![]() ![]() Boys' lovely little book Soap Bubbles (Dover, 1959), p 50.īesides illustrating the physics, the demonstration is a fine model for showing what would happen without the pulmonary surfactant inside the lungs. 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.įor 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. If possible, add the bubble solution to the trough the day before lecture and let it sit overnight. The closed box allows us to establish a humid, bubble-friendly environment. The bubble apparatus is contained in a closed box with a plastic see-through front and top. The valve allows one to blow up each of the two bubbles separately and then connect them together. The funnels are interconnected by Tygon tubing and a three-way valve. ![]() When air is blown into the stem of the funnel, the soap film becomes a spherical bubble attached to the funnel. Two inverted glass funnels are simultaneously dipped into a soap solution so that a soap film forms across the wide opening. Since the pressure is inversely proportional to the radius, a small bubble is capable of blowing up a larger one. 1 For soap bubbles (which have an inside as well as outside surface), the gauge pressure is twice this: 4γ/r. 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. ![]() One can use this to demonstrate Laplace's law or the phenomenon of minimizing the surface area of a soap film. When two different size soap bubbles are connected together, the smaller diameter bubble will shrink and collapse to blow up the larger diameter bubble. ![]()
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