Question: Are objects such as butter or jelly solids or liquids?

Susan Cartwright answered on 21 Jun 2015:

Jelly is a gel (the similarity of the sounds is because both are derived from the word “gelatine”). A gel is basically a liquid, but with cross-linkages between long molecules (polymers) that give it some structural strength. The liquid part of the gel is bound to this three-dimensional polymer “scaffold” by surface tension forces – the same forces that cause water droplets to form little domes instead of spreading out.

Butter is a solid – mostly milk fat – with a low melting point, so that at normal room temperature it gradually melts. There is some water content in butter – about one-sixth – but not enough to qualify it as a gel.

The distinction between solid and liquid can be very fuzzy. For example, bitumen – the black stuff used in tarmac – behaves in most respects as a solid: if you have a lump of it, you can knock bits off with a hammer. However, it is actually better described as a very, very sticky liquid, with a viscosity about 230 billion times that of water. Two experiments, one in Dublin and one in Australia, show that bitumen can drip through a funnel, though *very* slowly: the Queensland one was set up in 1927 and has had 9 drops fall, the most recent one in 2014. The 2013 drip in the Dublin experiment can be seen in this video: http://www.nature.com/news/world-s-slowest-moving-drop-caught-on-camera-at-last-1.13418. Lord Kelvin, professor of physics at Glasgow University where I did my degree, started two pitch experiments in the 19th century: one was a “pitch pool” containing corks (which have by now floated to the top of the pitch) and bullets (which have sunk), started in 1882, and the other was a “pitch glacier”, in which a block of pitch slowly flows down a slope. I remember seeing this one – it used to live in the foyer of the Physics Department, though I think it has now been moved to the Hunterian Museum. Apparently, the existing one is not Lord Kelvin’s original, but a replica created by students in 1915 – which still makes it 100 years old!