Re: Why are carbonated bubbles on a straw larger at the bottom?

You are right about bubble size for freely-floating bubbles -- a bubble at the bottom of a tank of liquid will expand to a larger size as they travel toward the top because of the decrease in pressure around the bubble.

Things are a little bit different when the bubble is sitting on a surface, though. There are several interactions: friction with the surface, fluid membrane tension, fluid pressure, and weight of the bubble.

The simplest explanation of the bigger bubbles has to do in part with the surface the bubble is sitting on.

Normally, a free bubble has fluid on all sides of it. The fluid pressure at the bottom of a bubble is slightly greater than the fluid pressure at the top of the bubble, which creates a net upwards push on the bubble -- a "bouyant force", first described by Archimedes of Samnos.

When a bubble sits on a surface, there is no fluid pressing on its bottom and the bouyant force is significantly reduced. This lack of fluid pressure on the bottom tends to flatten the bubble slightly, so the upward effect of pressure near the base is reduced. Additionally, the downward pressure on the bubble pins it to the surface with a force that is proporional to its depth in the tank. The increased downward pressure at greater depth, plus the lower upward contribution to bouyancy produced by the fluid around the bubble, means the bubble has to become larger to overcome the downward pressure than it would need to if it were freely floating in the tank (or, in this case, the glass).

If you look at the bubbles attached to the side of a glass, you often see that the bubbles get larger toward the bottom, too. These bubbles are slowed by being in contact with the sides of the glass, so they require a slightly greater bouyant force.

I hope this is helpful.