Key Concepts
Physics
Materials
Mixture
Buoyancy

Introduction
Have you ever noticed that the dried fruits or nuts in your breakfast cereal are not evenly spread out inside the box—or that in a container of mixed nuts, Brazil nuts gather at the top? This phenomenon is commonly called the "Brazil nut effect," and the science behind it is surprisingly complex and far-reaching. This situation can be a nuisance when you want to fill silos, bags or bins with different types of materials. It can also be used to our advantage: an avalanche airbag uses the Brazil nut effect to keep skiers on top of the snow during an avalanche. In this activity you will challenge yourself to mix different kinds of granular materials. It's not as easy as it sounds!

Background
We encounter three states of matter: solid, liquid and gas. We know that rocks are solid, the water in the ocean is liquid, and the air we breathe is gas. But classification is not always that clear when it comes to the way some materials behave. You can pile sand like a solid, but you can also pour it so it flows more like a liquid. This is because it is a granular material, which is made up of many individual particles, with each particle being a grain.

Granular materials can range in size from small grains, such as sand, to very large objects, such as boulders. For a granular material to show characteristics of a liquid, there must be many grains close together. A couple of boulders rolling down a hill are not acting like a liquid, but thousands of boulders tumbling together down a hill during a landslide are. Similarly, a grain of sand on its own does not act like a liquid, but it can flow more like a liquid when you pour a bunch of it out of a pail.

If you put different granular materials in a container, you could shake the container or rotate it to mix them. The shaking or rotating motions make the grains jump up or roll over each other. As smaller grains fall through the spaces between larger grains, large particles tend to move toward the top in a process called percolation. Buoyancy also plays a role: buoyancy makes denser grains sink and less dense grains float to the top. Another contributing factor is granular convection, which is when granular materials that are vibrated move in convectionlike circulation patterns. The larger, denser pieces follow the circulation pattern up, and don't move down. Even the air between the particles and the container's shape are believed to make an impact.

Materials

• Small transparent cylindrical container with a lid. (Small food containers work well, as do petri dishes.)
• Single color of dessert sprinkles—enough to fill your container 1/3 full)
• Fine-grained material, such as extra-fine granular sugar or sifted dry sand (you can also use colorful sand from a crafts store)—enough to fill your container 1/3 full
• Measuring spoon
• Small spoon for stirring
• Tape (if needed to secure the lid of your container)
• At least four trail mix ingredients of different sizes (such as peanuts, raisins, sunflower seeds and dried strawberries) (optional)
• A larger container with a lid in which to make (and shake) the trail mix (optional)

Preparation

• Gather all of your materials in an area that can be easily cleaned if your granular materials spill.
• Add a tablespoon of sprinkles to your container, followed by about the same amount of your other granular material. Continue until your container is about 1/2 full. If needed, add tape around any openings or to secure the lid.
• Are the sugar (or other granular material) and sprinkles in your container well mixed? What do you think will happen if you shake your container up and down or from side to side?

Procedure

• Move your container up and down: first slowly and then gradually faster. Observe what happens. When do the sugar and sprinkles start moving inside the container? Do the sugar and sprinkles mix or separate when you shake the container?
• Open your container and use the small spoon to mix the sprinkles into the sugar. Close your container and reapply tape if needed.
• Move your container from side to side, first slowly and then gradually faster. Observe what happens. At what point do the sugar and sprinkles start to move inside the box? Do the sugar and sprinkles separate when you shake the container slowly? Do they separate when you shake the container vigorously? Why do you think this happens?
• Open your container, and use the small spoon to mix the sprinkles into the sugar. Close your container and reapply tape if needed.
• Next you will rotate your container. Do you think the mixture will stay well mixed, or mix better by this movement?
• Try it out! What do you observe? Why do you think this happens?
• Move your container in whatever way you like. Can you move your container in a way that mixes your sprinkles into the sugar?
• Extra: Make trail mix. Start with two ingredients, such as peanuts and raisins, and shake the container. How well do these mix? Add a third and then a fourth ingredient, such as sunflower seeds and freeze-dried strawberries. How well do all of the ingredients mix when shaken? What about rolled or poured? Try different ingredients. Which combinations mix well? Do more ingredients make a better-mixed trail mix?

Observations and Results
Regardless of the way you moved your container, as soon as you moved it vigorously enough, the sprinkles and sugar probably separated.

Fine sugar grains are much smaller than sprinkles, and they might be denser, too. If you shook the box vigorously enough, the particles inside probably started to jump up or glide alongside and over one another. The small sugar grains fell through small cracks between sprinkles, so the large sprinkles collected at the top and sides of your container. Moving the container did not help to mix the grains inside.

The separation and patterns are pretty, but this separation can be annoying when you want your trail mix or nuts to stay mixed!

More to Explore
Mixing Mystery: Why Does Tumbling Sometimes Separate Mixtures?, from Science Buddies
Candy Waterfalls: Can Candy Flow Like Water?, from Science Buddies
Why Brazils Always End Up on Top, from BBC News
The Brazil Nut Effect, from fyfluiddynamics
STEM Activities for Kids, from Science Buddies

This activity brought to you in partnership with Science Buddies