  The yielding behavior of a sheared Laponite suspension is investigated within
a 1 mm gap under two different boundary conditions. No-slip conditions, ensured
by using rough walls, lead to shear localization as already reported in various
soft glassy materials. When apparent wall slip is allowed using a smooth
geometry, the sample is shown to break up into macroscopic solid pieces that
get slowly eroded by the surrounding fluidized material up to the point where
the whole sample is fluid. Such a drastic effect of boundary conditions on
yielding suggests the existence of some macroscopic characteristic length that
could be connected to cooperativity effects in jammed materials under shear.
