Fluids Defined

What exactly is a fluid? It helps to know the characteristics!

  1. Fluids are a state of matter in which a substance cannot hold a shape.
  2. Fluids can be a liquid, a gas, or air.
  3. Newtonian fluids flow easily and continually under an applied stress.
  4. Liquids are said to flow in a continuum.
  5. Some fluids, such as air and gas, are said to be compressible.

Some fluids are Newtonian. These fluids have a:

  1. linear relationship between the shear stress  and the shear rate.
  2. definable coefficient of viscosity.
  3. constant coefficient of viscosity.
  4. predictable behavior.

Some fluids are Non-Newtonian. These fluids:

  1. do not behave like Newtonian fluids.
  2. are subject to shear stress from other factors such as temperature and pressure.
  3. coefficient of viscosity  that is not possible to calculate.
  4. have unpredictable behavior.

Wait! There’s more!


Aristotle (384-311 BC) was one of the greatest philosophers and scientists of all time. He didn’t focus his attention on the specific study of fluids in motion, but many of his observations of fluids and objects in nature play a role in our current understanding of fluids in motion. For instance, Aristotle correctly surmised that a fluid in a container completely fills the space it occupies, with an observable surface at the top if the container is not completely filled. He also understood that “something” was at work to bring a moving object to a stop. Today we refer to the former concept as continuum and the latter as resistance. Both are important in understanding fluids in motion. Continuum explains why a fluid can be tested at any point in the fluid. Resistance is fundamental to an appreciation of viscosity, the internal friction of a liquid.

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