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CAN LIQUIDS FORM FILMS?

We are going to make various copper wire forms such as those shown in the following drawing.

Last week we made a mystery liquid by putting four level teaspoons of soap powder into four cops of hot water. (If there isn't any soap powder, one teaspoon liquid detergent will do.) There really isn't any mystery about a soap solution, but it is a mystery until we know what we're going to do with it.

We'll dip the forms into the solution and then carefully take them out of the liquid. Take a look at that! Looks like that bubble stuff, doesn't it? It probably is very close to
being the same type of solution.

Instead of just admiring the pretty bubbles we would get by gently blowing through the film of liquid stretched across the wire form, we will try to figure out how it got there, and why it stayed.

To call this liquid on the wire a "film" really doesn't tell us what it is. Mr. Webster didn't help an awful lot this time, when I looked up "film," because he said "a thin
coating," and then also described the kind of film we use in cameras. The word came from an Anglo Saxon word which meant "skin," and that probably is the best description of what "film" means. Have you ever accidentally left a glass of milk on the countertop, and later, noticed a kind of coating on the surface of the milk? Or have you noticed the coating that forms on the surface of pudding if it is left uncovered? It is like skin, in that it covers the surface, and it is thin.

So a film of soap solution has formed on the wire we put into the solution. Let's dip the wire in again, and see if the film gets thicker, or if another film forms on it.

Our wires, of course, are of different shapes, so we know that the liquid will adapt to many shapes. We already learned that liquid has no shape of its own, and that
something else, like a container, has to give it a shape.
What can we do to change the shapes? The square form has a slider attached to it. Very gently pull the slider, and see what happens to the film on the wire. It stretches. That means it got longer and thinner. If you pull the slider too far, the film will break, so just pull it a little bit. What
happens when you let go of the slider? It goes back to its original position, doesn't it. How did that happen? The liquid was doing its trick--making the surface as small as
possible. The strength of the surface tension is what made the slider return.

It's almost like elastic, isn't it?

 

 
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