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. IntroductionThe handling of objects is a regular task in the industry. Holding objects of well defined size, shape and hardness can be done by robotic arms that present a gripper with a matching shape that fits the object to hold (e.g., hooks). Also, magnets can be used with ferromagnetic objects, and suction systems with objects presenting smooth surfaces. However, matching some of the gripping charac-

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. IntroductionThe handling of objects is a regular task in the industry. Holding objects of well defined size, shape and hardness can be done by robotic arms that present a gripper with a matching shape that fits the object to hold (e.g., hooks). Also, magnets can be used with ferromagnetic objects, and suction systems with objects presenting smooth surfaces. However, matching some of the gripping charac-