  A granular gripper is a device used to hold objects by taking advantage of
the phenomenon of Reynold's dilatancy. A membrane containing a granular sample
is allowed to deform around the object to be held and then vacuum is used to
jam the granular material inside the membrane. This allows to hold the object
against external forces since deformation of the granular material is prevented
by not allowing the system to increase its volume. The maximum holding force
supported by the gripper depends on a number of variables. In this work, we
show that in the regime of frictional holding (where the gripper does not
interlock with the object), the maximum holding force does not depend on the
granular material used to fill the membrane. Results for a variety of granular
materials can be collapsed into a single curve if maximum holding force is
plotted against the penetration depth achieved. The results suggest that the
most important feature in selecting a particular granular material is its
deformability to ensure an easy flow during the initial phase of the gripping
process.
