Double Bubble

What it shows

When two different size soap bubbles are connected together, the smaller diameter bubble will shrink and collapse to blow up the larger diameter bubble. One can use this to demonstrate Laplace's law or the phenomenon of minimizing the surface area of a soap film.

How it works

Laplace's law tells us that the gauge pressure of a spherical membrane is given by 2γ/r, where γ is the surface tension and r is the radius of the sphere.1 For soap bubbles (which have an inside as well as outside surface), the gauge pressure is twice this: 4γ/r. Since the pressure is inversely proportional to the radius, a small bubble is capable of blowing up a larger one.

Two inverted glass funnels are simultaneously dipped into a soap solution so that a soap film forms across the wide opening. When air is blown into the stem of the funnel, the soap film becomes a spherical bubble attached to the funnel. The funnels are interconnected by Tygon tubing and a three-way valve. The valve allows one to blow up each of the two bubbles separately and then connect them together.

double bubble

Setting it up

The bubble apparatus is contained in a closed box with a plastic see-through front and top. The closed box allows us to establish a humid, bubble-friendly environment. If possible, add the bubble solution to the trough the day before lecture and let it sit overnight.

Good technique is required of this demo, and the lecturer should either practice beforehand or ask one of the demo staff to be available to operate the apparatus. For best results make sure the big bubble is much bigger than the smaller one—go for as close as you can to a 2:1 ratio in radius, or else the slow change in volumes will be difficult for the audience to see when the bubbles are connected.


We copied this demonstration from C.V. Boys' lovely little book Soap Bubbles (Dover, 1959), p 50.

Besides illustrating the physics, the demonstration is a fine model for showing what would happen without the pulmonary surfactant inside the lungs. If the effect of large bubbles absorbing smaller ones were to take place in the lungs, the smaller alveoli would all collapse and the large ones would grow at their expense. This does not happen because the surfactant in the lungs is a phospholipid and its presence changes the surface tension as the radius of the alveoli changes, thus stabilizing the pressure between different size alveoli.2

1 Sternheim and Kane, General Physics, 2nd ed, (Wiley & Sons, 1991) pp 375-377.

2 J. Clements, "Surface Tension In The Lungs," Sci Am, December 1962, p 120.