What is the softest sand in the world? Why are some sand softer than others?
– Peter S., Brooklyn, NY
We do not know. No one understands how sand works.
This may sound absurd, but it is true. Understanding the flow of granular materials such as sand is a major unsolved problem in physics.
If you make an hourglass and fill it with sand grains with a known range of shapes and sizes, there is no formula to reliably predict that sand flows through the glass per hour How long it will take, or will it flow 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 challenging in part because grains have so many different properties, such as size, shape, roughness and more: “One reason we don’t have the general theory is that all of these properties matter.”
But understanding individual grains is only the beginning. “You have to care not only about the properties of the particles, but how they are arranged,” Dr. Daniel said. Loose rash may feel softer because they have space to flow around your hand, but when the same grains are tightly packed together, you have to adjust yourself to your hand again. There is no place to organize from, which makes them feel firm. This is part of the reason that the surface layers of beach sand feel softer than the bottom layers: in deeper layers the grains are pressed together.
Our failure to find a general theory of sand is not for lack of effort. For everything from agricultural processing to landslide prediction, understanding the flow of granular material is extremely important, and we are not very good at it right now.
Dr. “People who work particle handling in chemical engineering factories can tell you that those machines spend a lot of time,” Daniels said. “Anyone who has tried to fix an automatic coffee grinder knows that they get stuck all the time. These are things that do not work very well. “
Luckily, we are not completely in the dark, and can say a few things about making sand soft or hard.
Sand with rounder grains usually feels softer, as the grains slide past each other easily. Small rash also does not evoke the pinnip feeling of individual rash pressing into your skin. But if the grains are too small, the moisture clings them together, making the material feel sticky and firm.
Dr. Daniels stated that the softest granular material he had ever touched was a substance called Q-Cell, a silica powder used to fill dents in surfboards. The powder is made of hollow grains, so it feels extremely light, and the silica material remains dry, which prevents it from sticking. He likened the way a very fine, very dry beach sand bucket was dropped all around.
A beach made of Q-cell “sand” may be soft, but it will not be very pleasant. Fine, dry powder is dust, no sand, and inhaling them can be extremely dangerous to your lungs. The ideal beach sand will probably have a grain size and shape with balanced softness, dust, stiffness and many other properties that make the sand soft and good to move. With so many subjective factors to consider, it is difficult to say what would be the ideal soft beach sand.
You will just need to collect some experimental data.