Rheology is the study of how things...
A Polymerist Basics
Last week I wrote about the passing of my favorite thesis committee member Professor Chang Ryu and how he was a mentor when it came to understanding and utilizing rheology. If you were reading that and thinking: “What the hell is rheology?” Just know that you are not alone even amongst PhD chemists.
Rheology is the study of how things flow over certain conditions such as time, temperature, stress, and strain. To a certain extent everything flows over a long enough timeline. Rock can flow as magma under certain conditions of temperature and time as can mountains over a long enough period of time. If you’ve ever made a peanut butter jelly sandwich you know that if you don’t apply some stress with a knife across the bread then your peanut butter and jelly will not flow to the edges of the bread--this phenomena is known as shear thinning.
If you’ve ever tried to get some ketchup out of a glass bottle and struggled then you’ve dealt with a flow problem. Rheology principles apply at the level of glass ketchup bottles in diners all the way to a continuous polyethylene reactor in Baytown, Texas. Understanding how things flow under certain conditions can make significant differences in how a manufacturing operation runs in terms of up-time, maintenance, and productivity. This is true for polymers as well as the majority of the packaged food industry such as candy. Caramel, chocolate, nougat, and most things that go into candy bars are all carefully engineered to not flow (melt) while you are eating them.
We can separate rheology into two different disciplines: Linear and Non-Linear Rheology. Linear rheology essentially means that the majority of Newton’s laws pertaining to classical mechanics are true in that all of our fluids maintain Newtonian behavior and this makes the math easier so that people like me can do experiments. Non-linear rheology is when Newton’s laws do not always hold true and when you have to start deriving many of your own equations to describe your system. I stay in the linear rheology camp.
Under the sort of times and applied forces of everyday life we might see some examples of Newtonian fluids: honey, epoxy resins, maple syrup, or water. A non-Newtonian fluid: Ketchup, toothpaste, paint, or a molten polymer. The thing with rheology is you can also study things under time domains and applied forces that are not typical to everyday life such as being pumped through a pipe, being shot at with a bullet, or how pitch appears to be a solid brittle mass when you touch and feel it but on a very long time domain it actually behaves like a fluid.
The Pitch Drop Experiment is the longest running scientific experiment in the world. From the University of Queensland in Australia where the experiment was started:
At room temperature pitch feels solid - even brittle - and can easily be shattered with a hammer. But, in fact, at room temperature the substance - which is 100 billion times more viscous than water - is actually fluid.
So in conclusion rheology is the study of how things flow and most things we interact with on a daily basis will flow given the right conditions including things like glass. It’s just a matter of time, temperature, and applied stress. The basic understanding of rheology will help when I start talking about rheology modifiers such as clay and cellulose, which can give paints their anti-drip characteristics by turning them into shear thinning fluids. The application of shear (painting) thins out the material, but removal of the brush or roller sends the thickness of that fluid back up.
We can refer the how thick a fluid is by the term viscosity. Viscosity and rheology modifiers deserve their own post I think, but to give you an idea of how viscosity can change as a function of poylmerizations occuring the next time you use some five minute epoxy to glue something to something else notice how fluid the material is upon mixing and then as the epoxy cures the viscosity of the fluid goes up really fast until it doesn’t flow anymore. The inhibition of flow could also be considered a gelation point (also typically measured through a rheological test), but I think I’ve rambled on enough about rheology at this point.
Tony
For those interested in learning more about rheology from a blog check out It’s a Rheo Thing.
For those interested in a nice three part video series start here:
I'm one of those people who knew zilch about rheology before I got into the industrial R&D world. I still find it difficult to wrap my head around -- the simple concepts are easy to analogize (e.g. ketchup) but things get weird very quickly once oscillation, temperature changes, reactivity, etc. start popping up at the same time.