transport theory provides an alternative interpretation of viscosity in terms of momentum transport: viscosity is the material property which characterizes momentum transport within a fluid, just as thermal conductivity characterizes heat transport, and (mass) diffusivity characterizes mass transport. to see this, note that in newton's law of viscosity, formula_41, the shear stress formula_42 has units equivalent to a momentum flux, i.e. momentum per unit time per unit area. thus, formula_42 can be interpreted as specifying the flow of momentum in the formula_8 direction from one fluid layer to the next. per newton's law of viscosity, this momentum flow occurs across a velocity gradient, and the magnitude of the corresponding momentum flux is determined by the viscosity.