A 2 Liter bottle full of soap bubbles.
Material
A two Liter bottle, try to get one with vertical
sides.
A Marker Pen that will write on the bottle.
A liter or more of soap bubble solution:
100 ml "Dawn original scent" blue color.
900 ml water
optional 10 ml glycerin
Assembly
Fill the bottle with the soap solution.
Put the cap on the bottle.
Shake the bottle.
Take the cap off the bottle.
Invert the bottle into a large mouth jar, let it glug, glug, glug
out.
Put the cap back on the bottle.
To Do and Notice
Notice the pattern of bubbles in the bottle.
In particular notice the pattern of bubble walls that touch the sides of the bottle.
Notice how often three bubble walls come together at a point.
Use the marker pen to draw several of the patterns of bubble walls.
Notice that the angle between bubble films is near 120 degrees. (Measure this with a protractor.)
Optional you can even cut the bottle open and flatten out the side walls to study your patterns.
What's Going On?
When bubble walls meet at 120 degree angles the surface area of the bubbles is minimized.
This is true of the lengths of a network of lines connecting points in a plane. The length of the network of lines is often minimized when three line junctions are used in which the lines make 120 degree angles with each other.
Activity by Paul Doherty
From an original discovery by Marie Wallum.
Scientific Explorations with Paul Doherty 

1 Feb 2000 