The physics
A liquid surface pulls along its edge, like a stretched skin. The pull on each metre of edge is the surface tension S. Its unit is N/m.
Making new surface takes work. Each square metre of new surface stores energy S. So S is also the surface energy per square metre.
F = S × edge length E = S × new area
A soap film has two faces, front and back. So it pulls a straight edge with 2Sl.
A curved surface squeezes what is inside it. So the pressure inside a drop is higher than outside. A soap bubble has two surfaces, so its extra pressure is twice as big.
drop, or air bubble in water: ΔP = 2S/r soap bubble: ΔP = 4S/r
Under water, add the water's pressure ρgh too. Two bubbles that touch share a film. The smaller bubble pushes it into the bigger one.
A bubble's air keeps P × V the same at a steady temperature. In a room, 4S/r is tiny. In a chamber that a pump has almost emptied, 4S/r matters.
(P₁ + 4S/r₁) r₁³ = (P₂ + 4S/r₂) r₂³