What is viscosity?
Viscosity is the measure of how thick or syrupy a liquid is. In Dutch it is called viscositeit. It results from the internal friction between the particles of a material: the more resistance a liquid offers to a force applied to it, the higher its viscosity. Water has a low viscosity, syrup a high one.
How does it work?
- Definition: viscosity is the ratio between the shear stress (force per area) and the shear rate. The unit is Ns/m², that is pascal second (Pa·s). At 20 °C water has a viscosity of about 1 mPa·s.
- Temperature: for most liquids, viscosity falls as temperature rises. Bitumen is thick to solid at room temperature and becomes fluid when heated.
- Measuring: with a viscometer, for example using the Brookfield or Krebs method, in which the resistance of a rotating element in the liquid is measured. For paint, the efflux time from a flow cup is also used.
- Rheology: the science of flow behaviour. In some materials the viscosity changes with movement: shaken ketchup becomes thinner.
Application
- Paint and varnish: viscosity determines whether paint is easy to apply, how thick the coat will be and whether it sags or runs.
- Concrete and mortar: on site, workability is tested with methods such as the slump test, the flow table test and the V-funnel test. In underwater concrete (colloidal concrete), an admixture raises the cohesion and viscosity so that the cement paste is not washed out.
- Adhesives and sealants: a low viscosity penetrates deep into porous material, a high viscosity stays in place in vertical joints.
Tips
- Mind the temperature: paint, adhesive and sealant are thicker in the cold. Use them within the temperature range on the technical data sheet.
- Only thin as specified: too much thinner gives a coat that is too thin, with poor coverage.
- No extra water in concrete: adding water makes the mix more fluid but lowers its strength. Use a superplasticiser if the mix needs to flow more.
- Stir thixotropic products: some paints and adhesives only become workable after stirring.
Related terms
- Consistency class
- Flow table test
- V-funnel flow time
- Colloidal concrete
- Bitumen
- Adhesive
- Superplasticizer
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