In simplest terms, viscosity is defined as “the resistance to flow” and is often referred to as the thickness of a fluid. The concept of a fluid having a “thickness” has existed for thousands of years, however the term “viscosity” was not introduced until 1929. Even Newton referred to viscosity as “the lack of slipperiness of the parts of the liquid” (Sir Isaac Newton, Principia, 1687).

Measuring viscosity at a single and arbitrary shear rate provides limited information which can be difficult to interpret. A much more thorough understanding of a formulation is obtained by measuring the shear rate dependence. In addition to the practical relevance of predicting the behavior during application or processing, the details of a non-Newtonian response reflect the molecular interactions and resulting microstructure at therapeutic levels. Even protein solutions at moderate concentrations can exhibit a Newtonian plateau followed by a shear thinning region illustrating the importance of non-hydrodynamic forces and degree of structure formation.