The Rheologist's Toolkit

The Rheologist's Toolkit

Here’s a really simple approach to help you decide on the most relevant rheology and viscosity data you should be measuring for your products and applications:

Flow Regimes

At any time a product will be subjected to one of three flow regimes:

Regime 1 is when the product is at - or close to - rest. I call it the “Low-shear/No-shear Regime”.

Regime 3 is when the product is flowing, hence the name the “Flowing Regime”.

Regime 2, the “Transition/Yield Regime”, sits between 1 and 3. This is where the product is either commencing flow from resting, or returning to an at-rest condition from flowing.

The various conditions experienced during its lifespan of production, filling, storage and ultimate usage can be considered in terms of these regimes. Some examples of common flow processes and their relevant regimes are summarised in the first two columns below:

The third column shows a range of easily-obtained rheological parameters that describe behaviour under each regime. For example:

Zero-shear viscosity is the at-rest viscosity of a product and therefore clearly falls into Regime 1.

Yield stress is the stress that must be applied to a structured product to disrupt that structure and elicit a significant flow (in other words, get the product moving) so it falls into Regime 2.

Power Law Index is a measure of the viscosity’s dependence upon shear rate – a measure of “non-Newtonian-ness” – and therefore comes under Regime 3.

So much more than just viscosity

It is interesting to note that a simple viscosity value obtained on a lab viscometer only describes the product when flowing (regime 3) but says nothing about it under the other two regimes.

Think about your own products and the typical processes or flow events they experience, for example: spreading, stirring, pouring, storage, spraying, slumping or penetration into a substrate. In particular, think of the critical flow processes or behaviours, those where poor rheology would result in the product being deemed a failure, such as sagging of a paint, poor wash resistance of an ointment or inability to evacuate a tube of cream or bottle of lotion. Then decide which regime this type of process falls under and read off the relevant parameters from the table above.

The Rheologist’s Toolkit

These parameters, and the rheological test methods used to obtain them, make up a “rheologist’s toolkit” that I use daily for development, quality control and process design applications. When working with a customer on a specific need the toolkit is usually refined to one or two test methods providing three or four of the most critical parameters.

Effectively obtaining repeatable quantification of the toolkit parameters is dependent upon:

1. The correct choice of test method (flow, creep or oscillation, controlled stress or shear rate).

2. Correct choice of measuring system (cones, plates, spindles etc).

3. Correct interpretation and quantification of the results.

The Rheologist’s Toolkit in your lab

The implementation of the Rheologist’s Toolkit approach is now covered in my Practical Rheology for Chemists course and is proving an effective framework for learning and using rheology and viscosity testing. If you would like a toolkit of methods and parameters set up for your rheometer, with full training and ongoing support, please contact me for more details and prices.