Saturday, August 8, 2015

Patterns in the Sand

Crane Beach, MA (June 17, 2006)

One thing that I like of the beach environment is how water, wind and sand collude together to create infinite magical miniature worlds. Grain by grain, as in a real-life game of minecraft, the water and the wind draw intricate patterns in the sand. Valleys, mountains, dunes, a fantastic topography replicating on a tiny scale geological features etched on Earth's continents. Walking on the beach on the line where the sea meet the sand is like flying over a Martian landscape imaged by a NASA probe.

A miniature river delta
Making a sand castle
Looking at the patterns in the sand of a beach we are faced with the fractal nature of the world. Shapes repeat across scales, self-similar, in an intricate game of russian dolls. This similitude of patterns, however, betrays the actual complexity in the physics of sand. While the forces of nature act equally on a multitude of scales, their effects are scale dependent. Even objects that are made of the same substance, such as a grain of sand and a large block of sandstone, will react to external forces in different ways, because the effect of these forces will depend differently on a combination of their size, surface area and weight. This is why the slightest of breeze can blow away the tiny grains of a dry sand beach, but you won't be able to move a pebble made with the same material by blowing on it with all the strength of your lungs. At the small scales of a grain of sand, physics becomes very complicated. Sand particles are in fact subjected to forces that have no parallel in the macroscopic world, and that in some conditions make them stick together like a solid, while in others make them flow like a liquid. This is why, during earthquakes, solid ground can sometimes suddenly liquefy. Given the obliquity of granular materials in our everyday world, one would think that by now we would have figured out the peculiarities of the physics of sand. Not so; these phenomena remain among the most baffling problems in material science. Which fortunately doesn't prevent us to admire and enjoy the beautiful miniature landscapes produced in mysterious ways by the wind and the water on our favorite beach.

Crane Beach, MA (June 17, 2006)

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