What are Fluids?

A fluid is substance that can flow or continually deforms under an applied shear stress. A shear stress state is where the stress is parallel to the surface of the material. The term fluid therefore includes liquids and gas.

There are of course differences between liquids and gases. Gases are easily compressed whereas liquids are nearly incompressible. A liquid has definite size but a gas expands to fill any closed vessel containing it. However, in discussing the mechanical behavior of fluids we use only the properties of liquids and gases connected with their ability to flow. Therefore, the same basic laws control the static and dynamic behavior of both liquids and gases.

Unlike solids, fluids can change their shape readily, so their mechanical behavior cannot be described in terms of mechanics of a rigid body as is the case with solids.

There is a difference in the way a force acts on a fluid as opposed to a solid. A force can be applied to a single point of a solid and be sustained by it; but a force can only be applied to and sustained by a surface of an enclosed fluid. In a fluid at rest such a force is always directed at right angles to the surface. A fluid at rest cannot sustain a tangential force; the fluid layers would simply slide over one another when subject to such a force.
It is convenient therefore to describe the force acting on a fluid by specifying the pressure P, which is defined as the magnitude of the normal force per unit surface area.

P= Force/Area
Pressure is a scalar quantity.
Fluids display properties such as:

--not resisting deformation, or resisting it only lightly (viscosity), and
--the ability to flow (also described as the ability to take on the shape of the container).This also means that all liquids have the property of fluidity.
These properties are typically a function of their inability to support a shear stress in static equilibrium.

Solids can be subjected to shear stresses, and to normal stresses both compressive and tensile. In contrast, ideal fluids can only be subjected to normal, compressive stress which is called pressure. Real fluids display viscosity and so are capable of being subjected to low levels of shear stress.

In a solid, shear stress is a function of strain, but in a fluid, shear stress is a function of strain rate. A consequence of this behavior is Pascal's law which describes the role of pressure in characterizing a fluid's state.

Depending on the relationship between shear stress, and the rate of strain, fluids can be characterized as one of the following:

Newtonian fluids : where stress is directly proportional to rate of strain
Non-Newtonian fluids : where stress is not proportional to rate of strain

Rheology is the science of deformation and flow. One common factor between solids, liquids, and all materials whose behavior is intermediate between solids and liquid is that if we apply a stress or load on any of them they will deform or strain.

Test your Understanding:

1. The term fluids generally applies to:
a) gases only
b) liquids only
c) solids, liquids and gases
d) gases and liquids

2. What are the differences between gases and liquids
a) gases are easily compressed where liquids are not
b) liquids have a definite size where gases can expand to fill any closed container
c) gases have weak molecular force of attraction while liquids have a moderate force of attraction
d) All of the above

3. What statement best describes Non-Newtonian Fluids?
a) where stress is not proportional to rate of strain
b) they do not hold the shape of their container
c) where stress is directly proportional to rate of strain
d) classical example is water

4. Which of the following statements in not correct?
a) solids can be subjected to shear stresses
b) fluids display the property of flow
c) forces act on a fluid in the same way they act on a solid
d) a force can only be applied to and sustained by a surface of an enclosed fluid

5. The science of deformation of flow is called-
a) Physics
b) Rheology
c) Aerodynamics

See also:

Water Pressure Lab- What is the relationship between how far water squirts out of a hole in a container and the depth of the hole beneath the water?