Particles naturally form a cone-shape on a flat surface.

Slope ange is called the "angle of repose".

Stratification & Segregation

When sand or some other granular material with particles of nearly the same size and shape is poured onto a flat surface, it begins to mound up. As the pile grows, particles move down the slope and away from the center. The result is a pile with the general shape of a cone.

The pile has a characteristic slope that depends upon several factors, including the size and the shape of the granules. The angle of this slope is called the angle of repose.

When a mixture of two granular materials with particles of different angles of repose is poured, something more can happen. The particles often separate according to size, with larger particles ending up at the bottom of the slope. Sometimes the two materials will also arrange themselves in layers. In this activity we will explore some different examples.

First, let’s investigate the angle of repose for individual granular materials with grains of different sizes and shapes. Then we can investigate how mixtures of different granular materials might separate when they are poured.

To observe and measure the angle of repose consistently, we will pour the granular material into a Hele-Shaw cell, a box with two parallel sheets of rigid transparent material a few millimeters apart. You can make such a cell from a CD case and three pencils, or from other materials. Click here for the instructions.

Make a small Hele-Shaw Cell from a CD case and three pencils.

Dirt, rice, seed or beans are examples of granular materials you can use in your Hele-Shaw Cell.

Tools & Materials

For this investigation, you will need:

• Hele-Shaw cell (click here for building instructions)
• protractor
• index card or similar piece of card stock
• transparent tape
• about 50 ml (1/4 cup) each of several different dry granular materials (e.g., sand, salt, sugar, rice, lentils, other grains or beans less than 5 mm in diameter)

Note: If you constructed the larger Hele-Shaw cell, you will probably need more than 50 ml (1/4 cup) of granular material.

The Experiment

Here’s how to get started.:

1. Place your Hele-Shaw cell on a tabletop or other firm surface and hold it upright.

2. Slowly pour about 50 ml (1/4 cup) of granular material into the funnel. Keep the Hele-Shaw cell upright and motionless during this process. Observe how the slope of the pile of granular material is established. Stop pouring when the bottom of the slope approaches the far side of the cell.

3. Without moving the cell, carefully tape an index card or similar piece of card stock to the transparent plate so that its lower edge is lined up as closely as possible with the slope of the material. The lower corner of the card should rest on the tabletop. You might want to ask someone to help you with this.

4. Measure the angle of repose from the horizontal up to the lower edge of the card with a protractor. Make sure that the lower edge of the card goes through the center point of the protractor.

5. Record your protractor reading in the data table below. Remove the index card. Then return the granular material to its original container. Repeat Steps 2 through 4 twice more and record your results. Calculate the average angle of repose for your material.

6. Use this same method to determine the average angle of repose for several other granular materials. Click here for our results.

Angle of Repose Data Table

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