The standard example of a mechanical transport process is Newton's law of viscosity, which states that the shear stress S x y is linearly proportional to the strain rate. The strain rate γ γ is the rate of change streaming velocity in the x-direction, with respect to the y-coordinate, γ γ = d e f ∂ ∂ u x / ∂ ∂ y . Newton's law of viscosity states