15 minutes or fewer
Anyone who’s ever spent a day at the beach building sandcastles knows the type of sand you use makes all the difference. Dry sand doesn’t stick together or form clumps; instead it flows almost like a liquid. If you pick up a fistful of dry sand, it’s hard to keep it from streaming out between your fingers. But if you add a little water to dry sand, it suddenly becomes packable and moldable. Wet sand sticks to itself, holding its shape, and can be sculpted into towers or mermaids on the beach!
The combined properties of sand and water are what make wet sand act so differently from dry sand. Sand is hydrophilic, or “water loving,” which means that water molecules (H2O) can adhere, or stick, directly to it. The adhesion of water molecules to sand grains explains why sand behaves differently when it gets wet. But why does wet sand clump together? Water tends to stick to itself—a property called cohesion. That tendency causes water to form a tiny network of bridges between individual sand grains that holds them together.
What happens when you change the properties of sand so that it becomes hydrophobic, or “water fearing,” instead? Will it still stick together when it gets wet? Let’s find out:
Sand, split into two containers
Scotchgard (get the help of an adult to use this safely)
Tray or baking sheet
Tip: Cover your tray with newspaper to speed drying and to protect the container.
Two bowls of water
Well-ventilated work area
Optional: spoon, water dropper
Make weird sand
- Pour one container of sand out in a thin layer onto a covered tray or baking sheet. Save the other container of sand for comparison.
- In a well-ventilated area and with the help of a parent, spray Scotchgard evenly over the entire surface of the sand until it appears wet. Be sure to follow safety label instructions, and see our projects policy here.
- Let the sand dry in a well-ventilated area.
- Use your hands to mix up the sand. Then spread it out on the tray, and spray Scotchgard on it a second time for good measure. Let it dry completely in a well-ventilated area.
- Once the sand is completely dry, sprinkle water onto a small pile of it. What happens?
Be curious, ask questions, mess with stuff.
- Does the sand feel different in your hands compared to normal sand? Did spraying the sand with Scotchgard change its appearance or texture in any way? Describe what you observe.
- Try getting the sand wet with more water. Does the sand stick together? Are you able to sculpt it to form shapes or mounds? Would this sand make good sandcastles?
- Does it look like the sand is hydrophobic or hydrophilic, based on how it responds to water?
- What happens when you submerge the weird sand completely in water? Look carefully at the sand underwater. Does this weird sand look and behave differently from normal sand that’s been submerged in water? Why do you think that is?
- Scoop the weird sand out of the water. What does it look like now? Did the water make the sand wet, or did the sand repel it?
Aha! You’ve made hydrophobic sand!
Scotchgard is a spray-on coating that protects surfaces from stains. It’s highly hydrophobic, so it repels water-based liquids. When you coat sand with Scotchgard, you change it from being hydrophilic to hydrophobic, thus preventing water from adhering to it. That’s why water dropped onto the weird sand beads up like it would on a raincoat (another hydrophobic material). Hydrophobic sands are super fun—so fun that they have been sold as a special kind of toy, called “Magic Sand.”
Experiment more, make predictions. It’s what scientists do.
- Ask questions: What do you think will happen if you use a liquid other than water to try and get your weird sand wet?
- Modify the experiment: Try other liquids, like vegetable oil, to see if you can turn your weird sand into a castle-building material.
Next, use a different substance to make normal sand act weird! Mix ¼ cup cornstarch into your sand. Slowly add ¼ cup liquid dish soap to your sand-cornstarch mixture. How is this new sand different from normal sand? How is it different from the weird sand you created with the Scotchgard?
- Make an educated guess: Based on your observations of this cornstarch-and-dish-soap sand, do you think that there are “bridges” between sand particles like there are in normal wet sand? Why or why not?
- Predict what will happen: Make some observations of how dish soap interacts with water and how cornstarch interacts with water. Based on your observations, what do you think this cornstarch-and-dish-soap sand will do in the presence of water? How will it behave differently from the first weird sand that you made? Test out your prediction!
- Document your experimental results: Take photos of each type of sand you made, being careful to document its texture, appearance, and packability, and how it changed with the addition of water.
- Tell other people: Teach other people what you learned! Work as a team to find other hydrophobic or hydrophilic substances, and experiment with different sands and mixtures.
Did you do this nifty experiment? Show us your sand concoctions by tweeting them to @scifri with the hashtag #WeirdSand
Schiffer, P. (2005). Granular physics: A bridge to sandpile stability. Nature Physics, 1(1), 21-22.