A pressure difference across a liquid layer will accelerate it. For incompressible and inviscid motion, it is possible to describe the motion of the surfaces through boundary integral techniques. In particular, dipole distributions can be used together with an external flow that specifies the acceleration. The classical Rayleigh-Taylor instability and the creation of bubbles at an orifice are two important applications. A new method for the numerical approximation of the boundary integrals removes the difficulties associate with surfaces in close proximity.