A fluid is a substance whose molecules are held together by weak intermolecular forces.
The main characteristic of a fluid is that it cannot resist any shear force applied to it. Informally, a shear force is the kind of force applied by a pair of scissors. Some fluids are more deformable than others but when a shear force is applied, they continually deform: they flow.
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While a solid will respond with restoring forces under a shear force (or will break), a fluid will undergo a large continuous deformation at constant volume under small shear forces (see figure below). A solid reaches a constant deformation angle, but a fluid will have a deformation velocity (which can be constant or not).
In these pages we won’t discuss the intermolecular interactions within the fluid or between the fluid and its container. We will work with the so called continuum hypothesis:
A fluid is continuous, and therefore we won’t consider the voids and empty space between molecules.
The main properties of a fluid are:
- Weak intermolecular forces.
- They flow easily.
- They take the shape of its container.
- They are mostly incompressible (liquids).
- They can be either liquid or gases (in these pages we will deal mostly with liquids).