# How does an air bubble have 2 surfaces and a liquid drop has 1 surface?

My teacher told me that an air bubble has 2 surfaces and a liquid drop has only 1 which is the reason for the air bubble having twice the pressure difference as a liquid drop with same surface tension. But I couldn't get it how an air bubble has 2 surfaces isn't it just 1 spherical surface ? Please help

• I've removed a number of comments that were attempting to answer the question and/or responses to them. Commenters, please keep in mind that comments should be used for suggesting improvements and requesting clarification on the question, not for answering. Jun 21, 2020 at 9:45
• Are you thinking of an air bubble in water, or a bubble in air? (The former would have only one surface, the latter two.) Jun 21, 2020 at 21:49
• I was thinking about a bubble in air Jun 24, 2020 at 7:22

Think about what a microscopic observer would experience as they moved from somewhere outside the bubble to its centre.

With an air bubble they go from air (outside) to water (the "shell" of the bubble) to air again (centre). So they pass through two surfaces.

With a water drop they go from air (outside) to water (all the way from the surface of the drop to its centre). So they only pass through one surface.

• This doesn't explain the "twice the pressure difference" claim. Jul 15, 2020 at 13:14

I think there is a little confusion here between a bubble and a membrane.

What is referred here as an air bubble, is actually a membrane of liquid form enclosing a volume of gas, and floating inside another volume of gas (usually air).

And what people refer to as air bubbles, is this membrane, which like any other membrane has two surfaces, while a pure air bubble inside any liquid has only one surface.

A air bubble is hollow inside it. It doesn't pop or pops just due to pressure difference inside it and outside it. Hence it has 2 surfaces- the inside one and outside one

While water drop has only one surface i.e the outside one

• then why does a water drop pop ? Jun 21, 2020 at 9:42
• Who said a water drop pops? It doesn't pop . Its just a shape of water Jun 21, 2020 at 9:44
• A water drop can break due to stresses on it (like its own weight, and air resistance) overcoming its surface tension. A bubble is lighter, so it doesn't fall much against air resistance, and its insides don't have any more weight than the air outside. Jun 21, 2020 at 20:35

Think bubble as a bag. A bubble is an empty bag. The bubble has an inside and outside. The drop is a bag full filled with the same material of the membrane.