https://doi.org/10.1351/goldbook.V06650

A quantity ΞΆ which enters into equations at any point where the flow involves a change in volume, i.e. is dilatational. If the deformation is purely dilatational, the average of three normal stress components is: \[\sigma =\frac{1}{3}\ (\sigma _{xx}+\sigma _{yy}+\sigma _{zz})=- p+\zeta \ (\frac{\partial v_{x}}{\partial x}+\frac{\partial v_{y}}{\partial y}+\frac{\partial v_{z}}{\partial z})\] where p is the hydrostatic pressure at the point considered in the absence of motion.*Source: *

PAC, 1979,*51*, 1213. 'Terminology and Symbols in Colloid and Surface Chemistry Part 1.13. Definitions, Terminology and Symbols for Rheological Properties' on page 1216 (https://doi.org/10.1351/pac197951051213)

PAC, 1979,