Let R be the radius of curvature of common surface when bubbles A and B of radii RAandRB coalesce. The excess pressure.
in A and B are 4T.R_A and 4T.R_B, respectively.
∴pA=4TRAandpB=4TRB
If R is radius of common interface, then we must have
PA−pB=4TR
Or4TRA−4TRB=4TR
This gives R=RARBRB−RA=0.002×0.0040.004−0.002=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.