coal tar experiment

• Pitch Drop experiment

We're home to the famous Pitch Drop experiment, which holds the Guinness World Record for the longest-running laboratory experiment .

The experiment demonstrates the fluidity and high viscosity of pitch, a derivative of tar that is the world's thickest known fluid and was once used for waterproofing boats.

Thomas Parnell, UQ's first Professor of Physics, created the experiment in 1927 to illustrate that everyday materials can exhibit quite surprising properties.

At room temperature pitch feels solid - even brittle - and can easily be shattered with a hammer. But, in fact, at room temperature the substance - which is 100 billion times more viscous than water - is actually fluid.

The experiment explained

In 1927 Professor Parnell heated a sample of pitch and poured it into a glass funnel with a sealed stem. He allowed the pitch to cool and settle for three years, and then in 1930 he cut the funnel's stem.

Since then, the pitch has slowly dripped out of the funnel - so slowly that it took eight years for the first drop to fall, and more than 40 years for another five to follow.

Now, 87 years after the funnel was cut, only nine drops have fallen - the last drop fell in April 2014 and we expect the next one to fall sometime in the 2020s.

The experiment was set up as a demonstration and is not kept under special environmental conditions - it's kept in a display cabinet - so the rate of flow of the pitch varies with seasonal changes in temperature.

The late Professor John Mainstone became the experiment's second custodian in 1961. He looked after the experiment for 52 years but, like his predecessor Professor Parnell, he passed away before seeing a drop fall.

In the 86 years that the pitch has been dripping, various glitches have prevented anyone from seeing a drop fall.

See for yourself

To see the experiment for yourself, view the physical set-up in its display case in the foyer of the Parnell Building (Building 7).

Alternatively, you can watch the experiment's live video stream . More than 35,000 people from some 160 countries are registered to view the stream.

Professor Andrew White is the Pitch Drop's third and current custodian.

Email your Pitch Drop enquiry to  [email protected] .

Related links

UQ News explainer: the Pitch Drop experiment

Pitch Drop experiment paper (PDF, 252kB)

UQ Physics Museum

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July 19, 2013

World's Slowest-Moving Drop Caught on Camera At Last

The once-forgotten "tar pitch" experiment has yielded results after a seven-decade wait

By Richard Johnston & Nature magazine

How long would you be willing to wait for a drop of the black stuff in Dublin? After 69 years, one of the longest-running laboratory investigations in the world has finally captured the fall of a drop of tar pitch on camera for the first time. A similar, better-known and older experiment in Australia missed filming its latest drop in 2000 because the camera was offline at the time.

The Dublin pitch-drop experiment was set up in 1944 at Trinity College Dublin to demonstrate the high viscosity or low fluidity of pitch — also known as bitumen or asphalt — a material that appears to be solid at room temperature, but is in fact flowing, albeit extremely slowly.

It is a younger and less well-known sibling of an experiment that has been running since 1927 at the University of Queensland in Brisbane, which Guinness World Records lists as the world’s longest-running laboratory experiment (see: Long-term research: Slow science ). Physicist Thomas Parnell set it up because he wanted to illustrate that everyday materials can exhibit surprising properties. In the past 86 years that experiment has yielded eight drops, with the ninth drop now almost fully formed and about to fall.

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Long wait John Mainstone has been custodian of the Queensland experiment since 1961, and has yet to see or capture the fall of a drop — unsurprising given that it takes 7 to 13 years for a drop to form, but only a tenth of a second for it to fall.

Pitch-drop experiments involve heating a sample of pitch and pouring it into a sealed glass funnel. The pitch is given time — three years in Parnell’s experiment — to settle and consolidate before the sealed stem of the funnel is cut.

The origins of the Dublin experiment are now lost to history. It may have been part of a push by the physicist and Nobel prizewinner Ernest Walton, a professor at Trinity College Dublin, to promote science demonstrations for educational purposes. Over the years, the identity of the scientist who began the experiment was forgotten, and the experiment lay unattended on a shelf where it continued to shed drops uninterrupted while gathering layers of dust.

Watching it fall Physicists at Trinity College recently began to monitor the experiment again. Last April they set up a webcam so that anyone could watch and try to be the first person ever to witness the drop fall live.

At around 5 o'clock in the afternoon on 11 July, physicist Shane Bergin and colleagues captured footage of one of the most eagerly anticipated and exhilarating drips in science. “We were all so excited,” Bergin says. “It’s been such a great talking point, with colleagues eager to investigate the mechanics of the break, and the viscosity of the pitch”.

The Trinity College team has estimated the viscosity of the pitch by monitoring the evolution of this one drop, and puts it in the region of 2 million times more viscous than honey, or 20 billion times the viscosity of water. The speed of formation of the drop can depend on the exact composition of the pitch, and environmental conditions such as temperature and vibration.

Asked about the value of this demonstration, Bergin’s colleague Denis Weaire says, “Curiosity is at the heart of good science, and the pitch drop fuels that curiosity”.

Scientists used to believe glass to be a slow-moving liquid as well — in part because old church window panes are thicker at the bottom — but it is now considered a solid .

