Question

1. When you blow a bubble with either a soap solution or chewing gum, the bubble is a sphere. Why? Explain this in terms of gas particle behavior rather than surface tension.

2. When you blow a bubble under water (in a swimming pool, for example), the bubble is not a sphere- it’s either flattened or it looks like the cap on a mushroom, depending on it’s size. Why? (Hint: What do you feel in your ears as you dive deeper under water?)

Answer 1

1.

2. If a bubble is moving through another fluid, it can lose its spherical (round) shape because the forces pushing on the bubble from the outside change as it moves through a gas or liquid. For instance, if you go SCUBA diving you will notice that the air bubbles that you blow out into the water are less dense than the water and repelled from it, so they form spherical bubbles that begin to rise to the surface. When the bubbles rise to the surface, they expand and are pushed against by the water above them that they are moving through. If the bubbles are big enough and rise fast enough, then the force of the water pushing against the top of the bubble causes the larger bubbles to flatten. These bubbles change shape into a bowl, with a rounded bottom and a flattened top. This flattening effect also happens to large raindrops as they fall from the sky and are flattened by the force of the air pushing against them from below. The larger bubbles of carbon dioxide that form and rise in a soda bottle after opening it probably also flatten in this way.

The flattening of bubbles only happens to larger raindrops and air bubbles as they move, because in order for the forces pushing on the outside of the bubble to cause it to flatten, these forces have to exceed the surface tension that is trying to pull the drop into a spherical shape in order to minimize its surface area. The force of the fluid pushing against the outside of the bubble or raindrop is only greater than the surface tension for larger bubbles and raindrops. This is because the larger ones rise (in the case of bubbles) or fall (in the case of raindrops) faster than small ones and so the force on their outside is greater.