Do an internet search for the term “viscosity”, and you’ll find a variety of definitions for the word. For example:
- Wikipedia: The viscosity of a fluid is a measure of its resistance to gradual deformation by shear stress or tensile stress.
- The Free Dictionary: the state or property of being viscous
- (Physics / General Physics) Physics
- the extent to which a fluid resists a tendency to flow
- Also called absolute viscosity a measure of this resistance, equal to the tangential stress on a liquid undergoing streamline flow divided by its velocity gradient. It is measured in newton seconds per metre squared. Symbol η See also kinematic viscosity, specific viscosity.
- Hawai’i Space Grant Program: Viscosity is an internal property of a fluid that offers resistance to flow.
If you’re not already confused (good for you), read further into these definitions, and you’ll be confronted with a myriad of other terms – “rheology”, “shear rate”, “non-Newtonian fluid”, “thixotropy”, and “laminar flow”, to name just a few. It's enough to make your head feel as thick as cookie dough. What do they all mean, and why are they important to quality assurance testing?
CSC’s Guide to All Things Viscosity
First off, let’s define viscosity in laymen’s terms:
Viscosity is the measure of the resistance of a substance to motion under an applied force.
In other words, viscosity measures how hard it is to force a substance into motion.
Several factors can affect the viscosity of a substance. Temperature can affect the viscosity of all fluids, some more extremely than others, which is why it is important to maintain a consistent temperature for samples during viscosity testing.
For Newtonian fluids (that is, fluids that behave according to the formula identified by Isaac Newton), at a given temperature, viscosity will remain constant no matter how it is measured.
Sadly for Isaac, few fluids quite manage to live up to these expectations. We call these renegades Non-Newtonian fluids. This kind of fluid’s viscosity sees its shear rate vary disproportionately to the shear stress imposed on it.
- Shear stress = the force required to physically move or distribute molecular layers of a fluid in relation to one another.
- Shear rate = the change in speed between adjacent molecular layers of a fluid, relative to one another.
Just to make things even more complex, there are several different types of non-Newtonian fluids:
- Pseudoplastic: viscosity decreases with increased shear rate (i.e. printing ink, which is viscous enough not to drip from the applicator, but thins out as it is applied to the paper for a smooth flow)
- Dilatant: viscosity increases with increased shear rate (i.e. cornstarch suspended in water, which is hard when you strike it quickly, but if you push your hand onto it slowly, your hand will sink into the mixture)
- Plastic: fluid behaves as a solid under static conditions and requires a certain amount of force to be applied before flow is induced (i.e. ketchup – think having to shake a bottle in order to get ketchup to flow out of it)
- Thixotropy: viscosity decreases with time, when sheared at a constant rate (i.e. paint, which thins out as it is stirred)
- Rheopexy: viscosity increases with time, when sheared at a constant rate (i.e. whipping cream, which thickens as it is stirred)
I’ll end with one final definition of a word that is often linked to viscosity – rheology.
Rheology studies how matter changes in flow and form, which means that viscosity is a part of this field of study. A rheometer, therefore, measures more than a viscometer. It measures the way viscosity in a fluid changes in response to different parameters.
Some viscometers can test for rheology too. Always read the product features to find out if a viscometer is capable of doing double-duty.
Perplexed as ever,
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