Science & Environment

Tedium, tragedy and tar: The slowest drops in science

Pitch drop experiment, Dublin Image copyright Shane Bergin
Image caption In long-running experiments in Brisbane and Dublin, a drop of pitch falls about once every ten years

Life is speeding up. News travels fast, data is everywhere and buzzing gadgets keep our schedules ticking over. What can we learn from the physics of the very, very slow?

In two famous - and famously lengthy - experiments, scientists wait for pitch to drip from a funnel. It happens about once a decade, because pitch is a substance so viscous that it is, to all intents and purposes, a solid. Lives have come and gone while the drops went undripped, or fell unwitnessed.

Recently, a student project in London picked up the pace, using slightly runnier pitch in a similar set-up.

The invisible, inevitable progress of these inky half-fluids has captured the public's attention; the drops have acquired meaning and drama.

But alongside the storytelling, Dr Kostya Trachenko is adamant there are important measurements to be made. He and his undergraduate students at Queen Mary University of London put their pitch into five different funnels, with different sized openings.

Pitch, or bitumen, is the black muck left over from distilling crude oil, and can also be produced by heating wood. It is a liquid at high temperatures but becomes very hard when it cools down, which makes it useful in waterproofing and road surfacing.

At normal room temperature, in fact, a lump of pitch can be shattered with a hammer.

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Media captionTimelapse footage of 24 days in the life of Dr Trachenko's comparatively high-speed pitch drop project. Video courtesy of Kostya Trachenko / QMUL

"We convincingly saw that it behaves like a liquid," Dr Trachenko says as he shows me the set-up, clearly proud of his students' work. "And we were also able to quantify it - because in physics it's important to attach a number to the process."

As you would predict for a liquid, the bigger openings let more pitch through: almost 53g over the course of a year for the widest (6mm), compared to 5g for the narrowest (2.5mm).

Waiting game

This rate is positively heart-stopping by comparison with the best-known experiment of this kind, which began in Brisbane, Australia, in 1927. There, at the University of Queensland, Prof Thomas Parnell let some pitch solidify in the top of a funnel - and waited.

Prof Parnell, a Cambridge graduate and veteran of World War One, wanted to show his students that if you watched it for long enough, it would still flow like a liquid.

It was a long wait. The funnel dripped its first drop in 1938 and only eight more have fallen since.

Image copyright University of Queensland
Image caption The Queensland experiment, photographed in 1979
Image copyright Christian Aas / University of Queensland
Image caption Pitch, like glass, is hard and breakable but its molecular structure is jumbled and irregular, like that of a liquid

In 1961, just three drops later, a new lecturer called John Mainstone adopted the experiment after a colleague pointed it out, gathering dust in a cupboard. He eventually persuaded the university to put it on display and the drops became talking points.

Prof Mainstone, however, never saw the pitch in motion. In 1979, the sixth drop went on a weekend. In 1988, with the experiment proudly displayed at Brisbane's World Expo, Prof Mainstone was fetching a drink when the seventh drop fell. By 2000, a video camera had been set up to capture drop number eight, but it malfunctioned at the crucial moment.

When the ninth drop fell in April this year it was watched by three webcams and thousands of online enthusiasts - but not by Prof Mainstone, who died eight months earlier at the age of 78.

The Queensland experiment itself was pipped to the post of posterity in July 2013, when another long-standing funnel of pitch became the first to drip a drop in public. At Trinity College Dublin, a very similar set-up dating from 1944 was filmed as it shed a black blob into its own beaker, offering the first ever glimpse of such an event taking place.

Image copyright Christian Aas / University of Queensland
Image caption Prof Mainstone was the custodian of the Queensland pitch drop for 52 years

Dr Shane Bergin, a physicist and senior research fellow at Trinity, explains that it was the feeling of suspense in the Brisbane story that rekindled interest in the Dublin drop. "Eventually, when our one was caught on camera, it provided the world with a kind of scientific 'Aaaah' moment," he says. "As in, finally, we see it!

"In a world where we expect to expect things to happen very quickly, and stuff is demanded of us instantaneously, it's a little quirky to think that a lot of stuff just happens on a time scale that's much slower than we can normally appreciate."

Ongoing controversy

All this drawn-out drama could be accelerated, of course, if the pitch was warmed up. Pitch is one of a group of substances called "glasses" which, when cooled, become hard and solid-seeming, despite maintaining the jumbled molecular structure of a liquid. There is no obvious shift to a more rigid, crystalline organisation of molecules, which happens when water and other liquids freeze. Glasses just get steadily slower and stiffer as the temperature drops.

