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
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.
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.