Bingham flow

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:

$σ − σ B = η Δ D$

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.

Source:

PAC, 1979, 51, 1213 (Manual of symbols and terminology for physicochemical quantities and units. Appendix II: Definitions, terminology and symbols in colloid and surface chemistry. Part 1.13. Selected definitions, terminology and symbols for rheological properties) on page 1217