Let `R` be the radius of curvature of common surface when bubbles `A` and `B` of radii `R_(A)` and `R_(B)` coalesce.The excess pressure in `A` and `B` are `4T.R_(A)` and `4T.R_(B)`, respectively.
`implies :. p_(A)=(4T)/R_(A)` and `p_(B)=(4T)/(R_(B))`
If `R` is radius of common interface, then we must have
`p_(A)=p_(B)=(4T)/R`
or `(4T)/(R_(A))-(4T)/(R_(B))=(4T)/R`
This gives
`R=(R_(A)R_(B))/(R_(B)-R_(A)) =(0.002xx0.004)/(0.004-0.002)=0.004m`
As the excess pressure is always towards concave surface and pressure in smaller bubble is greater than that in larger bubble, the common surface is concave towards the centre of the smaller bubble.