What is the sign of a good rheologist?
Written by: Adrian Hill
As one of the rheologists here at Malvern, I thought you would like to know about one of the slightly unusual questions I was asked recently by a customer: ‘What is the sign of a good rheologist?’
Maybe my initial answer wasn’t the one that the customer expected, but it made me smile……it’s the rheologist’s “badge of honour”, also known as how a rheologist earns their stripes! This is a true sign of a practical rheologist, although technology (for the better) is starting to make this less apparent.
To explain, we need to consider the catalyst that made rheology the common and useful tool that we know it as today: Controlled Stress technology, such as that used in the original Bohlin CS rheometer which was launched in the mid-Eighties. Before this rheometers were controlled rate, where the speed of the measurement was controlled directly, and were also relatively expensive and compared to a modern rheometer, and not very sensitive. With these, it was not so easy to earn the badge…
Controlled stress technology, as the name suggests, now allowed us to apply the stress to the sample directly. As viscosity is the shear stress divided by the shear rate, apply the stress, measure the resultant shear rate from the sample, then we get viscosity. However, it is important to realise that applying a stress is like applying a rotational push, with the resistance against this push coming from the viscosity.
This works very well, and still to this day where modern rheometers such as Kinexus can directly apply the stress or the rate, stress is often the measurement of choice for the most sensitive measurements. However, at higher applied shears, this is where the rheologist earns their stripes.
The maximum shear stress that you can apply to a sample is dependent on the sample viscosity (the resistance to flow) as that controls the rate, which itself is directly related to the angular velocity. At high enough angular velocities a sample will, of course, literally fly out of the gap…. resulting in a stripe of sample all across the user’s lab coat; or the rheologist’s stripe! With early controlled stress rheometers this was actually quite easy as there was a kind of uncontrolled cascade when the first part of the sample started to come of out the gap. When this happens, the resistance to flow effectively decreases (along with the shear thinning nature of most samples), which then means that the same applied stress results in a higher angular velocity where a little more sample comes free from under the measurement geometry…. and all happening so fast (<1 second) that your badge of honour is handed out!
Nowadays, advanced rheometers such as Malvern’s Kinexus come with features such as triggers, which can have various affects (for example stopping the increase in applied stress and stopping the test) depending on real time events from the rheometers (such as the angular velocity getting above a set value). We now let the rheometer do the hard work for us.
Whilst there is a small part of me that is saddened by the decline in the industrial lab coat cleaning business, these triggers in the rSpace software are a very useful feature which are continuing to find more and more uses.
For example, a recent webinar I gave on Computer Aided Linear Viscoelastic Oscillatory Measurements uses the flexibility of triggers to help with the LVER determination. Possibly more useful than hindering the rheometer honours system!