Viscosity measurement - General principles

Viscosity characterizes a fluid's ability to flow. What is a fluid? Liquids and gases are fluids. Classically, and in contrast to a solid, a liquid occupies the shape of the container in which it is contained. But when we study more complex materials, i.e. those structured on different scales (microscopic, mesoscopic and macroscopic), the boundary between fluid and solid is not always so clear. For example, sand is made up of solid particles, but it flows. A fluid can be said to be a body capable of deforming indefinitely, provided that the resultant of the forces applied is not zero. This definition is very general, and liquids and fluid pastes are the focus of this article.

Knowledge of a fluid's viscosity is fundamental in many fields: fluid transport, processes such as coating, printing or spinning, paint application, the agri-food, cosmetics, biomedical and civil engineering sectors, concrete shaping, oil drilling... While viscosity measurement on samples can be used for manufacturing control, continuous measurement enables the process to be monitored and regulated.

However, it is important to warn the reader that a point measurement of viscosity is generally insufficient on its own to describe the properties of a flowing fluid; in what follows, other rheological quantities will be defined.

For fundamental concepts, please refer to the articles "Fluid Mechanics" [A 1 870] and "Non-Newtonian Fluids" [A 710].