  Granular grippers are highly adaptable end-effectors that exploit the
reversible jamming transition of granular materials to hold and manipulate
objects. Their holding force comes from the combination of three mechanisms:
frictional forces, geometrical constraints, and suction effects. In this work,
we experimentally study the effect of particle size on the suction mechanism.
Through X-ray computed tomography, we show that small particles (average
diameter d = 120 micrometers) achieve higher conformation around the object
than larger particles (d = 4mm), thus allowing the formation of air-tight
seals. When the gripper is pulled off, mimicking lifting of an object, vacuum
pressure is generated in the sealed cavity at the interface gripper--object. If
the particles used as filling material are too large, the gripper does not
conform closely around the object, leaving gaps between the gripper's membrane
and the object. These gaps prevent the formation of sealed vacuum cavities
between the object and the gripper and in turn hinder the suction mechanism
from operating.
