Sandcastle physics: discover the forces that make for perfect beach digs

Sandcastle physics: discover the forces that make for perfect beach digs – Earth/physical: sand/surface tension

Victoria Marcinkowski

Building a sandcastle involves nothing but sand, water, and your imagination–or does it? The physics behind what makes sandcastles stand certainly intrigues NASA. The agency has sent samples of sand aboard spacecraft on three missions to study its properties in the absence of gravity (attracting force between two objects). But you don’t need a space shuttle to figure out what makes sandcastles work.

“The two main components are compaction and moisture,” says professional sand sculptor Lucinda Wierenga. To compact sand, you scoop up wet sand and press it into a bucket, letting water ooze between sand grains like glue. “When you add liquid to sand, each drop of water seeks contact with the surface of two or three grains,” explains physics professor Peter Schiffer at Pennsylvania State University. Then surface tension (clinging force on the surface of a liquid) builds tiny bridges that make grains stick together.

The perfect water-to-sand ratio for building your dream castle takes experimenting: Too little water and there won’t be enough surface tension to counter the force of gravity–so the sand collapses. Too much water dissolves the bridges created by surface tension, and the castle becomes a soupy mess. “The best sand is fine grained,” says Wierenga. The smaller the grains, the better they stack up.

A sandy beach is really trillions of tiny grains of two substances: quartz, a hard mineral composed of the chemical compound silicon dioxide; and a compound called calcium carbonate that comes from seashells. Sand is worn down, or eroded, by millions of collisions between grains. To be classified as sand, a grain must measure between 1/12 and 1/20 of an inch across. Smaller particles are called silt or clay; larger particles are gravel.

Why would NASA want to study the stuff in space? Granular mixtures like sand have two different personalities: They can flow like liquid and be solid at the same time. (For example, you can both pour sand and walk on it.) If scientists can figure out how granular mixtures morph from solid to liquid, the results could benefit industries from building construction to food processing. “We’re trying to determine if wet sand sticks together in space the same way it does on Earth,” says Stein Sture, NASA’s lead investigator of the mechanics of granular materials.

You don’t have to wait for answers from NASA to make an awesome sandcastle on Earth. Follow these tips from Wierenga:

* Cut the bottom out of a big bucket and place it where you want to build your castle.

* Compress moist sand in the bucket and remove the bucket.

* Start carving and have fun!


Before you hit the beach, explore how surface tension holds grains of sand together with our hands-on activity.

PREDICT: Which will flow: dry sand or moist? Why?

YOU NEED: 2 wide-mouth 1-qt glass jars * dry sand * water * 25 pennies * sharpened pencil or chopstick


1. Fill one jar 3/4 full with dry sand.

2. Fill the other jar 1/4 full with dry sand. Add a little water and stir sand until moist. Repeat until the jar is 3/4 full of moist-not wet-sand. Pack sand flat.

3. Poke pencil or chopstick straight down into dry sand, then slowly remove. Does dry sand flow like a liquid or act like a solid?

4. Repeat step 3 with moist sand. What happens? Why?

5. Stir the moist sand and repack it. Insert a pencil at an angle of about 45 degrees from the top edge to the other side of the jar, then carefully remove the pencil.

6. Stack pennies on the surface. How many does it take to collapse the tunnel?


Scientists are trying to figure out how granular solids flow like liquids. Can you think of any reasons?


Did You Know?

* Fourteen phyla of tiny creatures, including microbes and sand fleas, live in sand. They keep beaches clean; without the tiny scavengers beaches would be mucky swamps.

* Surface tension is the principle that allows water droplets to form.

* Some sandcastles will stand up when they’re dry; that happens if the water used to moisten the sand contained salt or other minerals. The minerals form a crust between sand grains, holding them together even when water evaporates.

Cross-Curricular Connection

History: Most castles were built during the Middle Ages (ca. 1000-1500 A.D.). They usually housed a lord and his army. Find. out how real castles were constructed.

Critical Thinking: Why do you think rough-grained sand is better for building sandcastles than smooth-grained? (Hint: It has to do with surface area and surface tension.)


To learn more about what NASA’s doing with sand in space, visit this Web site:

Professional sand sculptor Lucinda Wierenga (a.k.a. Sandy Feet) offers castle-building tips on her Web site:

This article explores how weight gets divvied up in a granular mixture:

“A Fine-grained Look at Forces in Sand” by James Glanz, Science, April 11, 1997.

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