  We present experimental evidence, which contradicts the the standard boundary
conditions used in continuum theories of non-cohesive granular flows for the
velocity normal to a boundary u.n=0, where n points into the fluid. We propose
and experimentally verify a new boundary condition for u.n, based on the
observation that the boundary cannot exert a tension force Fb on the fluid. The
new boundary condition is u.n=0 if Fb.n>=0 else n.P.n=0, where P is the
pressure tensor. This is the analog of cavitation in ordinary fluids, but due
the lack of attractive forces and dissipation it occurs frequently in granular
flows.
