What You Always Wanted to Know About Viscosity
You’re certain to have heard the term viscosity before. But can you describe what it actually means off the top of your head? And what kind of a strange word is “viscosity” anyway?
Put simply, viscosity is the overall thickness of a liquid, or its flow resistance. This resistance is caused by the internal friction within the liquid and by intermolecular forces, such as Van der Waals forces. A thick liquid such as honey has a high viscosity. A thin liquid such as water has a low viscosity. People talk of high-viscosity and low-viscosity liquids.
Incidentally, the term originates from the plant genus Viscum, i.e. mistletoe. In ancient times, mistletoe berries were used to make birdlime. Birdlime is a viscous, sticky liquid that was used to coat rods in order to catch and trap birds. Freely translated, viscous means something like “as tough as birdlime”.
Seems pretty straightforward, doesn’t it? It’s a little more complicated in practice, of course, as a distinction is made between dynamic and kinematic viscosity:
Dynamic viscosity, also known as shear viscosity, is calculated from the ratio of the shear stress and velocity gradients within the fluid:
It is measured in the unit Pa·s (Pascal second).
To understand the formula, picture the following structure:
Two superimposed panels with the surface A are separated by a film of liquid. The distance is the y-coordinate.
The force F acts on the upper panel. With sufficient force, the upper panel moves accordingly. The upper layers of the liquid film move approximately at the speed of the upper panel, while the lower layers stay in their position.
This results in a speed profile with a gradient. The force generates a shear stress in the fluid. The more viscous the fluid is, the greater the force and the shear stress required to move the panel at the same speed.
The kinematic viscosity is the ratio of the dynamic viscosity to the density of a fluid
The unit of kinematic viscosity is St (Stokes) or m²/s (1 St = 10 -4 m²/s = 1 cm²/s).
Why do we distinguish between these two concepts? In the aforementioned experimental setup, the dynamic viscosity can be measured without further knowledge of the medium. The kinematic viscosity is normalised to the density, so to speak. At the same dynamic viscosity but a higher density, the value of the kinematic viscosity decreases.
As the molecules of the liquid are closer together at a higher density, however, a higher viscosity would be expected. This contradiction resolves itself because the intermolecular forces become smaller. The kinematic viscosity is therefore a material parameter.
A variety of physical measuring principles are available for measuring the viscosity. The classic falling ball viscometer devised by Höppler is one particular example. The liquid to be measured is located in a cylinder. A ball is dropped into the cylinder. A balance is established between the weight force and the frictional resistance between the ball and the medium, so that the ball drops at a constant speed. The viscosity can now be calculated from the fall speed.
The following table shows some typical values for the dynamic viscosity:
| Liquid | Temperature in °C | Viskosity η in mPa·s |
|---|---|---|
| Water | 5 | 1.52 |
| 10 | 1.297 | |
| 20 | 1.00 | |
| 25 | 0.891 | |
| Blood | 37 | 3 to 25 |
| Engine oil | 150 | ≈ 3 |
| 25 | ≈ 100 | |
| Glycerine | 0 | 12100 |
| 10 | 4500 | |
| 20 | 1490 |
What role does viscosity play in flow rate measurements?
A suitable measuring principle must be selected based on the viscosity of the liquid. While a Vortex or turbine flow meter can be used in cases of low viscosity, for high viscosity liquids, positive displacement flow sensors, oval gear flow meters or magnetic inductive flow meters are more suitable.
Did you know?
At SIKA, we are currently developing a new optical measurement principle that works completely independently of the viscosity. If you would like to find out more, please get in touch:
