A substance’s flow behavior depends on three factors:
- The substance’s inner - molecular – structure. The tighter the molecules are linked, the more the substance will resist deformation, i.e. the less it will be willing to flow.
- The outside or external forces acting upon the substance that deform it or make it flow. Both the intensity of the external force as well as the duration has an influence. Only Newtonian liquids are independent of the external force. The external force can have the form of wiping or pushing or tearing a substance; the simplest form is gravity, which pulls all substances down to earth. In viscometry, the external forces figure as shear rate or shear stress.
- The ambient conditions. The temperature and the pressure when the substance is stressed by external forces.
Depending on these factors the substance flows and develops different types of flow. Only one type of flow is suitable for testing a substance's viscosity.
Flow Conditions - Laminar or Turbulent
For testing a fluid's viscosity, defined flow conditions are essential. The fluid has to develop laminar flow. With laminar flow, the substance moves in imaginary thin layers in which molecules do not change from one layer to another. The flow has an orderly structure.
In turbulent flow, on the other hand, no recognizable structure or layers can be observed. Molecules move freely. The fluid forms vortices.
If testing a fluid under turbulent flow conditions, the results will give a falsely higher viscosity. (The turbulent movement of the molecules will be misinterpreted - so to speak - as higher flow resistance by a measuring instrument).
Practical examples: A shear rate that is too high for the tested substance can lead to turbulent flow. That means that e.g. too fast runtimes for glass capillary viscometers or spindles which turn too fast in rotational viscometers can cause turbulent flow.
The shear rate is an important parameter in defining viscosity (refer to the two-plates model) and also in specifying a substance's flow behavior.
The vital question is whether a change of shear rate does or does not change a fluid's viscosity. This question draws the line between Newtonian and non-Newtonian fluids.
Ideally viscous or Newtonian Liquids
If a fluid's internal flow resistance is independent of the external force – i.e. the shear rate - acting upon the fluid, it is ideally viscous. Such fluids are named Newtonian liquids after Sir Isaac Newton, who discovered the mathematical relation between viscosity and the external force acting upon a fluid. A viscosity function means plotting the viscosity over the shear rate. The viscosity function of a Newtonian liquid is a straight line (curve 1). Typical Newtonian liquids are water or salad oil.
If a substance is not ideally viscous, its viscosity changes with the shear rate. For such substances the apparent viscosity is specified. There are substances that show shear-thinning behavior (curve 2). Their viscosity decreases when the shear rate increases. For other substances the viscosity increases with increasing shear rate – that is called shear-thickening (curve 3).
For example yoghurt and shower gel show shear-thinning behavior, while starch solutions show shear-thickening behavior. These are just two of the most basic examples of potential flow behavior. Learn more about how shear rate can influence a substance’s flow behavior in World of rheology.
A fluid's viscosity strongly depends on its temperature. Along with the shear rate, temperature really is the dominating influence. The higher the temperature is, the lower a substance's viscosity is. Consequently, decreasing temperature causes an increase in viscosity. The relationship between temperature and viscosity is inversely proportional for all substances. A change in temperature always affects the viscosity – it depends on the substance just how much it is influenced by a temperature change. For some fluids a decrease of 1°C already causes a 10 % increase in viscosity.
In most cases, a fluid's viscosity increases with increasing pressure. Compared to the temperature influence, liquids are influenced very little by the applied pressure. The reason is that liquids (other than gases) are almost non-compressible at low or medium pressures. For most liquids, a considerable change in pressure from 0.1 to 30 MPa causes about the same change in viscosity as a temperature change of about 1 K (1°C).
Even for the enormous pressure difference of 0.1 to 200 MPa the viscosity increase for most low-molecular liquids amounts to a factor 3 to 7 only. However, for mineral oils with high viscosity this factor can be up to 20000. For synthetic oils, this pressure change can even result in a viscosity increase by a factor of up to 8 million. For example, lubricants in cogwheels or gears can be submitted to pressures of 1 GPa and higher. For better understanding, refer to the conversion equation for pressure units: 1 bar = 0.1 MPa = 105 Pa = 105 N/m2
For most liquids, viscosity increases with increasing pressure because the amount of free volume in the internal structure decreases due to compression. Consequently, the molecules can move less freely and the internal friction forces increase. The result is an increased flow resistance.
The Flow Behavior of Water under Pressure
The anomaly that water has its maximum density at +4°C is widely known. Such an anomaly can also be observed for the flow behavior of water under pressure. For temperatures >+32°C, water behaves like other liquids. Its viscosity increases with increasing pressure. Below +32°C and under pressures of up to 20 MPa, the water's viscosity decreases with increasing pressure. The reason is that the structure of the three-dimensional network of hydrogen bridges is destroyed. This network is rather stronger than the structures of other low-molecular liquids.
Thomas G. Mezger, 'The Rheology Handbook', 3rd revised Edition, (C) 2011 Vincentz Network, Hanover, Germany