The Curious Case of the T-Bar Viscosity
Those of us who measure the viscosity of pastes, creams and waxy products may be familiar with the Brookfield Helipath and T-Bar accessory. This is a very useful tool that enables us to take a measurement of a soft-solid with a simple viscometer. As we will see, the test can provide some very useful information but, if misinterpreted, the results can lead to confusion. It is useful then to understand the nature of the measurement and how it relates to alternative viscosity measurement methods.
An example will illustrate:
Which viscosity is the right one?:
Sudocrem: Looks soft but feels thick.
T-bar viscosity: 1052Pa.s
Cone/plate viscosity: 3.3Pa.s
Body Shop Intensive Foot Rescue cream: Looks thick but feels soft.
T-bar viscosity: 1374Pa.s
Cone/plate viscosity: 0.52Pa.s
Body Shop’s Intensive Foot Rescue cream applies with a light, silky touch when rubbed on the skin whereas Tosara's Sudocrem has a distinctly “draggy” heavy skin feel. However, when measured with a Brookfield viscometer fitted with a Helipath and T-bar accessory (T-D spindle at 1rpm) the results apparently contradict the in-use observation: Sudocrem comes in around 1052 Pa.s while the foot cream registers at a higher 1374 Pa.s.
So why the contradiction? How can the higher viscosity product be easier to spread?
The answer lies in the fact that the Helipath/t-bar "viscosity" is merely a measure of the force required for the spindle to “cut through” or disrupt undisturbed sample. A “true” viscosity measurement, on the other hand, measures the stress needed to shear a sample in a highly-defined geometric arrangement, usually between two surfaces such as in a cone/plate or concentric cylinder geometry. In actual fact a "defined shear" method such as this (a cone and plate viscometer running at 250s-1 shear rate) recorded viscosities of 3.3 Pa.s for the Sudocrem and 0.52 Pa.s for the foot cream. It is immediately apparent that the defined-shear cone and plate viscosity reading comes in at orders of magnitude lower than the t-bar viscosities. Furthermore, the relative relationship now correlates with the consumer’s experience of their respective "spread-ability".
T-bar "viscosity" measurements: The t-bar spindle is driven down through the sample and force required to disrupt the sample's structure is measured.
If it’s not viscosity...what is it?
Although the t-bar viscosity does not correlate with the specific attribute of application texture examined here it is, in fact, a very useful bit of information indeed. Because it is a measure of the force required to drive the t-bar spindle through the sample it is, in essence, a measure of structural strength or "yield". This is why the t-bar viscosity tends to correlate more closely with the undisturbed product condition rather than with how a material behaves in flowing situations such as pumping, spreading or filling processes. Such a knowledge of the product's soft-solid structure helps us understand how it suspensions, emulsions, waxes and gels will behave when in long-term storage such as in the container or on the shelf.
Non-destructive sample loading: A rheologist's dream.
Another often-unappreciated bonus to be gained from the Helipath and t-bar approach is that testing can usually be performed in the sample's original container. This means that as the spindle spirals down into the sample it meets, and measures, product that has not been exposed the disruptive agitation associated with other rheological methods such as cone/pate and concentric cylinders. The avoidance of any structural damage to a sample prior to measurement is often the holy grail of rheometry and research rheometers are often fitted with very expensive sample loading capabilities such as automated gap closures and axial stress limitation to meet these needs. The Helipath/t-bar method achieves this easily, albeit in a rather more work-a-day manner.