MadSci Network: Physics

Re: Why can soap film be flat.

Date: Mon Mar 2 15:33:32 1998
Posted By: John Christie, Faculty, School of Chemistry, La Trobe University, Bundoora, Victoria, Australia
Area of science: Physics
ID: 888709249.Ph

I have two problems. I know approximately what you are asking, but I am not 
quite sure exactly what your concern is. Please excuse me if I don't quite 
answer the right question. My second problem is that it is very difficult 
to answer a question like that. Why shouldn't soap film be flat?

Anyway, here goes: To get a flat soap film, make a flat loop of wire, dip 
it in a bath of bubble-blowing solution, and gently lift it out. You can 
get a film stretched right across the loop in the wire, and it will be 

Now, as you point out, if you make a soap bubble, the shape of the film is 
not flat, but spherical. Do soap films have to be flat or spherical? No. 
Can they be any shape they want to? No, though perhaps they can for a very 
short time. There are only a very small number of shapes that are stable 
shapes a soap film can adopt (as opposed to ones they can rapidly pass 
through while they are wobbling).

Let me show you another shape a soap film can adopt. Make a square loop of 
wire, and twist two opposite sides, so that the square is skewed, with two 
diagonally opposite corners sticking up, and the other two sticking down. 
If you put a soap film over this loop, you will find that it is saddle-

What is special about these three 'soapy' shapes -- the sphere, the flat 
surface, and the saddle-shape -- that makes them different from 'non-soapy' 
shapes like egg-shaped, or ripply, or elephant-shaped? It has to do with 
minimum area. Soap films are a bit like stretched rubber. They try to 
shrink as much as possible. The minimum surface area that covers a flat 
loop is a flat plane; the minimum surface area that covers a skewed square 
is a saddle-shape; and the minimum surface area that encloses a fixed 
volume of gas is a sphere.


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