November 03, 2003
Many people have noticed how wet sand seems to "dry out" beneath their footsteps, but few stop to think about what is happening. Many of my geology students at first tell me that my foot has somehow 'pushed the water away', but upon consideration they realize that I can't push the water any further than the edge of my shoe! When shear forces are applied to these 'dilatant sands', the individual grains slide up upon one another in such a way as to increase the porosity of the sand around your foot. The previously saturated sand now appears drier! When you lift your foot, the sand grains immediately relax (or settle) into a more compact alignment, and the sand instantly appears to be wet again. If you leave your foot in place for while, water will gradually seep in from the perimeter of the 'dry spot' to saturate the newly created pore spaces. Lifting your foot then will actually leave a pool of water where your foot was just previously positioned! Other materials, notably cornstarch and water, illustrate the dilatant property well, and in industrial settings handling dilatant fluids presents interesting problems.