  The first stages of finger formation in a Hele-Shaw cell with lifting plates
are investigated by means of linear stability analysis. The equation of motion
for the pressure field (growth law) results to be that of the directional
solidification problem in some unsteady state. At the beginning of lifting the
square of the wavenumber of the dominant mode results to be proportional to the
lifting rate (in qualitative agreement with the experimental data), to the
square of the length of the cell occupied by the more viscous fluid, and
inversely proportional to the cube of the cell gap. This dependence on the cell
parameters is significantly different of that found in the standard cell.

Statistical Mechanics