Many colloidal dispersions show Bingham flow which is characterized by a σ-D diagram as shown. At rates of shear greater than that at point A, the following relation applies: \[\sigma - \sigma _{\text{B}}=\eta _{\Delta }\ D\] where σ B (or τ B) is called the Bingham yield stress, η Δ is the differential viscosity, D is the shear rate, and σ is the average of three normal stress components if the deformation is purely dilatational.