A. Introduction

When a shearing stress is applied to a confined fluid, a velocity gradient develops within the fluid which then starts to move. The maximum velocity occurs where the stress is applied. Viscosity is a measure of the internal friction of the fluid which opposes dynamic change in the fluid; the higher the friction the higher the viscosity. Viscosity is formally defined as the ratio of the shearing stress per unit area over the velocity gradient. The SI unit of viscosity is N·s/m² (also Pa·s). In practice, the Non-SI poise (P) and cP (0.01 P) are widely used. 1 P is equivalent to 0.1 N·s/m².

The ratio of viscosity to density is known as kinematic viscosity. Its SI unit is m²/s. The non-SI unit of stoke is widely used. 1 stoke is equivalent to 0.0001 m²/s.

The viscosity is independent of shear rate in Newtonian fluids which include most gases and low molar mass fluids of engineering importance. Fluids for which the viscosity depends on the shear rate are called non-Newtonian fluids and are outside the scope of this paper.

Viscosity plays a major role in the design of pumping systems, heat and mass transfer applications and the optimal selection of process equipment. A review of the viscosity correlations of practical nature is given by Poling et al. (2001).