use of the greek letter mu (formula_10) for the viscosity is common among mechanical and chemical engineers, as well as physicists. however, the greek letter eta (formula_19) is also used by chemists, physicists, and the iupac. the viscosity formula_10 is sometimes also referred to as the "shear viscosity". however, at least one author discourages the use of this terminology, noting that formula_10 can appear in nonshearing flows in addition to shearing flows.