What is the softest sand in the world? Why are a few sands softer than others?
– Peter S., Brooklyn, NY
We don’t know No one understands how sand works.
That sounds absurd, but it’s true. Understanding the flow of granular material like sand is a big unresolved problem in physics.
If you make an outerglass and fill it with a grain of sand with a known range of shapes and sizes, there is no formula to reliably predict how long it will take for the hourglass to pass through, or whether it will flow everywhere. You just have to try it.
Karen Daniels, a physicist at North Carolina State University who studies sand and other granular materials – a field that is actually called “soft matter.” – Told me that sand is challenging in part because grains have many different properties like size, shape, roughness and more: “One common reason we don’t have a general principle is that all of these properties matter.”
But understanding individual grains is just the beginning. “You have to care not only about the properties of the particles, but how they are arranged,” Dani said. Daniels said. Scattered grains feel soft because they have space to flow around your hands, but when the same grains are packed fairly tightly, they don’t have room to rearrange to accommodate your hands, so they feel firm. This is partly why the surface layers of beach sand feel softer than the following layers: In deeper layers the grains are pressed together.
Our failure to find the general principle of sand is not due to lack of effort. For everything from agricultural processes to landslide forecasting, it is extremely important to understand the flow of granular matter, and we are not very good at it.
“People who work in particulate handling in chemical engineering factories can tell you that those machines break down a lot of the time,” Dani said. Said Daniel. “Anyone who has tried fixing an automatic coffee grinder knows they hang all the time. These are things that don’t work very well. ”
Luckily, we’re not completely in the dark, and can say a few words about what makes sand soft or hard.
Spherical grain sand usually feels softer, as the grains pass through one another more easily. Even small grains do not produce a pinprick feeling of individual grains pressing into your skin. But if the grains are too small, the moisture causes them to stick together, making the material look clumsy and firm.
Dr. Dani. Daniels said the softest granular material she ever touched was a substance called Q-Cell, a silica powder used to fill surfboards. The powder is made of hollow grain, so it feels very light, and the silica material stays dry, which protects it from corrosion. It revolves around a bucket full of very nice, very dry beach sand compared to it.
A beach made of Q-cell “sand” may be soft, but it will not be very pleasant. Nice, dry powders are dust, not sand, and inhaling them can be very dangerous for your lungs. Ideal beach sand may have a grain size and shape with balanced softness, dustiness, clamping and a variety of properties that make the sand soft and nice for walking. Despite taking into account so many subjective factors, it is difficult to say exactly what the ideal soft beach sand would be.
You will only need to collect some experimental data.
