What is the softest sand on the earth? Why is a few sand softer than others?
— Peter S., Brooklyn, N.Y.
We don’t know. No one understands how sand works.
That might sound absurd, however it’s kind of true. Understanding the circulation of granular supplies like sand is a serious unsolved drawback in physics.
If you construct an hourglass and fill it with sand grains with a recognized vary of shapes and sizes, there isn’t any formulation to reliably predict how lengthy the sand will take to circulation by the hourglass, or whether or not it can circulation in any respect. You have to only attempt it.
Karen Daniels, a physicist at North Carolina State University who research sand and different granular supplies — a area truly referred to as “soft matter” — advised me that sand is difficult partly as a result of the grains have so many alternative properties, like dimension, form, roughness and extra: “One reason we don’t have a general theory is that all of these properties matter.”
But understanding particular person grains is simply the beginning. “You have to care not just about the properties of the particles, but how they’re organized,” Dr. Daniels mentioned. Loosely packed grains would possibly really feel gentle as a result of they’ve room to circulation round your hand, however when the identical grains are packed collectively tightly, they don’t have room to rearrange themselves to accommodate your hand, making them really feel agency. This is a part of why the floor layers of seashore sand really feel softer than the layers beneath: the grains within the deeper layers are pressed nearer collectively.
Our failure to discover a common principle of sand isn’t for lack of attempting. For all the pieces from agricultural processing to landslide prediction, understanding the circulation of granular supplies is extraordinarily necessary, and we simply aren’t superb at it.
“People who work in particulate handling in chemical engineering factories can tell you that those machines spend a lot of time broken,” Dr. Daniels mentioned. “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.”
Luckily, we’re not completely at the hours of darkness, and may say just a few issues about what makes sand softer or more durable.
Sand with rounder grains normally feels softer, as a result of the grains slide previous one another extra simply. Smaller grains additionally don’t produce the pinprick feeling of particular person grains urgent into your pores and skin. But if the grains are too small, moisture causes them to stay collectively, making the fabric really feel clumpy and agency.
Dr. Daniels mentioned that the softest granular materials she had ever touched was a substance referred to as Q-Cell, a silica powder used for filling dents in surfboards. The powder is fabricated from hole grains, so it feels extraordinarily gentle, and the silica materials stays dry, which retains it from clumping. She in contrast the best way it sloshes round to a bucket stuffed with very wonderful, very dry seashore sand.
A seashore fabricated from Q-Cell “sand” could be gentle, however it wouldn’t be very nice. Fine, dry powders are mud, not sand, and inhaling them will be extraordinarily hazardous to your lungs. The best seashore sand would most likely have a grain dimension and form that balanced softness, dustiness, clumping and a wide range of different properties that make sand gentle and good to stroll on. With so many subjective elements to contemplate, it’s laborious to say precisely what the perfect gentle seashore sand could be.
You’ll simply want to assemble some experimental knowledge.