2. Viscosity

Viscosity coefficients are physical quantities that play an essential role in rheology. Their knowledge is sometimes sufficient to accurately characterize the rheological behavior of a material. Different viscosity coefficients are defined. The viscosity of a hydraulic fluid is the property that characterizes its resistance to flow. More precisely, viscosity results from the resistance of fluid molecules to forces tending to displace them by relative sliding within the fluid. A homogeneous liquid, whose density is assumed to be constant, undergoes relative deformations (sliding or shearing) to which it opposes a measurable resistance. Depending on their rheological behavior, fluids can be classified into two categories: "Newtonian" and "non-Newtonian" (see § 2.7