So the famous experiments, Dr Trachenko insists, could be made even drearier. "If you put it in the fridge, it would take thousands of years," he points out, with a dry smile. "For a theorist like myself, 70 years is actually not that long."

Image caption Dr Trachenko's pitch was more of a drizzle than a drop, but it provided data for his theory about the liquid glass transition

Window glass belongs in the same category, but debate has raged for years among physicists as to whether solid glass in fact represents a different "phase" of matter. Dr Trachenko believes that all of the pitch drop results, including those of his students' project, are important evidence to the contrary.

"Even though glass is a familiar system, explaining how it forms is a big deal in modern physics research," he says.

"Once we assume that the behaviour is essentially liquid-like... we can come up with equations to predict how long it would take for solid glass to flow, even though we would never ever see it."

Dr Trachenko is firmly in the camp that sees glass as "merely a very viscous liquid" and has published calculations and models in support of that view, based on the way it absorbs heat and behaves under pressure.

The popular myth, however, that ancient windows are thicker at the bottom because the glass has sagged with the centuries, is wide of the mark. They have thicker edges because they were made that way, Dr Trachenko is quick to explain.

To see glass flow would take an almost unimaginably long time, he says - but it would happen.

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

That is a long wait to win an argument. In the meantime, he relies on maths.

"For physicists, one billion years is not much different from one second. It's just a number. It's an extremely long number, but I can quantify the process."

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Media captionProf Mainstone on the Today programme in 2013

And to go with the numbers, he now has observations from the dribbles of bitumen inside the cabinet.

Their comparatively speedy flow, with multiple drips from multiple funnels, allowed Dr Trachenko's students to calculate - and publish - the average "viscosity" of their pitch, measured in units called pascal-seconds. (About eight million pascal-seconds, to be precise.)

By that measure, they estimate it is thirty times runnier than the pitch used in Queensland, a million times runnier than glass, and a hundred billion times thicker than water.

The long view

Relative runniness can be important. A recently rediscovered pitch drop experiment at Aberystwyth University in Wales actually predates the famous Queensland funnel by 13 years - but its pitch is stiffer and has never yielded a single drop.

In fact, it has barely entered the stem of the funnel and is unlikely to bear fruit for at least 1,300 years.

The Hunterian Museum at the University of Glasgow, meanwhile, contains two remarkable demonstrations set up in the 1800s by renowned physicist Lord Kelvin. Intrigued by some of the same questions as Drs Parnell, Mainstone and Trachenko, Kelvin placed bullets on top of a dish of hard, black pitch, and corks at the bottom: over time, the bullets sank and the corks floated.

Lord Kelvin also showed that the flow of pitch is genuinely glacial, with a mahogany ramp that allowed it to slide imperceptibly downward and form similar shapes and patterns to rivers of ice in the Alps.

Image copyright The Hunterian, University of Glasgow
Image caption Kelvin's "pitch pool", with floating corks and sunken bullets, and "artificial glacier" both date from the 1880s

The inexorable intrigue of these experiments exerts an obvious pull on our imagination.

"People are genuinely curious," says Dr Bergin from Dublin, where the Trinity College pitch drop is slated to be moved into a public exhibition space. "We want to use it as a hook to show that physics tries to understand the universe, from the nitty gritty to the super duper, and all of the wacky stuff that's in there as well.

"We're going to start a new, bigger pitch drop here in Trinity as well. We figured, why not?"

Back in London, Dr Trachenko similarly loves the idea of challenging the intuitions of his students - and the public. "It reminds people that the physical world is not about us," he says. "We are just passers-by."

But he also maintains that there are real findings to be made.

Image copyright Kostya Trachenko / QMUL
Image caption After this experiment, there are stickier plans still afoot for QMUL physics students

Modelling how materials like glass behave in the longest of long terms, Dr Trachenko says, could help plan for the safe disposal of nuclear waste that takes millions of years to decay. "That's not an experiment you can check in the lab."

He is clearly not finished with pitch - nor with his students.

"There are two jars on my shelf," he says with a hint of mischief. "The second one is more viscous."

As soon as the fastest flowing funnel in his current set-up is empty, Dr Trachenko is obviously itching to test out the even stickier stuff. He guesses that the next crop of students might need the entire length of their degree to get results.

"If they measure the viscosity of that one, then maybe they can hope for an extra grade."

And if they learn from history as well as physics, his students will be checking their camera twice.

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