Getting into the nitty gritty of sand  

Sand.

It has a way of getting everywhere on a beach day—and for some, it can make or break the whole trip. There’s the way it whips through the air just as sandwiches are being unwrapped, the way it turns blisteringly hot underfoot by midday, and the way it stubbornly clings to every piece of clothing.

But the same stuff people curse at the shore is quietly becoming one of the most important materials on Earth. And it’s starting to run short.

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“After water, sand is the second-most-used natural resource on the planet—the world goes through roughly 50 billion tons a year, says materials scientist Shu Yang. “That's enough to build a nine-story wall around the entire globe, and we’re pulling it from rivers, coasts, and seabeds faster than the hundreds of thousands of years Earth took to make it.”

Yang, who uses sand to create more sustainable concrete and building materials, says that while places like deserts are covered in sand, “not just any sand will do.” The wind-smoothed grains of dunes are nearly useless for construction, which is part of why a planet seemingly covered in sand can still face a shortage.

Penn Today caught up with Yang to get into the nitty gritty of what’s really going on beneath people’s feet.

Why some sand scorches your feet—and some stays cool

Whether a stretch of beach is pleasant or a barefoot hazard comes down to several factors, Yang says: composition, color, grain size, moisture, how tightly the sand is packed, and airflow.

“Darker sands absorb more solar radiation and can become much hotter than whiter ones,” she explains. Dry sand heats up fast because the air trapped between the grains is a poor conductor, so the heat stays near the surface; wet sand stays cooler because water conducts heat well and carries it off as it evaporates. Finer grains, with more surface area touching the skin, can also feel hotter underfoot.

If the air temperature is around 75° Fahrenheit, sand can climb above 100°—and on a 90° day, it can top 120°, well above the roughly 111° at which skin begins to burn. There’s even a name for the blistered soles that follow: “beach feet.”

“The standard advice to walk along the wet sand near the waterline is, it turns out, sound thermal physics,” Yang chuckles.

No two beaches feel the same underfoot

Why is one beach powdery while another is coarse? Most of it comes down to where the sand came from and how far it has traveled, Yang says.

Beaches near volcanoes or coral reefs tend toward rougher, larger grains, while shorelines pounded by constant waves get ground down into finer, smoother ones over time.

That same origin story is why beach sand comes in so many colors, Yang notes. Most of it is quartz, but it can be flecked with feldspar, basalt, or fragments of shell and coral. Hawaiʻi's Papakōlea Beach is tinted green by olivine crystals; the black sand beaches of Iceland and Hawaiʻi are pulverized by volcanic basalt; and the famous pink sands of Bermuda owe their blush to the red shells of tiny marine organisms called foraminifera.

For sturdier sandcastles, use water

When building sand structures, the biggest variable in how they behave, Yang says, is water.

Dry sand is governed by friction alone, so a dry pile won’t hold a shape. Add a little water, and everything changes: Thin films form “tiny liquid bridges between the grains, and the resulting capillary forces pull them together while surface tension resists pulling them apart,” Yang says.

It’s a phenomenon physicists call jamming, she explains. The same effect is why sand clings so stubbornly to wet skin—and brushes right off once skin has dried and those water bridges vanish.

Sand is everywhere, from drinking glasses to phones

Beyond construction, Yang notes, sand shows up “wherever glass is needed,” including in packaging as well as in water filtration and coastal protection.

Sand’s most consequential job, though, may be powering the device the one people are holding right now. Beach sand is mostly silica, and silicon refined from quartz sits at the heart of every computer chip. But to function as a semiconductor, silicon has to be refined to 99.9999999% purity, or about one stray atom for every billion atoms of silicon.

Getting there starts with unusually clean quartz, and somewhere between 70% and 90% of the world’s ultra-high-purity quartz comes from a single small Appalachian town where a continental collision some 380 million years ago forged quartz of exceptional purity. As the Wharton School’s Ethan Mollick puts it, “ The modern economy rests on a single road in Spruce Pine, North Carolina.”

If the right sand is so scarce, why not just make it?

Up to a point, Yang notes, people already do. The construction industry routinely manufactures sand by crushing rock, and chemists can synthesize quartz even purer than anything found in nature. But synthetic high-purity quartz can cost several times as much as the mined kind.

Sand’s next biggest job is also its plainest — and one that makes it a key component of an unsustainable practice. Concrete consists of sand and gravel bound together by cement, and it accounts for up to 9% of global greenhouse gas emissions.

Partnering with Masoud Akbarzadeh in the Weitzman School, Yang has published research on creating a more sustainable concrete using fossilized silica shells of microalgae called diatomaceous earth. She also founded Minerava, a startup that seeks to convert regional fine aggregates, biominerals, and waste materials into carbon-capturing, passive-cooling concrete to modernize infrastructure, in collaboration with Dorit Aviv of Penn Design and Akbarzadeh.

Shu Yang is a Joseph Bordogna Professor of Engineering and Applied Science in the Department of Materials Science & Engineering in the School of Engineering and Applied Science.