Mud cracks when it shrinks and become brittle. If you make a uniform layer of mud and dry it uniformly, you get a pattern of irregular polygons with edges that usually meet at 90 degrees. If the layer is very thin, you get polygons with meandering "diffusive" cracks at the edges. In either case, contrary to popular belief, most of the polygons are not hexagons. For some pictures, see the gallery. Near perfect hexagons are, however, found in the famous "columnar jointing" patterns in some lava flows. This phenomenon is brought about by a crack orienting effect which ocurrs near a propagating shrinkage front. There are some nice examples of long hexagonal columns at the Devil's Postpile in California and the Devil's Causeway in Ireland (see our Gallery Page).

To get at the crack orienting effect in a simplified system, we have been studying regular crack patterns in a two dimensional slab of mud (actually, we used nice clean Aluminum Oxide powder and water) after a drying front has moved through it. The experimental arrangement looked like:

A unidirectional breeze causes the layer to dry from one end to the other. We find that a very regularly spaced crack array is formed behind the front. The spacing depends on the strength of the adhesion to the substrate and the depth of the layer, but strangely enough, not on the speed of the front motion.

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The Experimental Nonlinear Physics Group, Dept. of Physics, University of Toronto,
60 St. George St. Toronto, Ontario, Canada, M5S 1A7. Phone (416) 978 - 6810.