2. Viscosity measurement devices

2.1 Techniques used and difficulties encountered

Measuring a transport property such as viscosity involves a number of difficulties not encountered when measuring a thermodynamic equilibrium property. These difficulties stem from the fact that it is necessary to bring the system out of thermodynamic equilibrium.

• Viscosity is determined from the relationship between a pressure gradient and the corresponding velocity flux. This flow appears in a hydrodynamic equation, which often contains additional non-linear terms. Thus, pressure gradients induce not only laminar flow in the fluid, but also secondary flow and energy dissipation. The system's response to the pressure gradient is therefore relatively complicated. The pressure gradient must...