# Fluid Definition and Examples A fluid is a material that flows under a shear force. Most fluids are liquids or gases. Examples include air and water.

A fluid is a material that flows or continuously deforms under a shear (tangential stress). In other words, a fluid has zero shear modulus.

Liquids, gases, and plasma are fluids. However, some solids behave as fluids as well. For example, pitch is a solid with a high viscosity that (very) slow flows. Silly Putty flows, but solidifies under a sudden force. Generally, solids are not fluids because they resist tangential stress and only deform to a point before reaching static equilibrium.

In biology, the definition of fluid includes the physical science definition, but also refers to body liquids, such as blood, plasma, and urine. Liquids given to replace body fluids, such as physiological saline solution and juice, are also fluids in this context.

### Ideal Fluid vs Real Fluid

An ideal fluid is incompressible and has no viscosity. In other words, it has constant density and there is no internal friction between layers. Ideal fluids flow without turbulence. A real fluid is a fluid with some viscosity and compressibility. Ideal fluids are imaginary — all actual fluids are real fluids.

### Examples of Fluids

Any liquid, gas, or plasma you can name is an example of a fluid. Some materials that appear solid are also fluids.

• Water
• Air
• Blood
• Honey
• Milk
• Oil
• Shampoo
• Mercury
• Gasoline
• Coffee
• Helium
• Quicksand
• Oobleck
• Mayonnaise

### Properties of Fluids

Fluids display two main properties:

• Fluids flow and take the shape of their container. Note, they do not necessarily fill the volume of the container.
• Fluids resist permanent deformation. If you poke a water or disturb air, it doesn’t stay where you put it.

### Types of Fluids

Two ways of classifying fluids are by viscosity and compressibility.

• Newtonian fluid – A Newtonian fluid is fluid that obeys Newton’s law of viscosity. It is a viscous fluid where the stress is directly proportional to the strain. Most familiar liquids and gases are Newtonian fluids.
• Non-Newtonian fluid – A non-Newtonian fluid does not obey Newton’s law of viscosity. Stress is not directly proportional to strain, so viscosity is not constant. Examples of non-Newtonian fluids include oobleck, ketchup, and yogurt. Applying a force or stress to these fluids changes their viscosity.
• Compressible fluid – A compressible fluid is one that experiences a reduction in volume or change in density under pressure. Gases and plasma are compressible fluids.
• Incompressible fluid – An incompressible fluid does not change its volume in response to changes in pressure or flow velocity. For the most part, liquids such as oil and water are incompressible fluids. However, they are not perfectly incompressible. Under enough pressure, real liquids do slightly compress.

### Superfluids

A superfluid is a special type of fluid that has zero viscosity so that no kinetic energy is lost when it flows. An interesting consequence is that superfluids climb or “creep” up container walls. Liquid helium-3 and helium-4 are examples of superfluids. Some Bose-Einstein condensates and ultracold atomic gases display superfluidity.

### Quiz Yourself

Do you think you understand what a fluid is? Which of the following statements is the definition of a fluid? A fluid is…

• a substance that fills the volume of its container.
• a material in the liquid state of matter.
• matter that flows because it has pressure acting upon it.
• matter that deforms under a normal stress.
• a substance that continuously deforms under a shear or tangential stress.

The first four definitions are incorrect.

• First, a fluid does not always fill its container. Water is a fluid, yet if you pour a cup of water into a bucket it doesn’t expand to fill the container.
• Liquids are not the only fluids. Gases, plasma, and some solids are fluids.
• Pressure (a normal force) isn’t necessarily the force that causes a fluid to flow. For example, if you have a ball of water on the space station that is exposed to atmospheric pressure it just sits there.
• Some fluids deform under a normal stress, while some do not. Gases deform under normal stress. Liquids generally don’t.

The final definition is the correct one. A fluid continuously deforms under a shear stress. The key points are that deformation is continuous and that the applied stress is tangential or shear.

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• Khalatnikov, Isaac M. (2018). An introduction to the theory of superfluidity. CRC Press. ISBN 978-0-42-997144-0.
• Thayer, Ann (2000). “What’s That Stuff? Silly Putty“. C&EN (Chemical & Engineering News). American Chemical Society. 78 (48): 27.
• White, Frank M. (2011). Fluid Mechanics (7th ed.). McGraw-Hill. ISBN 978-0-07-352934-9.