1.2 Viscosity
Viscosity is a liquid’s resistance to flow.
](files/ch10/viscosities.gif)
Figure 1.6: A less viscous fluid (left) vs. a more viscous fluid (right) – Wikipedia
Viscosity is larger in fluids with stronger intermolecular forces or whose particles are long and flexible. Long, flexible particles are able to entangle themselves around each other making them difficult to separate and flow.
| Name | Formula | m.m.* | Viscosity† |
|---|---|---|---|
peanut butter |
150k-200k | ||
molasses |
5k-100k | ||
motor oil (SAE 30) |
150-200 | ||
water |
H2O |
18.02 | 0.89 |
n–nonane |
C9H20 |
128.20 | 0.71 |
n–heptane |
C7H16 |
100.21 | 0.41 |
n–pentane |
C5H12 |
72.15 | 0.24 |
| * Molar masses in g mol–1 | |||
| † Viscosities in mPa · s at 25 °C | |||
Temperature also has a dramatic effect on the viscosity of a fluid. Consider the viscosity of two different motor oils below.
Figure 1.7: Viscosity of SAE motor oil across a range of temperatures
SAE 10W-40 has a higher viscosity at low temperature whereas SAE 0W-40 has a lower viscosity at low temperature. The ‘W’ stands for ‘winter’ or cold conditions and the lower the preceding number, the lower the viscosity. The second number following the hyphen indicates the viscosity at high temperature. Engine oil viscosity depends on how the engine was manufactured as well as the environment the owner generally lives in (cold vs. hot climates)!
Increasing length of floppy, flexible molecules lead to higher viscosities. Dispersion increases in longer molecules as well as the level of molecule entanglement.
Figure 1.8: Viscosities of some hydrocarbons for some temperatures.
Pentane
Heptane
Nonane
Water
The type and number of dominant intermolecular forces also affect viscosity.
Figure 1.9: Viscosities of some substances at 25 °C
Glycerol
Triethanolamine
Tetrachlorosilane
Cyclohexanol
1,2-Propanediol
Pitch Drop Experiment
The longest running scientific lab experiment began in 1927 by Prof. Thomas Parnell at the University of Queensland.
) – Professor of Physics at the University of Queensland](files/ch10/thomas-parnell.jpg)
Figure 1.10: Prof. Thomas Parnell – Professor of Physics at the University of Queensland
Pitch, a very viscous fluid, was placed in a funnel and allowed to flow starting in Oct. 1930.
](files/ch10/pitch-drop.jpg)
Figure 1.11: Pitch drop apparatus
Each drop of pitch has been recorded (see table below).
| Date | Year Interval | Event |
|---|---|---|
| 1927 | 0.0 | Experiment begins |
| Oct. 1930 | 3.0 | Pitch allowed to flow |
| Dec. 1938 | 8.0 | 1st drop |
| Feb. 1947 | 8.2 | 2nd drop |
| Apr. 1954 | 7.2 | 3rd drop |
| May 1962 | 8.1 | 4th drop |
| Aug. 1970 | 8.3 | 5th drop |
| Apr. 1979 | 8.7 | 6th drop |
| Jul. 1988 | 9.3 | 7th drop |
| Nov. 2000 | 12.3 | 8th drop |
| Apr. 2014 | 13.5 | 9th drop |