Physics in the fast lane

By Matin Durrani

Most of us want everything in life right here, right now. From fast food to fast cars, none of us can be bothered to hang about any longer than absolutely necessary. Where’s your reply to my e-mail I sent five minutes ago? Why haven’t you responded to my Tweet? Do you really expect me to read that 500-page novel for fun?

It was perhaps as an antidote to the ever-faster pace of life that so much has been made of two physics experiments that recently produced new data for the first time in years. I’m talking, of course, about the “pitch-drop” experiments at Trinity College Dublin in Ireland and the University of Queensland, Australia, which both consist of a glass funnel of sticky tar-like substance. A drop from the Trinity experiment finally fell last July, with a video of the event quickly going viral, while the Queensland set-up dripped this April for the first time in 13 years. (For more on why both experiments proved so popular, check out our great feature by Shane D Bergin, Stefan Hutzler and Denis Weaire from Trinity.)

But if you can’t be bothered to hang around for 10 years or more, you’ll be pleased to hear that physicists at Queen Mary University of London – led by Kostya Trachenko – have now set up a new pitch-drop experiment to explore the difference between solid and liquids on the much shorter timescale of just a few months.

The Queen Mary set-up is different to both well-known pitch-drop experiments. It uses a different bitumen that’s 30 times less viscous than the Queensland experiment, so that the flow can be seen faster. What’s more, Trachenko and two undergraduate students – Amy Widdicombe and Prathisan Ravindrarajah – have installed not one but five different glass tubes of varying diameters to give five speeds of flow, and set up web cameras to catch the drops in action.

“We’re using the pitch-drop experiment to inspire our students and make them question the fundamental nature between solids and liquids,” says Trachenko. “Because our experimental set-up is unique, we have proof that apparent solids such as bitumen can flow over long time scales – in this case, one academic year.”

You can find more details about the experiment in a new paper appearing today in the journal Physics Education, published by the Institute of Physics, which also publishes Physics World. There’s also a nifty “video abstract”, embedded above, featuring Widdicombe and Ravindrarajah, who used the set-up to measure the viscosity of bitumen as part of a summer project.

Well, they might be faster than the Irish or Australian set-ups, but the Queen Mary physicists still aren’t as quick as teacher Trevor Cawthorne from Queen Elizabeth’s Grammar School in Horncastle, Lincolnshire, UK. He recently set up his own pitch-drop-style experiment that drips about once a day.

Now that is fast.