Viscosity refers to the amount that a fluid (gas or liquid) resists changing shape. In other words, how much a fluid opposes flow. Therefore, the viscosity of water refers to how much water resists flow or movement.
A more technical way to describe viscosity is that it’s a measure of how much a given fluid can resist the deformation caused by shear or tensile stress.
The opposite (reciprocal) measure to viscosity is fluidity, which indicates how easily a fluid flows.
Viscosity encompasses some other physical factors such as internal friction (the contact forces between molecules) and how easily velocity is initiated and builds up within a fluid.
A simple comparison between the flow of water (low viscosity) and honey (high viscosity) can help to illustrate the principle.
Applications of viscosity
Viscosity is commonly used to assess lubrication in mechanical systems and to determine the opposition forces that arise when transporting fluids in pipelines.
It is also important when assessing and controlling liquid flow in industrial processes such as injection moulding and spraying.
The physics of viscosity
In many fluids, there is proportionality between the shearing stress that creates flow and the rate of deformation (or shear strain).
Another way to look at this is that when you divide the shear stress by rate of deformation, you get a constant value for a certain fluid held at a specific temperature.
The calculated constant for the fluid is known as the absolute or dynamic viscosity.
Dynamic viscosity = (Force x Time) / Area
The units of dynamic viscosity are Ns/m2 (Ns m-2), or mPa.s, or the SI unit Poiseuille (where 1 centi-Poiseuille = 1 mPa.s for water at 20oC).
Fluids that produce a viscosity constant are known as Newtonian fluids.
Fluids that don’t produce a constant are called Non-Newtonian fluids.
A different type of viscosity measure that is useful for some applications is kinematic viscosity which is calculated by dividing the dynamic viscosity by density and is measured in m2/s (m2s-1).
The effect of temperature
Viscosity changes with temperature in different ways depending on whether the fluid is a liquid or gas.
Liquid viscosity decreases quickly as temperature increases.
Gas viscosity increases as temperature increases.
To put it simply, when heated, liquids will flow more easily, and gases will flow more slowly.
What is the viscosity of water?
Water is a liquid, and each molecule contains one oxygen atom and two hydrogen atoms (H2O). It is the most abundant liquid on Earth, covering 71% of the Earth’s surface.
One reason that the viscosity of water is an important measurement is due to water being used widely in industry, engineering, agriculture and as a solvent in chemical production.
In fact, the reason it makes such a good solvent is that it has relatively low viscosity. As mentioned earlier the viscosity of liquids decrease as temperature rises, so it is often used at a raised temperature in industrial processes to improve viscosity. It is also important in biology as the viscosity of water is a crucial factor in blood circulation.
The viscosity of water at 20oC is commonly used as a primary standard. As mentioned earlier, the SI unit of absolute viscosity is Poiseuille, which relates to the viscosity of water at 20oC. Body temperature (37oC) is another commonly used viscosity measure.
Water has a low viscosity due to the low inter-molecular bonds. However, the strength of these bonds increase as water gets colder. Once it drops to 0oC and starts to freeze, it is clear that the viscosity is much higher. Ice still flows, but at a much slower rate than warm water.
Using the viscosity of water
Once the viscosity of one liquid is known at various temperatures, i.e. water, it’s easy to accurately determine the viscosities of others.
That’s why graphs and tables of water viscosity are readily available.
The main use for values of the viscosity of water is in comparison with other fluids. The viscosity of 0.00102 poise or 1.002 mPa.s at 20oC is the standard measurement that acts as a benchmark for experimentation with fluids.
This value was arrived at through careful, repeated experimentation using the capillary method, manometers and viscometers.
The viscosity of water at 20oC has become the standard measure of viscosity, to which most other fluids, especially liquids, are compared.
Experimentation which has led to a good understanding of viscosity helps in many industries. The design and maintenance of hydraulic systems require a sound understanding of viscosity for example.