Fluid Substance

In physics, a fluid is a substance that continually deforms (flows) under an applied shear stress. Fluids are a subset of the phases of matter and include liquids, gases, plasma and, to some extent, plastic solids.

Although the term “fluid” includes both the liquid and gas stages, in common usage, “fluid” is often used as a synonym for “liquid”, with no implication that gas could also be present. For example, “brake fluid” is hydraulic oil and will not perform its required function if there is gas in it. This colloquial usage of the term is also common in medicine and in nutrition.

Liquids form a free surface while gases do not. The distinction between solids and liquid is not entirely obvious. The distinction is made by evaluating the viscosity of the substance. Silly Putty can be considered to behave like a solid or a fluid, depending on the time period over which it is observed. It is best described as a viscoelastic fluid. There are many examples of substances proving difficult to classify. A particularly interesting one is pitch, as demonstrated in the pitch drop experiment currently running at the University of Queensland.