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What Makes Sand Soft?

Understanding how grains flow is vital for everything from landslide prediction to agricultural processing, and scientists aren’t very good at it.

By Randall Munroe

Nov. 9, 2020, 5:00 a.m. ET

What is the softest sand in the world? Why is some sand softer than others?

— Peter S., Brooklyn

We don’t know. No one understands how sand works.

That may sound absurd, but it’s sort of true. Understanding the flow of granular materials like sand is a major unsolved problem in physics.
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If you build an hourglass and fill it with sand grains with a known range of sizes and shapes, there is no formula to reliably predict how long the sand will take to flow through the hourglass, or whether it will flow at all. You have to just try it.

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Karen Daniels, a physicist at North Carolina State University who studies sand and other granular materials — a field actually called “soft matter” — told me that sand is challenging in part because the grains have so many different properties, like size, shape, roughness and more: “One reason we don’t have a general theory is that all of these properties matter.”
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But understanding individual grains is only the start. “You have to care not just about the properties of the particles, but how they’re organized,” Dr. Daniels said. Loosely packed grains might feel soft because they have room to flow around your hand, but when the same grains are packed together tightly, they don’t have room to rearrange themselves to accommodate your hand, making them feel firm. This is part of why the surface layers of beach sand feel softer than the layers underneath: the grains in the deeper layers are pressed closer together.
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Our failure to find a general theory of sand isn’t for lack of trying. For everything from agricultural processing to landslide prediction, understanding the flow of granular materials is extremely important, and we just aren’t very good at it.

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“People who work in particulate handling in chemical engineering factories can tell you that those machines spend a lot of time broken,” Dr. Daniels said. “Anyone who’s tried to fix an automatic coffee grinder knows they get stuck all the time. These are things that don’t work very well.”
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Luckily, we’re not totally in the dark, and can say a few things about what makes sand softer or harder.

Sand with rounder grains usually feels softer, because the grains slide past each other more easily. Smaller grains also don’t produce the pinprick feeling of individual grains pressing into your skin. But if the grains are too small, moisture causes them to stick together, making the material feel clumpy and firm.

Dr. Daniels said that the softest granular material she had ever touched was a substance called Q-Cell, a silica powder used for filling dents in surfboards. The powder is made of hollow grains, so it feels extremely light, and the silica material stays dry, which keeps it from clumping. She compared the way it sloshes around to a bucket full of very fine, very dry beach sand.

A beach made of Q-Cell “sand” might be soft, but it wouldn’t be very pleasant. Fine, dry powders are dust, not sand, and inhaling them can be extremely hazardous to your lungs. The ideal beach sand would probably have a grain size and shape that balanced softness, dustiness, clumping and a variety of other properties that make sand soft and nice to walk on. With so many subjective factors to consider, it’s hard to say exactly what the ideal soft beach sand would be.

You’ll just need to gather some experimental data.