We have been measuring the surface tension of the AB interface at zero pressure, in high magnetic fields and low temperatures below 0.2 T-c. We manipulate the phase boundary by controlling a magnetic field profile. We use the latent heat released/absorbed as the phase boundary moves to infer its position and velocity. We have observed that the motion of the interface through a small aperture is dependent on the magnetic field gradient. Here we extend numerical methods first used to calculate the shapes of liquid drops in a gravitational field to show that the gradient dependence can be accounted for by the deformation of the interface.