The pitch drop experiment is a long-term experiment which measures the flow of a piece of pitch over many years. Pitch is the name for any of a number of highly viscous liquids which appear solid, most commonly bitumen. At room temperature, tar pitch flows at a very slow rate, taking several years to form a single drop
The first Professor of Physics at the University of Queensland, Professor Thomas Parnell, began an experiment in 1927 to illustrate that everyday materials can exhibit quite surprising properties. The experiment demonstrates the fluidity and high viscosity of pitch, a derivative of tar once used for waterproofing boats. At room temperature pitch feels solid – even brittle – and can easily be shattered with a blow from a hammer. It’s quite amazing then, to see that pitch at room temperature is actually fluid!
In 1927 Professor Parnell heated a sample of pitch and poured it into a glass funnel with a sealed stem. Three years were allowed for the pitch to settle, and in 1930 the sealed stem was cut. From that date on the pitch has slowly dripped out of the funnel – so slowly that now, 80 years later, the ninth drop is only just forming.
The experiment was set up as a demonstration and is not kept under special environmental conditions (it is actually kept in a display cabinet in the foyer of the Department), so the rate of flow of the pitch varies with seasonal changes in temperature. Nonetheless, it is possible to make an estimate of the viscosity of this sample of pitch (R. Edgeworth, B.J. Dalton and T. Parnell, Eur. J. Phys (1984) 198-200). It turns out to be about 100 billion times more viscous than water! The picture above is of Professor John Mainstone, who currently maintains the experiment.
Table 1 Record of pitch drops.
1930 ———- The stem was cut
1938(Dec) — 1st drop fell
1947(Feb) — 2nd drop fell
1954(Apr) — 3rd drop fell
1962(May) — 4th drop fell
1970(Aug) — 5th drop fell
1979(Apr) — 6th drop fell
1988(Jul) — 7th drop fell
2000(28 Nov) — 8th drop fell
In the 80 years that the pitch has been dripping no-one has ever seen the drop fall. If you’re interested in trying your luck, or at least just having a look at the experiment, you can view it live below. You can also see students of The University of Queensland milling around outside the cabinet, so it is more exciting than watching grass grow!
Full Paper & Details available here: The Pitch Drop Experiment Abstract