Basic & Rotational

I had assumed that rheology was a continuum and that rotational, oscillatory and yield stress effects could all form a unified story. But in reality each effect is its own field. So here we look at the basics of viscosity and what happens when we measure it with a rotational device. But first some key basics that have always confused me.

Stress, Strain and Strain Rate

I have never forgiven physicists for choosing, for two key concepts, two words that start with the same three letters, and which are used in common language more-or-less interchangeably. I still have to stop and check every time if I'm using (or understanding) the right word, and I still get things wrong from time to time, to the confusion of myself and others.

  1. Stress. This is force (N) applied per unit area (m²), so is N/m² or Pa. A rheometer can work in "controlled stress" mode by making stress the key controlling parameter.
  2. Strain. This is the % increase in length when a stress is applied. If the original length is l and the applied stress makes it go to length l+Δl, then strain is Δl/l It is, therefore, dimensionless. A rheometer can work in "controlled strain" mode by making strain the key controlling parameter. Our language problem is that in everyday life we don't make a careful distinction. Although we can personally feel "under strain" we can also say that we are "straining hard" at something, with the second use really meaning that we are "applying a lot of stress" to something.
  3. Strain rate. This is the rate at which strain increases, and has units 1/s. Our problem is that in common rotational rheology we are interested in the strain rate from a shearing process, which is shear strain rate, but we commonly call this shear rate, which means that we can forget (it's not helped by the fact that "shear" starts with "s") that this is just a strain rate.

There is no easy solution to this nomenclature problem. My personal solution is that I can always seem to remember that Pa measure stress, so if I see Pa then the topic is not strain!