Hi this is Joey Hodges from the Centre for Industrial Rheology.
Zero shear viscosity is the viscosity of a material when it is effectively at rest.
Despite the misnomer, zero shear viscosity describes a plateau value which you can sometimes see when you use a high-powered rheometer to measure viscosity under extremely low shear conditions.
Here are a couple of typical examples and emulsions we measured with sample three in green having the highest zero shear viscosity, sample four having the lowest.
In the real world zero shear viscosity helps immobilise droplets in an emulsion or particles in suspension.
If you think about it, most materials spend a significant proportion of their existence under these really low shear conditions and so arguably this makes zero shear viscosity one of the best rheological metrics for describing how products behave in storage.