There is often a necessity to measure, or at least estimate, true viscosity values using non-standard measuring systems on a rotational rheometer. This may be to replicate a mixing or manufacturing process on a lab scale, to keep a sample dispersed and uniform during a measurement or to measure some rheological property that would be difficult or impossible with a standard configuration such as ‘cone and plate’ or ‘concentric cylinders’.
While non-standard measuring systems can be used on our Kinexus rotational rheometers (such as custom made systems attached via our universal geometry adapter) determining the true shear stress and shear rate for such configurations is far from trivial. Unlike ‘cone and plate’ or ‘narrow gap concentric cylinders’, where the shear rate and shear stress are generally well defined and can be calculated from raw torque and displacement data, for non-standard geometries such as mixers and even regular geometries submersed in a sea of fluid this is not the case and only raw data can usually be presented.
At Malvern we have developed a simple empirical method for converting torque to shear stress and angular velocity to shear rate for non-standard measuring systems allowing users to estimate true viscosity values as a function of shear rate for our range of dispersion tools or custom made measuring systems. We have recently published a paper detailing this work entitled “Simple method for determining stress and strain constants for non-standard measuring systems on a rotational rheometer”, which is available in the August print edition of Applied Rheology and through the Applied Rheology Website (Open Access).
Without elaborating on the details of the approach here (this is discussed in the paper), the two charts above demonstrate its validity. These show comparable flow curves obtained with cone-plate and a mixer-cup configuration (shown below), for a shower gel (left) and body lotion (right). Although it might not be possible to achieve the same degree of measurement accuracy using a mixer configuration compared with a standard measurement configuration, especially for non-Newtonian materials, comparable data can be obtained if the use of non-standard configurations is deemed necessary or beneficial for a particular application or sample type.
One such sample type is a smoothie! In a recent blog (Smoothie Wars!) we compared the viscosity profiles of freshly made smoothies produced using two popular smoothie makes – the Nutri Ninja Pro and the Nutribullet. As shown below (left) these pulp suspensions separate quickly when left to stand, hence the need to keep them dispersed for the duration of the measurement. The use of the above mixer configuration achieved this very well, keeping the sample suspended while giving good quality rheological data in the process (below right). The same approach could be used to evaluate a range of different suspensions that are prone to settling, creaming and syneresis, or alternatively for monitoring the evolution of viscosity during a mixing process, so extending the capabilities of the rheometer beyond standard testing.
Although the Kinexus rheometer is a precision engineered analytical instrument used primarily for determining the viscous or viscoelastic properties of samples ranging from low viscosity liquids through to stiff solids under controlled conditions, it is also a highly versatile instrument. In addition to its rotational capabilities, it has advanced axial capabilities that can be utilized for tack testing, squeeze flow measurements and filament stretching tests. The simultaneous application of axial and shear formation has also been used to replicate the chewing of food as described in a recent article in the Journal of Food Science and Technology entitled Chewing the Fat.
Such advanced capabilities are made possible by our rSpace software, which allows fundamental rheological measurement actions (or test building blocks) to be linked together with each other or other software/instrument actions, such as user feedback and choices, calculate values, loops and triggers, in order to build ‘intelligent’ tests. Basically you think it – Kinexus can run it!
Watch out also for the release of rSpace software (1.70) this month, which contains amongst other things an updated library of intelligent integrated sequences (rSolution) for addressing a range of application problems – complementary to our website application material.