|John Mainstone with the eighth drop in late 1990, about|
2.5 years after the seventh drop fell!
eensland in Australia, has never actually seen the pitch drop. He explains that he checks it about 5-6 times a day, but does "need some sleep"! In 1979, he missed the drop after skipping his usual Sunday campus visit, and in 1988 he missed it by 5 minutes after stepping out "to get a refreshment." The last drop, in 2000, was supposed to be captured on a film setup, but the equipment failed. The drop takes less than a tenth of a second, so even if he was present, if he blinked, he would have missed it.
Tar is distilled from wood, coal, or peat. It consists of hydrocarbons, resins, and alcohols. Because it acts like a solid, it was used in early days to preserve wooden ships against rot, the largest user being the Royal Navy. Tar may be best known to many people for the La Brea Tar Pits in Los Angeles. Many animals got trapped in the tar pits, died, and left behind their skeletons. I've not seen the tar pits in a very long time, but my memory is that they are a mixture of thick viscous tar and solid mounds.
Tars have been described as Bingham plastics between 85-180 C, having a defined yield stress and plastic viscosity. Anyone who has the misfortune (like me, twice) to drive on a road that is undergoing resurfacing or repair knows what happens when tar cools below this temperature. In the case of my car (just yesterday), the hot liquid material splashed up, cooled and solidified on the sides and back of my car. (Fortunately, there are products on the market to remove tar from vehicles, but they don't tell you that it also requires a lot of elbow grease!).
So, Professor Mainstone has shown that if you give even cold tar enough time (more than 80 years), tar isn't really a "solid" at all. Hmmmm....I wonder if I wait that long if it would fall off my car, or just drop down another inch or two!