3. Characteristics of Fluids
The principal difference in the mechanical behavior of fluids compared to solids is that
when a shear
stress is applied to a fluid it experiences a continuing and permanent distortion. Fluids
offer no permanent resistance to
shearing, and they have elastic properties only under direct compression: in contrast to
solids which have all three elastic moduli, fluids possess a bulk modulus only.
Thus, a fluid can be defined unambiguously as a material that deforms continuously
and permanently under the application of a shearing stress, no matter how small.
definition does not address the issue of how fast the deformation occurs and as we shall
see later this rate is dependent on many factors including the properties of the fluid
itself. The inability of fluids to resist shearing stress gives them their characteristic
ability to change shape or to flow; their inability to support tension stress is
an engineering assumption, but it is a well-justified assumption because such stresses,
which depend on intermolecular cohesion, are usually extremely small.....
Because fluids cannot support shearing stresses, it does not follow that such
stresses are nonexistent in fluids. During the flow of real fluids, the shearing
stresses assume an important role, and their prediction is a vital part of engineering
work. Without flow, however, shearing stresses cannot exist, and compression stress or
pressure is the only stress to be considered (A Physical Introduction to Fluid
Mechanics, by A. J. Smits, John Wiley &
So we see that the most obvious property of fluids, their ability to flow and
change their shape, is precisely a result of their inability to support shearing stresses.
Flow is possible without a shear stress, since differences in pressure will cause a fluid
lump to experience a resultant force and produce an acceleration, but when a fluid is
deforming its shape, shearing stresses must be present.
With this definition of a fluid, we can recognize that certain materials that look like
solids are actually fluids. Tar, for example, is sold in barrel-sized chunks which
appear at first sight to be the solid phase of the liquid which forms when the tar is
heated. However, cold tar is also a fluid. If a brick is placed on top of an open barrel
of tar, we will see it very slowly settle into the tar. It will continue to settle as time
goes by --- the tar continues to deform under the applied load --- and eventually the brick
will be engulfed by the tar. Even then it will continue to move downwards until it reaches
the bottom of the barrel. Glass is another substance that appears to be solid, but is
actually a fluid. The glass flows under the
action of its own weight. If you measure the thickness of a very old glass pane you would
find it to be larger at the bottom than at the top of the pane.
This deformation happens very slowly because the glass has a very high viscosity,
and the results can take centuries to become obvious.
Another example: silly putty behaves like an elastic body when subject to rapid stress (it bounces
like a ball) but it has fluid behavior under a slowly acting stress (it flows like a fluid under its own weight).