Five measurement tips for the viscosity lab
Here are some simple, easy-to-implement strategies you can adopt to improve the quality of your viscosity testing results:
Start using cone and plate, plate/plate or concentric cylinder accessories.
Defined shear viscosity testing is great for easy, repeatable measurements. In defined-shear methods the sample is held “captive” in a narrow gap between two moving surfaces where the shear rate and shear stress can be accurately controlled or measured. Typical defined-shear measuring systems are: cone and plate, plate-plate and concentric cylinder attachments. The plate- plate geometry does not impart a uniform shear rate throughout the sample in the same way the cone and plate does, however this relatively minor disadvantage is outweighed by the fact that the parallel surfaces can be roughened (serrated or sandblasted) to eliminate wall slip effects and the gap between the plates can be widened to accommodate particles or reduce structural damage on a sample prior to measurement.
This is particularly problematic when measuring large volumes of high viscosity samples where the equilibrating-effects of internal convection currents are arrested by the viscosity of the product. Around room temperature, the viscosity of water-based products (i.e. solutions or water-continuous suspensions/emulsions) changes by about 2.5% for every degree Celsius change in temperature so small temperature inconsistencies can affect repeatability.
Deal with thixotropy
A thixotropic material is one that undergoes viscosity change over time when either at rest or under constant shear conditions. This behaviour will manifest itself in the viscosity lab as measurements that seem to take a long time to achieve an equilibrium viscosity before a value can be recorded. It’s useful to think of thixotropic products as possessing a “memory” for how they’ve been handled prior to testing; recent high shear or agitation typically leading to lower measured viscosities and vice-versa. For this reason, it is important to define pre-handling (i.e. any stirring during a temperature-conditioning period), the length and intensity of any pre-shear conditioning steps and the test duration prior to data collection – how long you run the test for before recording the viscosity values. In simple single-point viscosity testing a 30s period before recording the value is typically chosen as a practical time-point however many suspensions and emulsions will continue to thin long after this point. It is important to note that although extending the shearing duration results in improved reproducibility, the longer the shearing period the less representative the sample is of the material we started with.
Watch out for slip effects in suspensions and emulsions
Slip - or wall-depletion, to give it its technical name - is an insidious little nasty that often goes un-detected but leads to erratic and unexpected viscosity values. The effect occurs when a sample slides over the smooth surfaces of the viscometer or rheometer, thus avoiding the bulk shearing necessary to obtain meaningful viscosity data. For many “qualifying” samples (suspensions, emulsions and some gels) the use of modified test methods, roughened plates or vane measuring systems is necessary to eliminate the phenomenon and deliver representative results.
Viscosity or Yield Stress?
Often an unexpected question: “Is it really viscosity that you need to measure?” If you are working with a semi-solid - such as a cream, wax, set gel or paste –then viscosity is not so relevant. In these cases we may actually be better off testing yield stress, the effort required to get the material moving in the first place. It’s not as difficult as it sounds, and in fact many people measure a kind of yield stress unwittingly with some viscometer set-ups. The simplest example of a basic yield-type test is to use the Brookfield T-bar/Helipath accessory. This simple attachment enables the measurement of soft-solid samples by spiralling a small horizontal pin through the structured sample. The “viscosity” displayed on the instrument screen is, in fact, an indirect measure of yield strength of the product. Other approaches include static yield tests such as stress ramps that impose increasing stresses on the sample until it yields or dynamic yield tests such as shear rate ramps with yield stress quantified by extrapolation methods such as the fitting of Bingham, Casson or Herschel-Bulkley viscosity models.