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What makes sand soft?

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

– Peter S., Brooklyn, New York

We do not know. No one understands how sand works.

It may sound absurd, but it is somehow true. Understanding the flow of granular materials like sand is a major unsolved problem in physics.

If you build an hourglass and fill it with grains of sand with a known range of sizes and shapes, there is no formula to reliably predict how long the sand will take to pass through the hourglass, or s ‘it will sink at all. You just have to try it.

Karen Daniels, a physicist at North Carolina State University who studies sand and other granular materials – an area actually called “soft matter” – told me that sand is a challenge in part because grains have so many different properties, like size, shape, roughness and more: “One of the reasons we don’t have a general theory is that all of these properties are important.

But understanding the individual grains is just the start. “You have to be concerned not only with the properties of the particles, but with the way they are organized,” Dr Daniels said. Loosely packed kernels may feel soft because they have room to flow around your hand, but when the same kernels are squeezed together they have no room to rearrange to fit your hand, which makes them. makes firm. This partly explains why the surface layers of beach sand are softer than the lower layers: the grains of the deeper layers are close together.

Our failure to come up with a general theory of sand is not for want of trying. For everything from agricultural processing to landslide prediction, understanding the flow of granular material is extremely important, and we’re just not very good at it.

“People who work in particle processing in chemical engineering plants can tell you that these machines spend a lot of time breaking down,” Dr. Daniels said. “Anyone who’s tried to fix an automatic coffee grinder knows it’s stuck all the time. These are things that don’t work very well. “

Fortunately we are not totally in the dark and can say a few things about what makes the sand softer or harder.

Sand with rounder grains is generally softer, as the grains slide over each other more easily. The smaller grains also don’t produce the pin-prick sensation of the individual grains being pressed into your skin. But if the grains are too small, moisture causes them to adhere to each other, making the material lumpy and firm.

Dr Daniels said the softest granular material she has ever touched was a substance called Q-Cell, a silica powder used to fill in bumps on surfboards. The powder is made of hollow grains, so it is extremely light and the silica material stays dry, preventing it from sticking together. She compared the way he swayed to a bucket filled with very fine, very dry beach sand.

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

You will just need to collect some experimental data.