Viscosity is a widely measured property and is critical in a wide range of industries and products.
The rheological properties of a fluid are important for many reasons.
- Is the product going to have the same look and feel my customer is expecting? For example, if a shower gel is too thin then when the customer tries to apply it, it will dissipate and they will have to use more product to be able to wash. Conversely, if it’s too thick they won’t use enough and it will also be very hard to get it out of the bottle.
- The viscosity of the fluid is critical where it’s pumped through a pipeline or dosing system; if the viscosity is too high, the pump won’t be able to cope.
- The amount of product dispensed in a process whether automated or manual is affected by its rheological properties.
- In printing, ink viscosity has huge effects on the print quality.
Newtonian fluids get their name from Sir Issac Newton. He described the flow behaviours of fluids with a linear relationship between shear stress [mPa] and shear rate [1/s]. This relationship is known as Newton’s Law of Viscosity, where the proportionality constant η is the viscosity [mPa-s] of the fluid:
Some examples of Newtonian Fluids: Water, honey, alcohol, oil for these liquids only temperature affects viscosity.
Non-Newtonian fluids, therefore their viscosity is dependent on the shear rate (Shear Thinning or Thickening) or the deformation history (Thixotropic fluids).
Non-Newtonian fluids display either a non-linear relation between shear stress and shear rate, have a yield stress, or viscosity that is dependent on time or deformation history or a combination of all the above.
Some examples of non-Newtonian fluids: Cornflour and water, Ketchup, adhesives, creams both the speed of the spindle and temperature affect the viscosity.
Kinematic viscosity is the ratio of dynamic viscosity to density of the liquid. It is a measure of fluid’s resistance to shear flow under the weight of gravity. It’s a measure of a fluid’s resistance to flow when no external forces except gravity are acting.
It is most useful in describing Newtonian fluids. Centistokes (cSt) is the most common unit of measurement for kinematic viscosity. Centisokes equals 1 millimetre squared per second (mm²/s).
Dynamic viscosity is the measure of a fluid’s resistance to shear flow when an external force is applied. It is useful for describing the behaviour of fluids under stress. Mainly, it is most useful in describing non-Newtonian fluids by observing how viscosity changes as the shear velocity changes.
How is viscosity measured?
There are two main types of viscometer and methods of classifying the rheological properties of the liquid. The main types are listed below; Within these types, there are many subtypes and methods.
Glass Capillary Viscometers and Flow Cups
These are the most simple of instruments for viscosity measurement. A set volume of liquid is passed through an orifice of known size, the time taken for this liquid is measured and (efflux time) is then calculated into a kinematic viscosities unit of measurement such as centistokes (cSt). Glass Capillary Viscometers and Flow Cups come in many types all manufactured to different standards.
Rotational viscometers work by measuring the torque created by the shaft and spindle.
Like the capillary viscometers, there are several types of rotational viscometer with many different spindle sizes and shapes suitable for just about any fluid viscosity measurement.