And the next one Mainstone, who has spent most of his life waiting to see a drop fall with his own eyes, congratulated the Trinity College team. “I have been examining the video over and over again,” he says, ”and there were a number of things about it that were really quite tantalizing for a very long time pitch-drop observer like myself.”

The University of Queensland pitch-drop experiment can be viewed live via a webcam and has a broad following across the globe. The next Queensland drop is predicted to fall some time in 2013.

This article is reproduced with permission from the magazine Nature . The article was first published on July 18, 2013.

Pitch Drop Experiment: The World’s Longest Running Lab Experiment

The pitch drop experiment began in 1927 when Professor Thomas Parnell of the University of Queensland in Brisbane, Australia, set out to demonstrate to his students that some substances that appear to be solid are in fact very high viscous fluids. He used tar pitch, or bitumen, a derivative of coal once used to waterproof boats, in an experiment to prove his point. At room temperature, pitch appears to be solid and can even shatter if hit with a hammer, but despite its look and feel, pitch can also flow, albeit extremely slowly.

Curious visitors inspecting the setup of the pitch drop experiment. Photo: University of Queensland

For his experiment, Parnell melted some pitch into a glass funnel with a sealed stem and allowed it to cool for three years. In 1930 he cut the sealed stem, hung the funnel over a beaker, and waited. It took eight year before the first drop fell into the beaker and another nine years before the second drop hit. Parnell didn’t live to see the third drop fall in 1954, as he passed away in September 1948. By then, the experiment was stored away in a cupboard of the physics department.

The pitch-drop experiment might have fallen into obscurity had it not been for John Mainstone, who joined the University of Queensland physics department in 1961. One day a colleague said, “I’ve got something weird in this cupboard here” and presented Mainstone with the funnel, beaker and pitch, all housed under a bell jar. Mainstone asked the department head to display it for the school’s science and engineering students, but he was told that nobody wanted to see it. Finally, around 1975, Mainstone persuaded the department to publicly display the experiment in a cabinet in the foyer of the department building.

The Pitch Drop Experiment with John Mainstone in a picture taken in 1990.

Today the experiment is broadcast on a live webcam . Unbelievably, no one had actually witnessed the pitch drop fall for 87 years, until April 2014, when the ninth drop fell. Thanks to round-the-clock monitoring by a webcam, the drop was seen touching down sometime between 9th and 14th of April—the wide time range is due to the fact that the pitch flows extremely slowly, and the exact moment of touchdown is uncertain. Although the webcam was present when the eight drop fell in November 2000, it couldn’t be recorded as the camera malfunctioned at the critical moment. The tenth drop was predicted to fall this year.

Timeline of the pitch drop experiment.

The experiment was not originally carried out under any special controlled atmospheric conditions, meaning that the viscosity could vary throughout the year with fluctuations in temperature. However, sometime after the seventh drop fell in 1988, air conditioning was added to the location where the experiment resided. The temperature stability has lengthened the interval between each drop.

The pitch drop experiment is recorded in the Guinness Book of Records as the world’s longest continuously running laboratory experiment, but it isn’t the only pitch drop experiment in the world. A pitch drop experiment was recently discovered at Aberystwyth University in Wales, that actually predates the famous Queensland experiment by 13 years. But the pitch used is stiffer and has never yielded a single drop.

Lord Kelvin’s pitch flow experiment. Photo: Geni/Wikimedia Commons

In 1944, an unknown professor started another one at Trinity College Dublin in Ireland. This physics experiment sat on a shelf in a lecture hall at Trinity College unmonitored for decades as it dripped a number of times from the funnel to the receiving jar below, also gathering layers of dust. In April 2013, physicists at Trinity College noticed that another drip was forming. They moved the experiment to a table and set up a webcam to record the falling drop. The pitch dripped on 11 July 2013, marking the first time that a pitch drop was successfully recorded on camera.

In the Hunterian Museum at the University of Glasgow, are two different experiments that demonstrate the incredible viscosity of pitch, installed by Lord Kelvin in the 19th century. Kelvin placed some bullets on top of a dish of pitch, and corks at the bottom: over time, the bullets sank and the corks floated. Lord Kelvin also showed that the pitch flows like glaciers, with a mahogany ramp that allowed it to slide slowly downward and form similar shapes and patterns to rivers of ice in the Alps.

Glass is viscous, but old church windows are not thicker at the bottom because the glass had sagged. Photo: Neil Lang/Shutterstock.com

There is another material that exhibit liquid like properties despite being seemingly solid. That material is glass.

Although glass has all the mechanical properties of a solid, its molecular structure is jumbled, like that of a liquid. Laboratory experiments have confirmed that even at room temperature, glass has a tendency to flow, but the viscosity of glass is several magnitudes higher than pitch. The popular myth that ancient windows are thicker at the bottom because the glass has sagged with the centuries is untrue.

To see glass flow would take an almost unimaginably long time, explains Dr Kostya Trachenko of Queen Mary University of London, who has been carrying out his own pitch drop experiments.

“If you wait longer than the age of the universe, you'll see this as a liquid.” Dr Trachenko said, tapping the glass cabinet that houses his students' pitch funnels. “It would flow. And that would be the end of it.”

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