Why are bubbles round?

Not only bubbles, but also rain drops. As far as I know this is because in such form (sphere) they have the smallest volume, compared to other objects with same dimensions.

pressure is exerted equally in all directions

Thanks you =]

“Surface tension in a bubble’s skin of soap and water molecules makes it pull in as tightly as it can, like a stretched balloon. It tries to hold the gas inside with the most stable structure possible and in a shape with the least amount of surface area. This shape is a sphere. Scientists call this a minimal surface structure.

A sphere is the shape that has the smallest surface area compared to its volume. It requires the least amount of effort (or energy) to form. Nature often does things the easy way or by seeking the lowest energy level.”

@uber: what is the source of that clear and scientifically accurate quote ?

@gailcalled http://www.seed.slb.com/qa2/FAQView.cfm?ID=1087

@uber: tnx

There’s a fun, topical, and incredibly difficult mathematical extension of your question: once you know that bubbles seek their minimal surface structures, you might ask what the minimal surface structure of two bubbles stuck together is. We know the answer roughly, of course—just pour out some bubble bath and stick them together. But to get the answer in a mathematically precise way is very difficult; the so called “double bubble” problem was solved only in 2000.

Pictures and explanation of double bubble are available at http://mathworld.wolfram.com/DoubleBubble.html. Here’s a portion of their description:

It had been conjectured that two equal partial spheres sharing a boundary of a flat disk separate two volumes of air using a total surface area that is less than any other boundary. This equal-volume case was proved by Hass et al. (1995), who reduced the problem to a set of 200260 integrals which they carried out on an ordinary PC. Frank Morgan, Michael Hutchings, Manuel Ritoré, and Antonio Ros finally proved the conjecture for arbitrary double bubbles in early 2000. In this case of two unequal partial spheres, Morgan et al. showed that the separating boundary which minimizes total surface area is a portion of a sphere which meets the outer spherical surfaces at dihedral angles of 120 degrees.

This is just another one of those ways in which unbelievably hard problems arise by pushing a little harder on the most innocuous questions.

Spheres minimize surface energy, as the other posters mentioned. You could also think about it from a dynamical perspective. A sphere has symmetry. Any other shape has some assymetry (some parts of the surface have higher curvature than others) and if there is no external field sustaining that, it will tend to move towards a symmetric (uniform curvature) shape. Just like when you pour milk in your coffee, you would not expect the milk to stay in one corner of the cup.

Of course, there is one common external field – gravity. Which is why falling rain drops are not perfect spheres.