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Located at the University of Queensland, there is an impressive display showcasing the longest-running laboratory experiment in existence. It has been running for such an extended period that two of its custodians unfortunately passed away before witnessing any results.
The experiment was started in 1927 by Thomas Parnell, the university’s inaugural professor of physics. As a demonstration of highly viscous materials, Parnell conducted an experiment with pitch, a residue obtained from the distillation of coal tars. He warmed the pitch, poured it into a sealed glass funnel, and patiently waited for three years for it to take the shape of the container. Waiting for the experiment to begin may seem like a lengthy process, but considering the duration of the demonstration, it was just a brief moment.
In 1930, Parnell made a deliberate incision in the stem of the funnel, causing the thick liquid to gradually drain from the lower part. The experiment has been running for an extended period of time, at an exceptionally sluggish pace. The first drop finally fell after eight long years of waiting, and five more did so over the following 40 years. The experiment has been ongoing for close to a century, with various individuals overseeing its progress. Parnell and his successor, Professor John Mainstone, both passed away before witnessing the drop-off, with Professor Andrew White now serving as the current custodian.
However, a webcam is now closely observing the experiment, so it’s possible that someone will see the next event. In 2014, there was another drop, which you can observe in this fast-paced video.
Although the experiment lacks the desired level of control, with factors like room temperature fluctuations and the inability to accurately measure the internal diameter of the stem without risking damage, it still manages to hold a few surprises for us.
By considering various factors, it is possible to make a reasonable estimation of the viscosity of pitch.
“The viscosity of pitch is then calculated to be q = (2.3 +0.5) x 108 Pa s, which is significantly higher than that of typical liquids,” explains a paper detailing the experiment. “At a temperature of 20°C, the viscosity of water is 1.0 x 10-3 Pa.” It’s worth mentioning that (excluding superfluidity), the value is quite close to the average range that physicists consider. The Earth’s effective viscosity is approximately 1020 Pa.
This contradicts previous predictions.
“The viscosity result obtained from the pitch drop experiment does not align with the predictions made using previous measurements, despite taking into account the significant variation of viscosity with temperature and the uncertain temperature history of the experiment,” the team explains. “The likely reason can be attributed to the varying viscosities of different pitch samples. It is possible that these samples contain different amounts of volatile hydrocarbons, which in turn impacts their viscosity.”
If you’re interested in observing the experiment in real time, it is possible for you to do so. Right now, there’s a rather substantial blob taking shape, but we wouldn’t advise observing it for an extended period since the next drop is projected to occur sometime in the 2020s, and there’s still a significant portion of the decade remaining.
