Scientists from Trinity College Dublin captured the falling drop of very high viscosity bitumen or asphalt in a pitch-drop experiment. As Rheologists say, “Everything flows”. Well…in the case of pitch, you need to be extremely patient. This material appears to be an elastic solid to the naked eye. As the drops from these experiments show, however, it is actually an extremely viscous fluid! The Dublin pitch-drop experiment was set up in 1944. This drop took so long to form and fall (1 drop/10 years) that the scientists actually forgot about the experiment and only recently, started recording it with a webcam. Viscosity estimates from the last drop put it at 2 x 107 Pa-s or twenty billion times greater than water. Imagine sitting in front of the funnel with your notebook and your stop watch. Now imagine that you fall asleep and then the drop finally falls!
These experiments involve heating the tar sample to lower its viscosity (which at room temperature is about 20 billion cP) and pour it over a sealed glass funnel. Then a bit of time is allowed (three years!) to let it settle and consolidate before it is allowed to flow through the bottom of the funnel. And all this is just the preparation for the long wait required for a drop to form and fall. Looking at this kind of experiment, I can’t help to think that there must be a better and faster way of measuring viscosity.
Johnston R (2013) World's Slowest-Moving Drop Caught on Camera at Last. Nature, Nature Publishing Group, Available from: http://www.nature.com/news/world-s-slowest-moving-drop-caught-on-camera-at-last-1.13418 (accessed 2015).