Factors affecting viscosity
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Factors Affecting
Viscometry
Factors Affecting
Viscosity
Learn about factors
affecting viscosity - i.e. a
substance's flow behavior and viscosity measurement.
Page Content
Which Factors?
Flow Conditions: Laminar or Turbulent
Shear Rate
Temperature
Pressure
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Factors influencing a substance's flow
behavior: Inner structure, external forces,
ambient conditions
Which Factors?
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
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
substance is stressed by external forces.
Depending on these factors the substance flows and develops different
types of flow. Only one type of flow
viscosity.
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Flow Conditions - Laminar or
Turbulent
For testing a fluid's viscosity, defined flow conditions are essential. The
fluid has to develop laminarpowered
flow. by
With
laminar flow, the substance Legal
moves
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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.
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Laminar flow: Molecules move in separate
layers | Turbulent flow: No recognizable
structure or layers
Shear Rate
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.
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Ideally viscous or Newtonian
Liquids
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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.
Non-Newtonian Liquids
If a substance is not ideally viscous, its viscosity changes with the shear
rate. For such substances the apparent viscosity
substances that show shear-thinning behavior (curve
decreases when the shear rate increases. For other substances the
viscosity increases with increasing shear rate – that is called shearthickening (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
rheology.
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Viscosity function (dynamic viscosity over
shear rate):
1 ... Newtonian liquid
2 ... shear-thinning substance
3 ... shear-thickening substance
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Temperature
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.
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Temperature influence on viscosity:
-1°C in temperature => +10 % in viscosity
Pressure
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 mineralpowered
oils with
high viscosity this factor can Legal
be up
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Factors affecting viscosity
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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 = 10
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.
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Pressure influence on viscosity: approx.
+30 MPa in pressure => +10 % in viscosity
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Literature References
Thomas G. Mezger, 'The
Rheology Handbook', 3rd revised
Edition, (C) 2011 Vincentz
Network, Hanover, Germany
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