  One of the oldest and most intriguing problems in the handling of materials
is how a collection of solid grains packs together. While granular packing is
normally determined by how grains are poured or shaken, we find that a
systematic and controllable increase in packing is induced by simply raising
and lowering the temperature, e.g., without the input of mechanical energy. The
results demonstrate that thermal processing provides a largely unexplored
mechanism of grain dynamics, as well as an important practical consideration in
the handling and storage of granular materials.
