Zero-shear-rate viscosity definition

The viscosity of polymer solutions is usually determined by measuring the flow time of a definite quantity of solution through a capillary. The driving force is the height of the fluid in the viscometer. A difficulty arises because polymer solutions are sufficiently oriented in ordinary capillary viscometers so that even at a low rate of shear the viscosity determined does not correspond to its real value at zero shear. In order to get values which are reproducible, regardless of the viscometer used, the viscosities, therefore, have to be determined at several rates of shear and extrapolated to zero shear rate as well as to zero concentration. This is particularly import t for high molecular-weight polymers where the shear dependence of viscosity is most pronounced. 

The superscript indicates that the shear rate was increased from zero at time t = 0. This definition is only meaningful if the fluid at time r = 0 is in a well-defined state, usually a stress-free state, and if this starting state and the transient viscosity are reproducible from one run to the next. The superscript -p is sometimes omitted. Measurements of t) give information about rates of structural rearrangement within a deforming complex fluid. 

A lower Newtonian region exists at low shear rates or stresses, and this leads to the definition of zero-shear viscosity (// ), i.e., value of / as y -> 0. 

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