The short answer

Peanut butter straddles the line between solids and liquids based on the forces applied to it. But since peanut butter has a fixed volume, takes the shape of its container, and can flow under applied force, it is best described as a liquid rather than a solid.

The long answer

On a trip to Detroit many years ago, I suffered a great tragedy.

The TSA confiscated my half-eaten jar of peanut butter I had packed in my carry-on. Apparently it fell under the TSA's definition of a liquid, and I was forced to abandon my peanut travel companion.

But is the TSA correct? Is peanut butter really a liquid?

What state of matter is peanut butter?

Let's begin with a refresher on states of matter, which are the distinct forms in which matter can exist.

• Solids have both a fixed volume and shape, with molecules packed closely in a stable arrangement.

• Liquids have a fixed volume and variable shape depending on the container. The molecules in a liquid are close together with no regular arrangement and can move past one another freely, which allows the liquid to flow and change shape.

• Gases have neither a fixed volume nor fixed shape. The molecules in a gas are much farther apart than in solids and liquids and move independently of each other, which allows gases to spread out freely to fill the space available to them.

So which state of matter does peanut butter fall under?

Technically, peanut butter is a heterogeneous mixture, with solid peanut particles suspended in liquid oil. In other words, peanut butter is a substance containing both liquid and solids.

Is peanut butter more like a liquid or a solid?

But "both" is not a satisfying answer, certainly not for the TSA which has taken the side of peanut butter being a liquid. So then the question becomes: Does peanut butter act more like a liquid or more like a solid?

To answer that question, we need to dive into the world of fluids.

A fluid is a substance that flows when a shearing force, such as a cutting action, is applied. For example, when a swimmer's arm cuts through water, it causes the water to flow.

One key difference between solids and liquids is that liquids are fluids, meaning they can flow. A solid cannot be a fluid because its tightly packed molecules prevent it from changing shape unless a force physically deforms it.

There are many types of fluids, but two common ones are Newtonian and non-Newtonian fluids.

Newtonian fluids, like water and air, act predictably in that their viscosity (i.e. flow rate) is constant. For example, water flows out of a glass the same whether or not you've stirred it beforehand.

Non-Newtonian fluids, on the other hand, have variable viscosity that changes as shearing force is applied to it. There are two types of non-Newtonian fluids:

1. Shear-thickening fluids: These fluids become more viscous (i.e. thicker) the more you agitate them. A great example of this is oobleck, or corn starch and water, which starts acting as a solid the more force you apply to it (​you can even run across it without sinking​).

2. Shear-thinning fluids: These fluids become less viscous (i.e. thinner) the more you agitate them. An example of this is ... spoiler alert ... peanut butter! The more you stir peanut butter, the runnier it gets and the easier it is to flow. But once you stop stirring it and let it settle, *peanut butter will act more like a solid again. ​Here's a short, simple video showing how peanut butter's flow changes depending on the force applied​.

So to recap, peanut butter is considered a non-Newtonian fluid because its viscosity is variable depending on how much and how long you stir it. Being a fluid, it acts more like a liquid than a solid (even though, technically speaking, it's a mixture of both states of matter).

Therefore, peanut butter can best be described as a liquid. And the TSA will definitely take it from you.

🧠 Bonus brain points

Peanut butter is what's known as a colloid, a chemical term for a substance consisting of insoluble particles (peanut) suspended in another substance (peanut oil).

Interestingly, if you ventured into the peanut butter business, you would need a piece of equipment called a colloid mill. I don't know if anyone else will find this fascinating, but I did!

Check out some other curious questions:

Sources

Alexandra, E. (2020, May 21). Non-Newtonian Fluid: A Homemade Peanut Butter Activity. Kid Minds. https://kidminds.org/homemade-peanut-butter/

Althouse, A. (2023, May 8). Peanut butter is a liquid. https://althouse.blogspot.com/2023/05/peanut-butter-is-liquid.html

Hall, N. (Ed.). (2021, May 13). Phases of Matter. NASA. https://www.grc.nasa.gov/www/k-12/airplane/state.html

Heindel, T. (2023, May 9). Here’s the Weird Physics That Makes Peanut Butter a Liquid. Scientific American. https://www.scientificamerican.com/article/heres-the-weird-physics-that-makes-peanut-butter-a-liquid/

Joyner, H. (2018, August 24). Yield stresses and peanut butter (rheology video example). YouTube. https://www.youtube.com/watch?v=MF933oquQFM&ab_channel=HelenJoyner

Turnbull, B. L., Sealion, D., & AbsolutePixel. (2023, May 14). Food Theory: The Hidden DANGERS of Peanut Butter!. YouTube. https://www.youtube.com/watch?v=K7yvSFXsK3w

The University of Waikato Te Whare Wānanga o Waikato. (2010, April 12). Non-Newtonian fluids. Science Learning Hub. https://www.sciencelearn.org.nz/resources/1502-non-newtonian-fluids