Our lab is looking at large aspect ratio (large length to diameter ratio) granular materials. These materials' resistance to disturbances is striking. One can easily run fingers through a bucket of sand, but not through nails. When the aspect ratio is large, a solid "plug" forms that holds its shape even when the initial container is removed. This is seen in the pile below, which was formed by putting acrylic rods (3" long, 1/16" in diameter) in a big cylinder and then lifting the entire pile by a small (1/4") ball at the bottom. When the cylinder is removed, the pile retains its cylindrical shape, supported only by the ball at the bottom.

This plug forms despite the presence of large voids into which particles could move. The rigidity results from particle entanglement, which greatly constrains the ability of particles to rotate.

Writing assignments are used as a pedagogical tool in nearly every discipline, including physics. Often these assignments are used merely to assess student understanding. The thought processes used to generate a written document, however, may also be instrumental in helping students internalize and master more difficult topics. Thus students, when writing, not only display their knowledge, but may also learn. Our lab is investigating the mechanism by which this learning occurs --- learning which currently is widely assumed but unexplained.

EiP is published by John Wiley & Sons. Visit the EiP web page for more information.