Kenneth Asks:

I picked up a jar of an unknown (to me) substance that has unusual
properties. Color is clear. At around 70 F it flows like syrup. At about 90
F it appears as if it is set up. This can be cycled over and over. Substance
was in with a bunch of lab items. Any idea what this may be? What is the
property called where normal flow/temperature properties are reversed?

It won’t be possible for me to even guess what this substance is, even if I had it in front of me. We may be able to narrow it down to some possibilities if you can give any other information about it, especially in regards to its rheological properties. Try cooling the substance further to see if the viscosity continues to decrease. If it does, then you are onto something.

As for the terminology, I haven’t found anything, although I have some inquiries out in the lab near where I work. There are terms for how viscosity changes with changes in shear and also those that change with shear AND with time. But all of these terms assume a constant temperature.

• Newtonian Fluids: Viscosity does not change with shear or with time.
• Non-Newtonian Fluids: Viscosity is dependent on shear and/or time.
  • Time Independent: Viscosity changes with shear, but does not change over time.
    • Pseudoplastic: Shear thinning (the faster you mix it, the thinner it is)
    • Dilatant: Shear thickening (the faster you mix it, the thicker it is)
  • Time Dependent: Viscosity changes with shear and with time.
    • Thixotropic: Time thinning (the longer you mix it, the thinner it is)
    • Rheopectic: Time thickening (the longer you mix it, the thicker it is)

As I said, I have some inquiries out at the lab, and I will update this once I have some more information. If you are inclined to do some more experiments, please do, and post them here.

Thanks for the question!