I was having a discussion with my mates about what would happen if you filled a swimming pool with jelly and jumped in. Some of the group believe you would sit happily on the surface. Others, myself included, reckon you would sink, and risk drowning as the jelly collapsed around you. We wouldn鈥檛 want anybody to be harmed, so we don鈥檛 recommend experimenting to find out. But do any New 杏吧原创 readers have a theoretical answer to the question?
鈥� The active ingredient in jelly dessert or jello is gelatin, a protein-based gelling product made from collagen.
Gelatin comes in different grades, or Bloom numbers, as measured by the force required to push a plunger into a solution of the stuff to a predetermined depth: the more rigid the sample, the higher the Bloom number. Jelly babies 鈥� a popular British sweet shaped like a miniature baby 鈥� have a high Bloom number, so there is little danger of drowning in a pool of the mixture used to make them.
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The density of jelly is typically 10 per cent higher than water, so a swimmer would float higher in a pool full of jelly than in water. Jelly is also more viscous than water, meaning that someone diving into jelly might have difficulty surfacing. However, two researchers from the University of Minnesota, Minneapolis, won the 2005 Ig Nobel prize for chemistry for showing that people could swim just as quickly in water spiked with guar gum, an edible thickening agent, as in ordinary water. The spiked liquid has double the viscosity of water, yet the increased drag is cancelled out by the increase in thrust that swimmers can generate in it.
鈥淛elly is more viscous than water, so someone diving into jelly might have difficulty surfacing鈥�
While we鈥檙e on the subject of desserts, custard is interesting as it becomes much more viscous under pressure. It is possible to walk across a pool full of the stuff, as demonstrated on the UK TV series Braniac ().
Mike Follows, Willenhall, West Midlands, UK
鈥� Jelly is an interesting substance because it behaves like both a solid and a liquid. We did some simple experiments to pin down its behaviour.
We found we could stack five 20-cent coins on top of some jelly before they punctured the surface, meaning the jelly was able to support a pressure of 700 newtons per square metre. This is only around one-tenth of the pressure a person would exert if they tried to sit on a pool of jelly, so presumably such an attempt would fail.
Once the surface broke up, we found the jelly behaved as a very viscous fluid, flowing around a small capsule we pushed into it. From the force required to move the capsule, we estimated the jelly鈥檚 viscosity to be around 50 pascal seconds (50 times that of castor oil).
This is so large that a person jumping into a pool of jelly would be slowed dramatically in the first fraction of a second 鈥� doing a belly-flop onto jelly would hurt. They would then sink gradually until their buoyancy became neutral, although they would float higher than in water as jelly is a little denser.
Finn Lattimore and Ruth Mills, Canberra, ACT, Australia