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.
That sounds ridiculous, but it’s true. Understanding the flow of granular materials such as sand is a major unresolved problem in physics.
If you build an hourglass and fill it with grains of sand of all known sizes and shapes, there is no formula to predict with certainty how long the sand will flow through the hourglass. or whether it will melt or not. You have to try it.
Karen Daniels, a physicist at North Carolina State University who studies sand and other granular materials – a real field called “soft matter”. – telling me that sand is a challenge partly because particles have so many different properties, like size, shape, roughness and more: “One reason we don’t have a general theory is all of these properties are important ”.
But understanding each particle is only the beginning. “You have to not only care about the properties of the particles, but also how they are organized,” said Dr. Daniels. Loosely packed seeds may feel soft as they have room to flow around your hand, but when identical seeds are tightly packed together they have no room to rearrange to fit your hand. make them feel solid. This is part of the reason why the sand layers on the beach’s surface feel softer than the underlying layers: the particles in the deeper layers are pressed closer together.
Our failure to come up with a general theory of sand was not due to lack of effort. For everything from agro-processing to forecasting landslides, understanding the flow of granular material is extremely important and we are not very good at.
“The people working in chemical engineering plants can tell you that the machines have been broken a lot of time,” said Dr. Daniels. “Anyone who tries to fix an automatic coffee grinder knows that they are always in trouble. These are things that don’t work very well ”.
Fortunately, we are not completely in the dark and can say a few things about what makes sand softer or harder.
Sand with rounder particles is usually softer, because the grains slide through each other more easily. The smaller particles also don’t feel like the needles of individual particles press against your skin. But if the particles are too small, the moisture will cause them to stick together, making the material feel lumpy and firm.
Dr. Daniels says the softest granular material she has ever touched is a substance called Q-Cell, a type of silica powder used to fill dents in surfboards. The powder is made from hollow particles so it feels extremely light and dry silica helps prevent clumping. She compared how it plunged into a bucket full of very fine and very dry beach sand.
Q-Cell “sand” beach may be soft, but it won’t be very pleasant. Fine, dry powders are dust, not sand, and inhaling them can be extremely dangerous for your lungs. The ideal type of beach sand can have a balanced grain size and shape between softness, dirt, clumping, and many other properties that make the sand soft and easy to navigate. With so many subjective factors to consider, it’s hard to say exactly what the ideal soft beach sand would be.
You just need to collect some test data.