Facts You Should Know: Why does wet sand become dry around your feet?
This is because your body weight causes a distance that is measured by the Reynolds number. The latter gives a measure of the ratio of inertial forces to viscous forces, and consequently quantifies the relative importance of these two types of forces for given flow conditions. You can easily observe this phenomenon at the seaside, at low tide.
Every time you push your feet on the wet sand, a clearer line surrounds it, as if the water were sucked out under your footprint. Intuitively, we would expect the opposite: When we walk on a wet sponge, the water drains out of it. In 1860, the physician Osborne Reynolds managed to explain this phenomenon.
The sand is made of grains and contains interstices between the grains. At low tide, the to and fro movement of water compresses the sand to the maximum: The sand grains are perfectly arranged, and hence the space between them is minimal. Under the pressure exerted
by the foot, the grains spread, and therefore, the spaces between them increase. In other words, the sand dilates. The water present all around the foot is sucked up to fill the void, as if in a kind of “water inflow.” Consequently, this almost dry sand line surrounds your feet.
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