Most low molecular weight substances such as organic and inorganic liquids, solutions
of low molecular weight inorganic salts, molten metals and salts, and gases
exhibit Newtonian flow characteristics, i.e., at constant temperature and pressure,
in simple shear, the shear stress(σ) is proportional to the rate of shear (γ˙) and the
constant of proportionality is the familiar dynamic viscosity (η). Such fluids are
classically known as the Newtonian fluids, albeit the notion of flow and of viscosity
predates Newton [40]. For most liquids, the viscosity decreases with temperature
and increases with pressure. For gases, it increases with both temperature and pres