  We report the experimental study of the degassing dynamics through a thin
layer of shear-thinning viscoelastic fluid (CTAB/NaSal solution), when a
constant air flow is imposed at its bottom. Over a large range of parameters,
the air is periodically released through series of successive bubbles,
hereafter named {\it cascades}. Each cascade is followed by a continuous
degassing, lasting for several seconds, corresponding to an open channel
crossing the fluid layer. The periodicity between two cascades does not depend
on the injected flow-rate. Inside one cascade, the properties of the
overpressure signal associated with the successive bubbles vary continuously.
The pressure threshold above which the fluid starts flowing, fluid deformation
and pressure drop due to degassing through the thin fluid layer can be simply
described by a Maxwell model. We point out that monitoring the evolution inside
the cascades provides a direct access to the characteristic relaxation time
associated with the fluid rheology.
