Key Concepts
Physics
Materials
Compression
Geology

Introduction
Summer is a nice time to take a stroll at the beach and walk barefoot along the shoreline. While doing that, have you ever looked at your footprints in the wet sand? If so, you might have noticed that with every step it looks like the sand around your feet dries out. Why is that? These dry footprints are caused by the pressure of your feet. You will find out exactly how this happens by trying this beachy activity!

Background
Many beaches are made of sand, which comes from—rocks that have been ground into tiny particles by water and wind. Materials such as sand that are made of many separate tiny particles are called granular materials. Even when sand particles appear to be directly touching each other, because they are irregularly shaped, there are tiny spaces in between them. (Think about how a pile of larger rocks has similar spaces between them.) These spaces are called pores. There are many pores between all the sand particles at the beach.

If you pour water on the sand, the water seems to disappear into the sand. It doesn't actually disappear—it drains into the tiny pores between the grains. Once all these pores are filled with water, the sand is saturated, which means that the sand cannot take up any more water. When you squeeze this saturated sand, you would probably expect the water in the sand to come out of the pores again, similar to what happens when you squeeze a wet sponge. However, this is not what happens. The exact opposite is the case. More water seems to disappear into the sand! The reason for this is something called dilatancy of granular materials.

Dilatancy means that a material expands when you squeeze it (put it under pressure) instead of contracting. This happens because under pressure the sand grains actually push each other slightly farther apart, which makes more space between them. This means there is more space for water to flow into, resulting in a dry footprint on the beach. Once the pressure is released, the sand grains settle closer together again, leaving less room for water. In this activity you will demonstrate this wet-sand effect—and you don't even have to be at the beach!

Materials

  • Bowl
  • Sand
  • Water bottle (narrow mouth)
  • Two large balloons (ideally they will be transparent)
  • Two transparent straws
  • Two rubber bands
  • Paper
  • Spoon
  • Water
  • Towel
  • Workspace that can tolerate spills
  • Wide-mouthed plastic water bottle (optional)
  • Ruler (optional)
  • Tape (optional)
  • Permanent marker (optional)
  • C-clamp (optional)
  • Two pieces of scrap wood (optional)


Preparation

  • Use the piece of paper to make a funnel, and place the funnel into the mouth of the narrow-mouth water bottle. Then spoon the sand into the paper funnel, filling the water bottle all the way up with dry sand.
  • Inflate one of the balloons. Then stretch the balloon's neck over the mouth of the water bottle. Flip the bottle upside-down, and pour the sand into the balloon.
  • Once all the sand is in the balloon remove the balloon from the bottle, and let the remaining air out. The balloon should now be filled with sand only.
  • Add water to the sand inside the balloon until the sand is saturated and cannot absorb any more water. (You can use the same inverted bottle technique that you used for the sand.) The sand inside the balloon should look darker from all sides once it is saturated with water. Where does the water go when you pour it on the sand?
  • When the sand is saturated, insert a straw far enough into the neck of the balloon so that the end of the straw is in the wet sand. Attach the straw tightly in place with a rubber band around the neck of the balloon.
  • Fill the second balloon with water. Then insert the second straw into the neck of the balloon so that the end of the straw is in the water. Again attach the straw tightly with a rubber band.


Procedure

  • Hold the balloon filled with water at its neck where it is connected to the straw. Hold it over the bowl in case it spills. Then add water to the straw until it is filled half way. What do you think will happen to the water in the straw when you squeeze the balloon?
  • Squeeze the balloon slightly with your hands. Observe the water inside the straw. What happens to the water inside the straw? Did you expect this to happen?
  • Put the water-filled balloon aside and pick up the sand-filled balloon. Again add water to the straw until it is filled up half way. What do you expect to happen this time when you squeeze the balloon?
  • Squeeze the balloon with both hands as much as you can. Observe what happens to the water inside the straw while you compress the saturated sand. Does the water level in the straw rise, fall or stay the same? Can you explain your observations?
  • Now release the pressure on the balloon and shake it slightly while observing the water level in the straw. Does the water level change again? How?
  • Extra: Try a simpler version of this activity. Add sand to a wide-mouthed plastic water bottle until it is three quarters full. Add water until the sand is saturated and you have about a quarter inch of water standing on top of the sand. Then squeeze the water bottle with your hands. What do you notice? While squeezing turn the water bottle upside down over a bowl. Do you see water dripping into the bowl when inverting the bottle? Then stop squeezing the bottle and shake it slightly. What happens? Now turn the water bottle upside down without squeezing it. Does water get into the bowl this time?
  • Extra: What other granular materials can you use to demonstrate the wet sand effect? Try clay, glass stones or "magic sand." Do you get similar results?
  • Extra: Try to quantify how much water disappears into the sand depending on the pressure you apply to the balloon. Hold a ruler next to the straw and make marks every quarter inch with a permanent marker. Then instead of using your hands to apply pressure to the balloon, use a C-clamp that you wrap around the middle of the balloon. To apply pressure to a larger area of the balloon, you can put a scrap wood pieces on each side of the balloon before you attach the C-clamp. Write down how much the water level changes with every turn of the C-clamp screw.

Observations and Results
When you squeezed the water-filled balloon you probably saw water rise up the straw as you expected. When you squeezed the sand-filled balloon, however, the water level probably went down, which seems counterintuitive. This happens because under pressure the sand particles pushed each other farther apart, making the sand expand in volume. This creates more pore space between the sand particles, which the water inside the straw can drain into.

When you release the pressure on the sand and shake the balloon a little bit, the sand particles go back into their previous, denser arrangement. As a result the water inside the straw starts to slowly rise again as the pore space between the sand particles decreases. This is exactly what happens when you make a dry footprint on wet sand. As your foot applies pressure to the saturated sand underneath, the grains of sand move which creates more pore space for the water to disappear in. The sand around your foot appears dry. When you remove your foot—and the pressure on the sand—the water comes back out of the sand.

Cleanup
Remove the water from the water-filled balloon and dispose both balloons, including the straws, in your regular trash.

More to Explore
Dilatant Sand, from Universities Space Research Association
Beach Bum Science: Compression of Wet Sand, from Science Buddies
All Mixed Up? Discover the Brazil Nut Effect, from Scientific American
STEM Activities for Kids, from Science Buddies

This activity brought to you in partnership with Science Buddies

Science Buddies