Thixotropy Hysteresis Loop
Thixotropy (time-dependent viscosity) can be measured in many ways. Here we use a hysteresis loop.
Thixotropy Hysteresis Loop
There are plenty of papers with theoretical treatments of thixotropy hysteresis loops. I've chosen the approach from a team in Barcelona1 as it is mathematically tractable and theoretically clear. This starts with the Shear Viscosity app, using the Cross model to create the "equilibrium" line which is what you would measure if your thixotropic fluid had been infinitely well-sheared and had no time to recover. The plot goes up to your chosen maximum shear rate.
The Up line is what you would expect from your thixotropic liquid which starts at near-zero shear (0.1/s) with a viscosity of ηstart.The rate at which it approaches the equilibrium line depends on three factors.
- How fast you scan your shear rate - the slower you scan, the longer it takes to get to your maximum shear rate and, therefore, the sooner (visually, in terms of shear rate) you reach the equilibrium line.
- The timescale for breaking up your structure, τbreak. The longer this is, the more thixotropic your system will be.
- This is combined with how much τbreak changes with shear rate, expressed as γ̇ power. If this is 1 then the time decreases proportionally to shear rate
In principle, your "down" curve will be very different from both your up curve (of course) and your equilibrium curve. This depends on τbuild, the timescale for re-building your original structure. This is assumed to be shear-rate independent. You only see significant effects (i.e. different from the equilibrium line) with fast scans and if τbuild is quite large.
You may be surprised that the curves don't look too much like the ones we always see in simple explanations. The reason is that there are lots of ways of looking at the data, and lots of "equilibrium" curves, so you have to first get you equilibrium curve right, which might need one sort of scale (e.g. log-linear viscosity), then view the thixotropy via a log-log plot with shear stress instead of viscosity. When testing the app against published papers I often thought the results were wrong, but then found I was using the wrong scale in the app. Effects that look really interesting in log-log can look quite dull in log-linear.
The area S between the up curve and down curve (though here it's between up and equilibrium) is some sort of indication of the thixotropy of the system. However, the area has no absolute meaning because it depends on scan rate and maximum shear rate.The relative area Srel which is S divided by the area under the up curve (Smax) is said to be rather more independent of the experimental variables. Both values are calculated for you.
The title of the paper is "A Simple Theory", so it is easily described - though implementing it was tricky. The core assumption is that the rate of change of viscosity η towards the equilibrium value ηe at any given shear rate is given by:
δη/δt = (η-ηe)²/τs
τs is the "structure breaking" or "structure making" timescale and for breaking it is given by tbreak/(γ̇ )p, where p is a power dependence.
1Elaine Armelin, Mireia Martí, Elisabet Rudé, Jordi Labanda, Joan Llorens, Carlos Alemán, A simple model to describe the thixotropic behavior of paints, Progress in Organic Coatings 57 (2006) 229–235