ABSTRACT
Viscosity in a fluid requires energy dissipative structures at the molecular level. The collisional momentum transport theory has been quite successful in describing the viscosity of gases at low pressures. The classic Einstein calculation of the intrinsic viscosity of hard, solvent-impenetrable spheres provides the basis for the theoretical description of more complicated models. It also provides the conceptual basis necessary for understanding intrinsic viscosity as a hydrodynamic property of colloidal or macromolecular solutes. The fluid velocity field is used directly to calculate the energy dissipation under shear both in the pure solvent and in the suspension. The difference is related to the viscosity increment due to the solute and hence to the specific viscosity. A few rotational viscometers are designed as constant-shear-stress instruments in which the shear rate depends on the fluid viscosity. In such devices, the stator, whether inner or outer, is fixed, and a constant driving torque is applied to the rotating element.
