Sector boundary drift
The expansion velocity can be stochastic, and depend on the shape of the front (curvature and slope in a "gradient" expansion):
The generic scaling of fluctuations of the above equation is super-diffusive:
Uniform (isotropic) growth: the coalescence rate is obtained by method of images as
Sector boundaries drift on a sloped surfaces, as
Stochastic fluctuations of the rough surface thus lead to random drift of the sector boundary, such that