A polymer solution or a polymer melt is non-Newtonian, which means its viscosity changes under different shear rates. For a Newtonian fluid like water, its viscosity is unchanged for a given temperature and pressure, when it is under different shear rates. When you use smooth parallel plates to measure viscosity, you need to be cautious about wall slip (Your measured viscosity will be smaller than it should be). In order to minimise wall slip, rough surfaces are recommended. However, if the shear rates are high (>1 s-1), the wall slip is less prevalent.

Please see the attached file that gives the formula for estimating the shear rate in the Brookfield viscometer.

On the parallel plate rheometer, find the viscosity at the same shear rate as that estimated on the Brookfield rheometer. It might not be an exact match but it should be close enough.

For a non-specialist in rheometry and rheology, it might be helpful to glance through couple of chapters that discuss the basics in the following books.

1. Aroon V. Shenoy, Rheology of Filled Polymer Systems, Kluwer Academic Publishers, Netherlands (1999).

2. A. V. Shenoy and D. R. Saini, Thermoplastic Melt Rheology and Processing, Marcel Dekker Inc., New York (1996).

Hi Christophe,

the formula, Aroon sent is perfectly correct but it based on the assumption to have a linear velocity gradient between the spindle and the cup. Depending on the properties of the sample, the limit for such a geometry is somewhere around a gap of 2 mm. Since this spindle does not come with a matching cup, I assume you used a suitable lab beaker or something alike. If this is correct then any estimate of a real shear rate is highly vague.

Thank you all. I have enough informations yet to begin new trials.

Christophe