Solid or Liquid? Non-Newtonian Fluids​

Solid or Liquid? Non-Newtonian Fluids

Can a substance behave both as a solid and as a liquid? It’s usually pretty easy to tell if a substance is solid or liquid. We take it in our hands for an assessment, if it flows, we call it liquid. If it does not flow and stands firm, we can classify it as solid. But this distinction is not always easy. Today we are going to discuss a confusing fluid group that changes its form under pressure.

Revolt of Fluid to the Fluid Regime: Viscosity

Yes, before we move on to non-Newtonian fluids, we need to explain one thing: Viscosity. This interesting name means the resistance of a liquid to fluidity. Liquids do the act of flowing, as we experience it every day. Like the flow example, we observe when pouring water from a jug into a glass. However, this flow rate and demand are not the same for every liquid. The flow rate of honey is much slower than water. In other words, the resistance of honey to flow is higher. Honey is more conservative and does not like to move much. When we explain this phenomenon scientifically, we say that “the viscosity of honey is higher than that of water“.

Honey Image

Newton’s Law of Anti-Pressure Viscosity

These fluids, which we will talk about in a moment, are not sure whether they are solid or liquid, and they do not like the law of viscosity. The law that these outlaws had to abide by was entered into the constitution by Isaac Newton. Basically, Newton’s law says that fluids must proceed in direct proportion at every point. This feed rate should not change with pressure. That is, the fluids should not rebel and continue to flow, no matter how much pressure there is. For example, consider a water bottle with a small hole in the cap. When we turn this bottle upside down, we do not apply any pressure, but it starts to flow and the flow rate is constant. Now let’s consider mayonnaise. Most of the time, when we open the lid and turn it upside down to squeeze on our fries, it does not want to flow. You need to shake it up and down quickly and then squeeze the bottle a little to get the mayonnaise moving.

Non-Newtonian Fluids

We learned that non-Newtonian fluids can change their fluidity based on pressure. These fluids look like liquids when viewed from the outside, but behave like solids on impact. In case of pressure, they are suddenly clamped together and act like a wall. But they are not always this nervous. If you approach it calmly, with slow movements, it behaves as a liquid, as if you were touching the water. Under pressure, they turn into a solid wall, but when spoken to, they turn into a cotton-hearted liquid.

Many of these fluids are actually shaped entirely by your approach. If you act harshly, you will receive a harsh response. If you come calmly, you will get a calm answer. Mayonnaise is also an example of non-Newtonian fluids. But this time it’s the opposite. While it becomes fluid under pressure, it has no intention of flowing if you don’t nudge it.

Examples of non-Newtonian fluids are:

Honey, ketchup, mayonnaise, toothpaste, shampoo, cornstarch-water mixture, pudding, and even slime…

Toothpaste Visual

How Tough Can They Be?

Okay, we’ve seen some of these fluids harden under pressure. But how tough can they be? There is no exaggeration in the analogy of a wall. Indeed, they can become indistinguishable from a wall. As you can see in the video below, it can create a solid ground that you can jump on. More interestingly, these fluids can even create a tough environment that is bulletproof. Pudding is a good example of non-Newtonian fluids. In this video, the bulletproof property of a bowl full of pudding is tested. And the results are pretty surprising.

In fact, this durability at impact makes these fluids potential safety equipment. Many researchers are investigating the use of these fluids in many pieces of equipment such as helmets and protective equipment.

Why Do They Behave Like This?

We can understand better why these fluids solidify under stress with an example from daily life. Suppose you are in a crowded environment right now and you want to cross the street. Or you have to pass through the people dancing on the floor during the wedding and reach across. If you start running in a hurry or go fast, you will definitely bump into someone dancing and pause. Striking a non-Newtonian fluid is similar. But if you go slowly, and calmly (perhaps you do a few fake dance moves as you cross the floor) you’ll pass people and reach your destination. Just as you can easily reach the bottom of the container by slowly immersing your hand in the non-Newtonian fluid.

We can say that the main reason for this unexpected behavior is the gaps in the structure of these fluids. Viscosity and fluid mechanics in general are very broad concepts. As mentioned above, even non-Newtonian fluids are divided into two: those that harden due to stress (dilatant) and, on the contrary, those that become fluid (pseudoplastic). It is not easy to explain the differences in the behavior of these fluids simply without going into terms and mathematical operations. If you are interested in the reasoning part, we recommend that you take a look at the “References and Further Readings” section at the end of the article.

You Can Make Your Own Non-Newtonian Fluid At Home, Too!

Our ingredients are just cornstarch and water. Yes, it’s that easy. You get a slurry mixture by taking a few spoonfuls of cornstarch and mixing it with a little water. If this slurry mixture looks like a liquid but acts like a hard solid when you hit it, congratulations! You now have a non-Newtonian fluid.

Have fun 🙂

References and Further Reading

Deshmukh, S. S., & McKinley, G. H. (2006). Adaptive energy-absorbing materials using field-responsive fluid-impregnated cellular solids. Smart Materials and Structures, 16(1), 106–113.

Eberhard, U., Seybold, H. J., Floriancic, M., Bertsch, P., Jiménez-Martínez, J., Andrade, J. S., & Holzner, M. (2019). Determination of the Effective Viscosity of Non-newtonian Fluids Flowing Through Porous Media. Frontiers in Physics, 7.

Wikimedia Foundation. (2021, May 10). Non-Newtonian fluid . Wikipedia, the free encyclopedia.,thus%20a%20Non%20Newtonian%20fluid.

You can access the sources of the images used by clicking on the images.

The proofreading has been done by Asu Pelin Akköse and Mete Esencan.

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Mete Esencan

Hello everyone! I'm Mete Esencan. I am a graduate student in the Department of Chemistry at METU. I was planning to establish a platform by combining the research knowledge I gained during my basic science education and the management experience I gained in the METU Chemistry Society, which I was in charge of for three years. For this purpose, in February of 2021, I took the first step and established the OkButWhy, a platform where we can write articles as if to chat about science, art and philosophy. I wish everyone a pleasant reading!

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