Frequent air travellers may be familiar with the warnings that they can only carry liquids with a volume of less than 100 milliliters. But peanut butter lovers rioted when the U.S. Transportation Security Administration (TSA) recently confiscated a jar of peanut butter under this rule. Some skeptics may suspect that hungry officers just want to feed themselves. But the TSA claims that peanut butter is a liquid, and a full-size jar is over the 100-milliliter limit.
Is peanut butter liquid?
Although this story quickly spread and caused a lot of controversy on the internet, from a scientific definition point of view, we can say that peanut butter is indeed a liquid.
But in order to define whether something is a liquid or not, we first need to define what a fluid is. Fluids are defined as any material that flows continuously when a shear force is applied. You can think of shear as a shearing action that causes matter to flow continuously. For example, moving your arm causes the surrounding air to deform and flow out of your arm’s path. This behavior can be more easily seen in the movement of the water during swimming.
There are many types of fluids. Some move very predictably and move smoothly, just like air or water. These are called Newtonian fluids, named after Sir Isaac Newton. A Newtonian fluid is, scientifically speaking, fluids in which the shear force varies in direct proportion to the stress (shear stress) exerted on the material. In a Newtonian fluid, the substance’s resistance to flow (viscosity) is constant at a given temperature.
Other types of fluids do not move so smoothly and easily. In the case of some substances, such as peanut butter, a minimum shear force may be required to keep it flowing, and this can vary non-linearly with shear stress. Imagine mixing a jar of peanut butter. If you mix too quickly with more shear force, the peanut butter will become more runny, while if you mix it slowly, the peanut butter will stay firm. Such fluids are called non-Newtonian or non-Newtonian fluids. Peanut butter’s flowing behavior can also be seen as more of a sticky behavior than a normal flow.
Peanut butter would be a great example of a non-Newtonian fluid, as it doesn’t flow as easily as air or water, but is runny when applied with enough force, such as when spread on bread with a knife. How easily it flows will also depend on the temperature. For example, you can apply it more comfortably on a warm bread and then you can see it dripping from the bread like water.
Our daily lives are filled with unexpectedly fluid substances. In general, if a substance can flow, it is a fluid. And eventually it will take the shape of the container it is in.
Can all fluids be considered liquids?
Some surprising fluids like cream, mayonnaise, and cookie dough are other examples from the kitchen, like peanut butter. You can also find other everyday examples, such as toothpaste. Other strange fluids such as lava, mud floods, avalanches, and swamp sand are found in the natural world.
At this point, some objections may arise as the TSA has defined peanut butter as a liquid, not a liquid.
Fluids are divided into two general categories, gases and liquids. Both gases and liquids can be deformed and poured into containers and take the shape of the container they are in. But gases are compressible, while liquids are (at least easily) incompressible.
Peanut butter can be poured into the container and deformed or take the shape of that container. And you probably know that peanut butter can’t be compressed easily. If you put the peanut butter between two pieces of bread or crackers and press it, it will flow out the sides with almost no jamming.
So we can safely say that peanut butter is not a flowing gas (probably without the need for such an experiment). For this reason (and for other reasons), the other type of fluid, liquid, is among the fluids, meaning it is a delicious liquid.
So liquid bans on airplanes apply to peanut butter as well as other tasty options like jelly…
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