20 July 2011

Building on Sand

This is one of Eric Wilbur’s photographs for the Boston Globe of this month’s sand-sculpting contest at Revere Beach. I was struck by how many of this year’s entries had large arches, tunnels, and other hollow (or cantilevered) structures in their designs.

The Sandscapes team says:
Wherever we go we invariably hear someone in the crowd say that the "Big Secret" is: sugar water, hairspray, unicorn tears or Mountain Dew. Other than attracting every ant within five miles and throwing piles of cash out the window these magic additives would accomplish little, if anything. . . .

In a word the big secret is "friction". More specifically, the sum total of all friction between the grains acting on each other. This is why compaction is so important. When you compact sand you increase the friction between the grains.

Uncompacted sand has relatively large pore spaces between the grains but compacted sand shrinks these spaces increasing points of contact between the individual grains and thereby increasing the friction between them. The more friction there is, the more resistant the grains are to separation.
While that article pooh-poohs the importance of the surface tension of water within the sand matrix, NASA says that’s definitely a force in how sand castles stick together.
Damp sand sticks together because water forms little grain-to-grain bridges. Surface tension--the same force that lets some insects walk on the surface of a pond--acts like rubberbands between the grains. Adding water to damp sand fills spaces between the grains. The bridges vanish and the sand begins to flow more easily.
In sum, there’s an equilibrium state between wet and dry, which probably varies for a particular form of sand. The sand at Revere Beach might be particularly well suited for this type of sculpture.

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