Sand behaves pretty weirdly when you step on it. Helen explains why.
There’s something about walking barefoot along a sandy beach, just above the line of the waves, that is always special. Being the first to step on newly smoothed sand is liberating, and it’s as though you’re making a fresh start with every step. But something odd is happening. As each foot presses on the damp sand, you see a circle of lighter coloured dry sand spread out around it, which shrinks back as your foot lifts off. It’s weird, because you might expect that pushing the sand downwards would allow water to flood in. If anything, you’d expect the sand near your foot to get wetter, not drier. But sand is strange stuff, and that dry layer is showing you one of its oddest properties.
Look at a handful of sand, and you’ll see that it’s made up of tiny fragments of rock and shell in a range of shapes and colours. The shapes don’t fit snugly together, so even in tightly packed sand, the spaces between the grains account for around 40 to 50 per cent of the volume, and those spaces are filled with air or water. The rough surfaces of the grains lock them together, so the beach is solid enough to walk on.
When you step, your foot presses down on this interlocked pile, and this is where things get interesting. The tightly packed solid grains can’t squish downwards – there’s nowhere for them to go. But the area around your foot isn’t being pressed. The sand can’t be squashed, but it can still move out of the way by sliding diagonally. Therefore, as the sand right underneath your foot moves downwards, the sand to the side of that slides round and up to the sides. The key here is the process of nudging sideways – layers of interlocked grains need to move over each other, ever so slightly. But these grains are efficiently packed – each one sits in a small hollow made by its neighbours. Imagine a flat layer of apples in a grocery shop box, with just one apple on top. The apple starts off by sitting in a dimple in the layer below. You can push it sideways, but in order to move, it must rotate and lift up as it rolls over one of the apples below. The same thing is happening in the sand. Under your feet as you walk, hundreds of thousands of sand grains are pivoting over each other as the sand slides out of the way. As the grains roll out of their hollows, they get further apart overall.
This is the fundamentally weird thing about sand – as you force it to slide sideways like this, it increases in volume. As you step on it, the sand beneath your foot takes up more space, not less, and the expansion happens beneath that circle of lighter sand that you can see spreading outwards. The water drains downward into the larger gaps below, leaving a much drier top layer. Once you lift your foot off, everything rotates back and the water comes back to the surface.
So if you’re lucky enough to spend some time wandering along a beach this summer, take a moment to watch your steps, and to poke the sand and experiment. You never have to look far to find a fascinating piece of physics, because you live every day in the best toy box we have, and it’s all right there in front of you. All you have to do is take a moment to notice, and then let yourself play.