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Picking Up Where Sir Isaac Newton Left Off: Newtonian vs. Non-Newtonian Fluids

Posted by Mike Bonner

Nov 26, 2017 12:52:00 PM

Not every fluid behaves like water. This might seem like a commonsense idea, but there’s quite a bit of physics behind it — it sums up the difference between Newtonian and non-Newtonian fluids.

If you’re a manufacturer dealing with printing, painting, coating, or other finishing processes, you likely already understand just how important it is to be monitoring your fluid’s viscosity using a viscometer.

Tracking viscosity is simple enough, but it gets more complicated when looking at the differences between Newtonian and non-Newtonian fluids — and these differences can affect how you approach your process.

Let’s dive in.

A brief history lesson

history written on blackboard

Newtonian fluids and non-Newtonian fluids are so named because of physicist Sir Isaac Newton. Newton, among his many well-known accomplishments (including the discovery of gravity), also discovered the basic principles of viscosity.

Newton’s observations led him to believe that a fluid’s viscosity was simply a function of shear stress and temperature — and that it would remain unchanged no matter the shear rate. In other words, Newton thought that no amount of stirring or similar actions would change the viscosity of a fluid.

If you’ve eaten yogurt recently, you know that this isn’t quite the case. Stirring your yogurt makes it less thick. On the other hand, you can stir water endlessly and its thickness or thinness will never change — so what gives?

The answer is that Newton only had half the picture.

What’s the difference?

There are a range of fluids that don’t behave according to his observations. These are aptly categorized as non-Newtonian fluids, and they have different properties than Newtonian fluids. Instead of changing viscosity according to temperature, they thicken or thin in response to the amount of force applied to them. Examples include:

  • Yogurt
  • Honey
  • Ketchup
  • Cream
  • Corn flour
  • Toothpaste
  • Ink
  • Paint

The fluids that do behave according to Newton’s theory are known as Newtonian fluids, and include:

  • Water
  • Alcohol
  • Gasoline
  • Thin motor oil
  • Mineral oil

Types of non-Newtonian fluids

Non-Newtonian fluids come in a number of different types: thixotropic, rheopectic, pseudoplastic, and dilatant.

Thixotropic: In thixotropic fluids, force applied over time decreases viscosity, or makes the fluid become thinner. An example of a thixotropic fluid is honey, as the more you stir it, the thinner it becomes.

Rheopectic: In rheopectic fluids, force applied over time increases viscosity, or makes the fluid become thicker. An example of a rheopectic fluid is cream, as the more you stir it, the thicker it becomes — simply stirring it once does nothing.

Pseudoplastic (shear thinning): In pseudoplastic (also known as shear thinning) fluids, applied force decreases viscosity, and it does so on a much more immediate time scale than force applied to thixotropic fluids. An example of a pseudoplastic fluid is ketchup (remember the last time you shook a ketchup bottle?).

Dilatant (shear thickening): In dilatant (also known as shear thickening) fluids, applied force increases viscosity, and much more quickly than with rheopectic fluids. An example of a dilatant fluid is a suspension of corn flour dissolved in water, which creates a substance called oobleck. Amazingly, if you move quickly enough, you can walk across a pool filled with oobleck, as seen in this video.

How does it affect your processes?

Non-Newtonian fluid viscosity can’t always be measured by everyday viscometers, though this is also dependent on what specific measurement you need. If you’re working with a non-Newtonian fluid and need to measure its viscosity, you might need a rheometer. However, in many cases, a well-placed viscometer capable of imparting shear on the fluid is more than sufficient.

If you need advice on how best to measure viscosity in your particular application, contact Saint Clair Systems.

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Topics: Viscosity, viscometers, Viscometer Series, newtonian fluids, non-newtonian fluids

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