Rheology School

Neil Cunningham, Rheology School
www.rheologyschool.com

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:

1. The easiest way to deal with this is to always record the viscosity value after a pre-determined test duration. So a test would read:
   
   Brookfield RVDVII+, spindle 3 @ 10rpm, 25 deg C, record after 30s.
2. Even the most conscientious workers, however, can find themselves tempted to stretch or contract the test time just enough to ensure the reading is well within the QC spec! If you would like to ensure this doesn't occur then one approach is to use a feature often overlooked on most digital Brookfield viscometers: the Timed Stop facility. This is available on the DVI+, DVII+ and DVIII+ viscometers. On the DVI+ you need to switch the motor off then simultaneously press the Set Speed and Select Spindle buttons as you turn it back on. On the DVII+ and DVIII+ press the Options button and scroll through the menu to get to it. You can then set a time period for the test, say 30s, and the viscometer will run for the set period and "freeze" the viscosity value after the pre-set time on the display.
3. Another way to do this is to simply plug a printer into the viscometer and have the instrument print the output. The DVII+ and DVIII+ viscometers can both be plugged directly into a parallel printer. You can then obtain a hard copy of the results from which the viscosity at the given time period can be easily read.

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.