  We report a simple experiment of two-dimensional pattern formation in a
circular Hele-Shaw cell, showing the appearance of a return branch that is
equivalent to the upward-connecting leader of lightning. Injecting water from
the center into a foam filled cell, we obtained patterns similar to dendrites
of two-dimensional dielectric breakdown experiments. When we repeat this
experiment allowing the presence of water in the outer (low pressure) region,
dendrites grow initially as in a normal experiment, but when a branch is near
the outer boundary, the low pressure water begins to penetrate the foam against
the pressure field, forming several return branches.

Fluid Dynamics;Pattern Formation and Solitons