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Let’s take a walk along the edge of the beach. We see that the sand is wet
here because it is darker than higher up away from the water’s edge. As we
stroll along, we see that the sand suddenly begins to look brighter and lighter
in colour in a kind of halo pattern around our footprints. The halo indicates
that in those regions the sand is drying out. Why should that happen? Shouldn’t
your weight squish the water up around your feet and make the sand wetter,
rather than drier, in the region where you walk? Something funny is happening:
physics is afoot.

Sand can flow as it does when it sifts between your fingers. This granular
flow is, however, different from flows in ordinary liquids such as ocean water.
If we put force on a sand pile to make it move, the grains in one layer
interlock with the grains in the layer below it. If nothing else were to happen,
we would have to put enough force on the sand so that the tops of the sand
grains would shatter before flow could commence. But we know that this cannot
be: we have to push on sand only a tiny bit before it begins to move. The way it
does this is to dilate slightly making each grain wider but not so
deep—just enough so that the two adjacent layers, where the flow takes
place, separate sufficiently to no longer interlock. Now they can move past each
other with ease.

As you put your weight on the wet sand, the sand must move. It does so by
dilating slightly in the regions where the motion takes place. This dilation
leaves gaps within the sand that will then be filled up by the water above it.
The top layers of the sand then dry, becoming lighter in colour. You can see
this same effect in a simple experiment.

Let’s fill a plastic squeeze bottle three-quarters full with beach sand.
Now let’s take some of the ocean water and fill the bottle with just enough
water so that the water line is slightly, about a quarter of an inch, above the
top of the sand. Make sure that the water has filled in all of the interstitial
space between the particles. Now squeeze the bottle. What do you think should
happen?

Before I began to study sand, I would have thought that the water level
should rise. But it doesn’t. As you squeeze the bottle, the water level does
just the opposite and falls below the top surface of the sand.