The experiment has been running for nearly a century, slowly dropping viscous material along the way. (Image credit: Jsjsjs1111 via Wikimedia Commons (CC BY-SA 3.0))
In 1927, Thomas Parnell started a laboratory experiment at the University of Queensland, and it's been running ever since. In fact, two custodians passed away before they could see any results. The experiment is meant to demonstrate highly viscous materials. At the time, Parnell warmed up residue from the distillation of coal tar and inserted it into a sealed glass funnel, where it took three years to take the container's shape.
He then cut the funnel's stem so the extremely viscous liquid could move out of the bottom. It took eight years for the first drop to fall, and five more dropped over a forty-year span. Unfortunately, Parnell and Professor John Mainstone, his successor, weren't alive to witness any drops. Almost a hundred years have passed since the experiment began, with various custodians overlooking it. Professor Andrew White is currently the custodian in charge.
Although it's not as controlled (room temperature fluctuations can affect it, and the stem's internal diameter can't be precisely measured) as one would like, the experiment is still surprising. With a few factors to consider, the team estimated the pitch's viscosity.
"The viscosity of pitch is then calculated as q = (2.3 +0.5) x 108 Pa s, which is enormous compared to that of common liquids," they explained in the paper. "Water at 20°C has a viscosity of 1.0 x 10-3 Pa s. It should be noted, however, that (ignoring superfluidity) it is close to the geometric mean of the range of values that physicists consider – the effective viscosity of the Earth is of the order of 1020 Pa s."
Of course, there are some disagreements with predictions based on previous measurements.
"The result for the viscosity from the pitch drop experiment does not agree well with the predictions based on [previous] measurements, even allowing for the enormous variation of viscosity with temperature and the rather unknown temperature history of the experiment," the team wrote. "The probable explanation lies in the differing viscosities of different samples of pitch – these could have dissimilar proportions of trapped volatile hydrocarbons, and this would affect the viscosity."
Anyone can watch live footage of the pitch drop experiment, which has a large blob formation. Keep in mind that the next drop could fall within the 2020s, so it might be a while until that happens. The latest drop occurred in 2014.
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