxed dogs; squirrels; and even zonked-out babies can meet the definition of a fluid.
Now, you might be objecting: But the TSA didn’t call peanut butter a fluid; it said it’s a liquid!
Fluids are divided into two general categories: gases and liquids. Both gases and liquids can be deformed and poured into containers, and will take the shape of their container. But gases can be compressed, whereas liquids cannot—at least, not easily.
Peanut butter can be poured into a container, at which point it deforms, or takes the shape of that container. And every 5-year-old knows that peanut butter does not compress: When you squish your PB&J or peanut-butter crackers together, the peanut butter does not smoosh into a smaller volume. No—it squirts out the sides and onto your hands.
So, the verdict on peanut butter: delicious liquid. If you plan to make a PB&J sandwich mid-flight, count on bringing 3.4 ounces of liquid peanut butter or less. And the same goes for its liquid cousin, jelly.