Pitch Drop Experiment, Waiting around ten years for one drop

We use Ring-and Ball test to get the softening point of bitumen. The softening point is defined as the temperature at which the two disks soften enough to allow each 3.5 gram steel balls fall a distance of 25 mm. During the Ring-and Ball test, the liquid is heated with a certain temperature increasing speed. This is because bitumen is brittle at lower temperature and will slowly change to slow-flowing materials or viscous liquid. Without heating to high temperature. the bitumen need long long long… time to fall down.

How long will it take? The Pitch Drop Experiment can tell you!

The Pitch Drop experiment was designed to test the fluidity and high viscosity of pitch, a derivative of tar once used for waterproofing boats (kind of hard bitumen binder). At room temperature pitch feels solid – even brittle – and can easily be shattered with a blow from a hammer. It’s quite amazing then, to see that pitch at room temperature is actually fluid!

In 1927 Professor Parnell of the University of Queensland in Brisbane heated a sample of pitch and poured it into a glass funnel with a sealed stem. Three years were allowed for the pitch to settle, and in 1930 the sealed stem was cut. From that date on the pitch has slowly dripped out of the funnel – so slowly that now, 80 years later, the ninth drop is only just forming.

The Pitch Drop experiment was set up as a demonstration and is not kept under special environmental conditions (it is actually kept in a display cabinet in the foyer of the Department), so the rate of flow of the pitch varies with seasonal changes in temperature. Nonetheless, it is possible to make an estimate of the viscosity of this sample of pitch. It turns out to be about 100 billion times more viscous than water!

In October 2005, John Mainstone (Prof who is current carrying on this experiment) and the late Thomas Parnell were awarded the Ig Nobel Prize in Physics, a parody of the Nobel Prize, for the pitch drop experiment.

It’s been more than 12 years and four months since the last time this the drop stretched down on 28 November 2000. And it seems that the ninth drop is coming.

In the past 80 years that the pitch has been dripping but no one has ever seen the drop fall. If you’re interested in trying your luck, or at least just having a look at the experiment, you can view it live below.

Reference: AASHTO T 53 and ASTM D 36

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