The essay below 
The Conversation, an online publication covering the latest research.
These Transportation Security Administration requirements are on the minds of all frequent flyers. He can carry liquids with a capacity of less than 3.4 ounces (100 milliliters) each.
But when the TSA recently confiscated Jif jars under this rule, peanut butter lovers were outraged. Some security skeptics might suspect that the hungry cops just wanted to build their own PB&Js. However, the TSA claims that peanut butter is liquid and that Jif’s full-size jar exceeds his 3.4-ounce limit.
Much like Americans’ favorite legume-based sandwich ingredients, the story and the outrage it caused began to spread. However, I am a mechanical engineer who studies fluid flow and the TSA action makes sense to me. By scientific definition, peanut butter is indeed liquid.
Think fluid first
To define a liquid, we first need to define a fluid. Any substance that flows continuously when a shear force is applied is a fluid. Think of shear as a cutting action that causes a material to flow continuously. For example, moving your arm will change the shape of the air around you (deform, to use the physics term). The same thing happens in the water when the arm swims and swims.
There are many types of fluids. Some, like air and water, move very predictably and smoothly. These are called Newtonian fluids, named after Sir Isaac Newton. Scientifically, a Newtonian fluid is a fluid whose shear force varies in direct proportion to the stress (known as shear strain) exerted on the material. For Newtonian fluids, the fluid’s resistance to flow, ie its viscosity, is constant at a given temperature.
Other types of fluids do not move so smoothly and easily. Like peanut butter, it may require minimal shear or cutting force to flow, and may vary non-linearly with shear strain. Imagine stirring a jar of peanut butter. If you stir very quickly with high shear, the PB will become more fluid, while if you stir slowly, the PB will remain firm. These types of fluids are called non-Newtonian fluids. Peanut butter is more likely to stick than flow.
Peanut butter is actually a good example of a non-Newtonian fluid. It doesn’t flow as easily as air or water, but it does if enough force is applied, such as when spread on bread with a knife. Ease of flow also depends on temperature. You may have experienced peanut butter dripping after you put it on warm toast.
We’re surrounded by strange liquids
Our daily lives are filled with substances that are unexpected liquids, with the exception of our carry-ons. In general, if it can flow, it is a fluid. And finally it will be in the form of a container.
There are also some surprising liquids next to peanut butter, such as whipped cream, mayonnaise, and cookie dough. Like toothpaste, there are other things in the toilet. Other strange fluids exist in nature, such as lava, landslides, avalanches, and quicksand.
Gravel can be considered fluid. Although the individual particles are solid, a collection of gravel particles can be poured to fill a container. This is called a granular fluid because it has fluid-like properties. The same goes for cereal and sugar that you pour out of the box and into a bowl.
Traffic flows on busy highways and people flow out of crowded sports venues.
When a cat lying in the sun flattens and fills its skin like a container, it can even be considered liquid. can satisfy the definition of
Liquid is a kind of fluid
Now, you may object: But the TSA didn’t call peanut butter a liquid.
Fluids fall into two general categories: gases and liquids. Both gases and liquids can be deformed and poured into a container, taking the shape of the container. Gases are compressible, but liquids are not. At least it’s not easy.
When you pour peanut butter into that container, it deforms or takes on the shape of that container.And any five-year-old knows that peanut butter doesn’t compress. If you crush the peanut butter too tightly, it won’t shrink down smoothly. No, it pops into your hand from the side.
So, the peanut butter verdict: delicious liquid.
If you plan to make a PB&J sandwich on board, bring less than 3.4 ounces of liquid peanut butter. The same is true for its liquid cousin, jelly.
This article originally appeared on The Conversation. Please read the original article.
