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Asked by Meg to Chris, Josh, Rebecca, Rob, Susan on 14 Jun 2015.
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Susan Cartwright answered on 14 Jun 2015:
The sphere is the shape that gives you the least surface area for a given volume – for example, a cube of the same volume as a sphere has 24% more surface area than the sphere does. The surface of a liquid has a property called surface tension: the molecules at the surface exert an attractive force on each other, and try to pull the surface into the most compact possible form. Because a sphere has the least surface area, it’s the most compact form, so a sphere is what you get, either for a bubble (formed by a thin film of liquid, with two surfaces close together) or a liquid droplet like a raindrop (contrary to popular belief, raindrops are spherical – it’s only when they hit something that they become elongated).
This property of a sphere also makes it the most compact shape for a given mass of material. This is why planets and stars are spherical: their gravity pulls their mass inwards to the most compact possible shape – and that’s a sphere.
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Rob Temperton answered on 15 Jun 2015:
Good morning!
I agree entirely with Susan’s explaination. Surface tension in liquids is pretty much what I researched for a year when doing my masters degree – it is a very important phenomenon.
The only thing I will add is a diagram which helps explain it:
https://upload.wikimedia.org/wikipedia/commons/f/f9/WassermoleküleInTröpfchen.svgThis is showing that the molecules deep in the bulk (in the middle) of the liquid are balanced – they are pulled equally in every direction by the surrounding molecules. At the surface of a liquid, there are only molecules of the liquid on one side so the pulling force is unbalanced and acts inwards. This is the origin of surface tension – it acts a bit like a pressure which forces the liquid into a state where the surface area is minimised.
Rob
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