The location of each bubble is incremented by its velocity, given in equation (24) as a function of the radius, growth rate, local material properties of the polymer melt, and distance from the sample surface, multiplied by the timestep duration. This calculation is performed in a geometry containing only the polymer melt, to reflect the assumption that bubbles are sufficiently scattered so as not to influence each others' motion. If the bubble is held at the surface to allow for thin-film draining before bursting, it is not allowed to travel further than the position at which its radial velocity is equal to zero.