Friday, 17 April 2009

Kitchen physics - making cross bedding

This demonstration - 'Making cross bedding' is fun.
A mixture of sand and sugar will separate into its components if poured slowly and evenly into a pile. The process can also be tried with different materials - sand, glass beads, sugar crystals - and the results are clear. Typically, larger grains will have a steeper angle of repose than smaller ones, and they will roll down the slope more energetically. The smaller grains tend to get stuck at the top of the pile, the larger ones at the base—they spontaneously segregate. But things become more complicated. As the different angles of repose of different grains are reached and exceeded, successive avalanches will be made up of different-sized grains. The cascades of smaller grains will stop first, to be then covered by a layer of the larger grains still on the move. The process repeats itself over and over, creating a layered pile. Different (and unpredictable) results can be achieved by varying the size, density, and shape of the grains and therefore their angle of stability or repose.

No comments: