Get a Grip

    Kids, do your parents or other adult friends enjoy coffee?  Here is something to try with their bag of vacuum-packed coffee grounds.  Before opening a new bag, notice that it is rather solid.  Cut off a corner of the bag and what happens?  The coffee grounds will become instantly fluid-like.  Wikipedia tells us that this phenomenon is called “jamming”, which is the physical process by which some materials, such as granular materials, glasses, foams, and other complex fluids, become rigid with increasing density.

    Please note:  All chemicals and experiments can entail an element of risk, and no experiments should be performed without proper adult supervision.

    The so-called “jamming transition” happens when density is increased1. The crowding of particles makes the aggregate material behave like a solid. The density at which systems jam is determined by many factors including shape, deformability, friction, and dispersity.  Cutting the corner off the coffee bag instantly decreases the density by allowing air to flow inside.

    Granular materials that readily change between fluid and solid states are the basis for a new universal "gripper" technology recently invented by chemists at the University of Chicago. Most robots are designed with grippers that resemble the human hand and are extremely complex. Such robots need hinges and motors to control movement as well as image-processing systems so the gripper knows when to grab an object and sensors so the gripper knows not to crush it.

    The new idea is to design a bag-like gripper that, like the bag of coffee grounds, can readily change between fluid and rigid states. This allows the gripper to conform to the shape of the object to be lifted and then to be made rigid in order to lift and hold that object.  The inventors wanted a low-cost, adaptable solution, something that would enable the gripper material, itself, to make decisions.

    What the inventors found most satisfying about this project was starting from a basic physics idea (the jamming transition), extending it in a new direction that has real-world applications, and finally translating it into a working prototype. Their next goal is to see the universal gripper technology adopted as an alternative or supplement to the hand-based model for robotic grippers.

    ** The jamming transition has been proposed as a new type of phase transition, with similarities to a glass transition but very different from the formation of crystalline solids.


    Kathleen Carrado Gregar, PhD, Argonne National Labs
    January 2011


    2.  Prof. Heinrich Jaeger, the William J. Friedman and Alicia Townsend Professor of Physics at the University of Chicago: