Key concepts

Have you ever poured sand out of a bucket or cereal out of a box and noticed it seems to flow' a lot like water? This is because both sand and cereal are granular materials. That means they're made up of solid particles, but they can actually flow like liquids! Candies such as Skittles, M&M's, Nerds and many others are also granular materials. In this science activity you'll investigate how the size and shape of granular materials affect how they flow. And what better way to do this investigation than with some sweets! So get ready to put your Halloween candy to some good scientific use.

Solid matter (such as sand) that is made up of many individual small particles is called a granular material, and the individual particles are called grains. Granular materials can range in size from small powders such as sugar and flour to large objects such as rocks and boulders. Note that the word "grain" doesn't just refer to things you'd traditionally call grains, such as sand or rice; it can be any object or particle in a granular material.

For a granular material to behave like a liquid there must be many, many grains close together. For example, a single boulder rolling down a hill is not acting like a liquid; but thousands of rocks, boulders and dirt particles flowing down a hill during a landslide do behave like a liquid. When granular materials flow like a liquid, it's called granular flow. Understanding granular flow is important for many industries that put things like candy, cereal or pills into bottles or bags. In these factories granular materials usually flow out of a large container called a hopper and through a funnel. To put the right amount in each bottle or bag engineers need to know the granular flow rate of the materials through the funnel.

• Clear plastic water bottle, 500 milliliters (one pint)
• Scissors
• Ruler
• Measuring cup (A graduated one with a spout works best.)
• Adult helper
• Bowl, medium to large in size
• At least three types of candies with different sizes, such as Nerds, Junior Mints and M&M's. You'll want at least one cup of each type. (Alternatively, you could use other types of small, solid materials. Tip: For the best results, try to only use candies with similar surface textures and avoid very lightweight candies such as ones that are hollow or air-puffed).
• Sheet of paper and pen or pencil
• Stopwatch
• Calculator

• Have an adult prepare the bottle so it can be used as a funnel. To do this, carefully cut the bottom off (as close to the end as possible) and carefully cut the top off until the opening size is about 3.3 centimeters (1.3 inches) in diameter.

• Measure out at least one cup of the largest type of candy you want to test. The more candy you use, the better your results will be. Exactly how many cups of candy did you measure out? Write this down on a piece of paper.
• Take the bottle you cut and flip it upside down. Have a helper hold the funnel over a bowl and plug the 1.3-inch-wide opening (which should now be at the bottom) with their hand. Pour the measured candy into the top and make sure none leaks out the bottom.
• Get the stopwatch ready and then have the helper quickly remove their hand and gently shake the funnel. Time how long it takes all of the material to go through the funnel and into the bowl below. How long did it take for all of the candy to leave the funnel? Write this down on your piece of paper. Tip: If the material jams the funnel, have an adult make the opening a little larger and try this again. Also be sure the helper is gently shaking the funnel during the entire time the candy is flowing.
• Calculate the volumetric flow rate of the candy. To do this, divide the volume of the candy by the time it took to finish flowing through the funnel. For example, if you used one cup of M&M's and it took two seconds to flow through, the volumetric flow rate would be 0.5 cup per second. What is the volumetric flow rate of your candy?
• You may want to try this process a few more times with the same type of candy to see how accurate your results are. Each time you test the candy be sure to hold the funnel from about the same height above the bowl and shake the funnel in the same way.
• Try this entire process with two other types of candy that are different sizes. What are their volumetric flow rates?
Overall, do you see a correlation between the volumetric flow rate and the size of the candies you tested? Do you think other factors, such as surface texture and shape, might affect the volumetric flow rate?
Extra: In this activity you looked at how size affects volumetric flow rate, but other factors affect the rate as well. To investigate this try testing materials that are the same size but have a different surface texture (such as smooth versus rough or bumpy candies) or are different shapes, such as conical Candy Corns and spherical malt balls. How do other factors affect a material's volumetric flow rate?
Extra: You could do this activity again but rather than measuring volumetric flow rate, you could measure the mass flow rate. What you would need to do is weigh your samples on a scale (or calculate their weight based on the packaging) instead of measuring them in a measuring cup. How does the volumetric flow rate compare with the mass flow rate?
Extra: You could investigate the bulk density of each material. The bulk density of a granular material is its mass per total volume that it occupies (including air space). Does packing density correlate with the volumetric (or mass) flow rate of the materials?

Observations and results
Overall, did the smaller candies have a faster volumetric flow rate than the larger candies?

Because granular flow rate is complex, it is difficult to accurately calculate; it is affected by a number of factors, including the grains' surface texture, density both as a group and individually, and shape and size, along with the funnel opening size. To try to only investigate the effect of grain size on the granular flow rate of different granular materials you should have only used candies with similar surface textures and avoided very lightweight ones (for example, hollow or air-puffed). Under these conditions you should have found that the smaller candies, such as Nerds, generally had a greater volumetric flow rate than the larger ones, such as Junior Mints. If you also investigated the bulk density of the candies you tested (which is measured in mass per total volume occupied, including air, such as in grams per milliliter or grams per cubic centimeter), you may have also seen that there is generally a positive correlation between bulk density and the flow rate. (In other words, the greater a material’s bulk density, the greater its flow rate).

More to explore
Granular materials, from the University of California, Santa Barbara, Physics Department
Particle size distribution and hopper flow rates, by Edward D. Sumner, et al., University of North Carolina, Chapel Hill, Journal of Pharmaceutical Sciences
The new physical-mechanical theory of granular materials, by Mester Laszlo
Making a Candy Waterfall: Can Solids Flow Like Liquids?, from Science Buddies

This activity brought to you in partnership with Science Buddies