volume viscosity (or dilatational viscosity)

A quantity ζ 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:

$σ = 1 3 ( σ x x + σ y y + σ z z ) = − p + ζ ( ∂ v x ∂ x + ∂ v y ∂ y + ∂ v z ∂ z )$

where $p$ is the hydrostatic pressure at the point considered in the absence of motion.

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 1216