Neil Cunningham, Rheology School
Are you plagued by viscometer readings that never seem to settle down? Do you measure a sample and watch in frustration as the reading just drops and drops and drops, seemingly forever? If this sounds like you then don't panic; you are not alone and there are solutions at hand.
What you are seeing is a manifestation of thixotropy. (see fig). A thixotropic fluid is one that reacts relatively slowly to shear, it takes time to reach a stable viscosity (known as an equilibrium viscosity) when subjected to constant shearing - and subsequently takes time to recover to its original viscosity after the cessation of shear.
Now this causes big headaches for QC lab staff and managers as it becomes a matter of guesswork when to record the viscosity value for the product under test. It also demonstrates how significant the sample preparation is with thixotropic fluids. Mixing and homogenizing produce changes that require time to recover from, so the handling of the sample must be very consistent, or enough time for full recovery must be allowed for reliable measurements to be made.
Here are three ways you can deal with the measurement part of this frustrating and potentially costly situation:
Hint: You'll get far better reproducibility if you use a defined-shear measuring system such as the Small Sample Adaptor or a cone and plate instrument such as the Cap 2000, RS-CPS or the Wells-Brookfield systems.