Frontiers Physics World  January 2018

Gyrated dice achieve perfect packing

(Diego Maza )

A new and rapid way to pack identical cubes in a dense configuration has been discovered by physicists in Spain and Mexico led by Diego Maza of the Universidad de Navarra. They poured 25,000 small plastic dice into a clear cylindrical barrel with a radius of 8.7 cm (left) before repeatedly twisting it back and forth. Slow twists tended to align the dice at the edges, while leaving those in the middle disordered. When the twist acceleration increased to 0.52g, however, the dice ended up in horizontal layers after about 10,000 twists, and were arranged in nearly perfect concentric rings within each layer (right). The twisting itself does not agitate the dice; they’re instead jolted by the change in direction, which induces shear. The team also ran experiments with more and fewer dice: the heavier load of more dice competed with the shear process to stop the dice from densely packing together, while fewer dice required a far smaller acceleration to become aligned. The work could be useful in industry as pharmaceutical powders, for example, are packed by repeatedly tapping them with ever-decreasing intensity, but this is slow and inefficient (Phys. Rev. Lett. 119 228002).

Benjamin Skuse