Focus question

• How do granular materials behave when poured?
• What can cause the particles in a mixture of two different granular materials to form layers?
• What might cause particles of the same size from a mixture of different granular materials to collect together?

Essential materials

• Hele-Shaw cell
• protractor
• index card or similar piece of card stock
• cellophane tape
• at least 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 5mm in diameter)

Main ideas and background information

• Granular material does not behave completely like a liquid or a solid. It can be poured like a liquid, but its individual particles behave more like a solid.
• When granular material is poured onto a flat surface, it mounds up into a pile in the general shape of a cone. Particles bounce, roll, or slide down the side of the pile until they come to a stop.
• The angle that the side of the pile makes with the horizontal plane is called the angle of repose. Each granular material has its own characteristic angle of repose. The size of this angle depends on the size and the shape of the grains, among other things.
• When a mixture of two granular materials with different angles of repose is poured, sometimes the result is spontaneous stratification or segregation. With stratification, alternating layers of the two materials form parallel to the sloping side of the pile. With segregation, larger grains end up at the bottom of the slope while smaller grains remain near the top of the slope. For more information, see Avalanches.
• Stratification behavior may be a factor in the formation of striped sandstone. For more information, see Stratification and Segregation: Why Does Some Sandstone Have Stripes?

Procedural tips

• It is probably easier for students to construct their Hele-Shaw cells from CD cases than from a sheet of acrylic plastic. You might want to construct a larger cell from acrylic plastic to use for classroom demonstrations.
• Pour granular material into the funnel in a small steady stream instead of a quick “dump.” This will allow the grains to fall freely and to arrange themselves consistently, resulting in data that is more usable. Stop pouring when the slope of the material reaches the opposite cell wall.
• Material with larger grains (such as lentils) may tend to clump in the funnel, even if poured slowly. Use a straightened paper clip to break up such clumps as they occur.
• Place pieces of transparent tape at the upper corner and on the upper edge of the index card prior to the angle of repose measurement. When the lower edge of the card is lined up parallel with the slope of the material, fix the card in place by pressing the tape gently onto the side of the cell.
• While positioning the index card parallel to the slope of the material, take care not to disturb the Hele-Shaw cell. Bumping the cell can set off small avalanches and change the slope of the material. If the cell is bumped, just have the students pour the material back into the original container and pour it into the cell again.
• When reading the angle of repose, be sure that the lower edge of the index card extends through the center point of the protractor.
• After measuring the angle of repose, pull off the index card. Pour the granular material out of the cell and back into your original container. Then you will be ready to repeat the process.
• When exploring with mixtures of two different granular materials, focus on which variables could be independent, that is, under the control of the experimenter. For example, the selection of the specific granular material and the proportion of one material to another in the mixture are variables that could be controlled. Design experiments that vary only one independent variable at a time, so that the effect of that variable on the behavior of the poured mixture of granular materials can be observed and documented.
• When instances of spontaneous stratification and segregation occur, take extra time to observe the behavior of the poured mixture closely and discuss the observations. See also the web page on Avalanches for details of the stratification and segregation mechanisms.
• Encourage students to explore the virtual Hele-Shaw cell. Suggest that they first click on the bottom three buttons of the virtual cell and measure the angles of repose of these three individual materials (medium faceted, large spherical, and small spherical particles) by carefully placing their protractors directly on the computer screen. Discuss how these angles of repose compare with the ones they have measured previously. After this, their exploration of mixtures of these materials (top two buttons) will probably be more meaningful.

Discussion

1. How would you describe the way granular material behaves when it is poured onto a flat surface? (The material pours like a liquid. It piles up, forming a cone-shaped hill that appears to be solid. The individual grains tumble or slide down the hill until they come to a stop. The slope of the hill is fairly uniform from the top to the bottom.)

2. How does a Hele-Shaw cell help you study the way material piles up? (The cell gives a sort of “cross sectional” view to the process. The motion of the grains can be seen clearly through the transparent cell wall. The final angle of repose can be measured fairly easily. However, the cell gives only a two-dimensional view of the process.)

3. Why might the angle of repose not be the same for different granular materials? (Granular particles will bounce, roll, or slide down the slope in different ways, depending on their shape and their size, among other factors. Material with particles of rough, blocky shapes might create a steeper pile with a larger angle of repose than material with smoother particles because rough, blocky particles do not move down the slope as readily as smoother particles.)

4. How would you explain the formation of sandstone that has several evenly-spaced layers? (The different materials in the sandstone layers might have different angles of repose. If a mixture of two different types of sand was subjected to some kind of pouring action (as in a small avalanche) before the sandstone was formed, the layers in the rock might have resulted.)

5. What might cause a mixture of granular material to separate when poured? (A larger particle may not find a resting place big enough until it has rolled down past the smaller particles near the top of a pile. For more information, see Avalanches.)

Assessment

• Are students able to describe how granular materials behave when poured? (Granular material behaves both like a liquid (it pours) and like a solid (it mounds up). The pile of material is generally shaped like a cone. The side of the cone has a definite slope characterized by its angle of repose.)
• Can students explain what can cause the particles in a mixture of two different granular materials to form layers? (When a mixture of two granular materials with different angles of repose is poured, sometimes the two materials form layers parallel to the slope of the pile.)
• Can students explain what might cause particles of the same size from a mixture of different granular materials to collect together? (When a mixture of large and small granular particles is poured, sometimes the larger particles will collect toward the bottom of the slope and the smaller particles will tend to remain near the top of the slope.)

Extensions and further investigations

• Have students repeat the experiment using
  - granular materials of different sizes and shapes not yet tested.
  - both dry and moist granular materials.
  - materials poured from different heights.
  - mixtures of three different granular materials.

• Before the students carry out each additional experiment, challenge them to predict what they think will happen and why. Then have them compare the results with their predictions and develop possible explanations for any discrepancies.

Career connections

• Career connections
• Geology and other careers related to earth science
• Civil engineering
• Designers of concrete and other useful mixtures
• Careers dealing with granular materials, such as granary operations, rock crushing, and powder production

Correlations with Standards

United States: This activity correlates with portions of NSES Content Standard A, Science as Inquiry, and Content Standard G, History and Nature of Science, Grades 5-8 and 9-12, and with the following additional standards:

Grades 5-8
Standard B – Physical Science: B1, B2
Standard D – Earth and Space Science: D1

Grades 9-12
Standard B – Physical Science: B4
Standard D – Earth and Space Science: D2

Britain: This activity correlates with the English National Curriculum standard Sc1, Science Enquiry, and the following additional standards:

KS3, Sc3, 2e, 2f

Glossary/vocabulary

granular material
angle of repose
mixture
stratification
segregation
independent variable

Resource links

http://www.uoguelph.ca/~earnaud/photos/talus.htm

This is a photograph of a talus slope, a formation that is created by the erosion of cliffs or other steep landforms. As the eroded material falls, it creates this sloping pile. Notice the larger material near the bottom and the finer grained material farther up the slope, an example of natural segregation.

http://www.quanterra.org/guide/guide1_5.htm

The talus slope in this photograph has an angle of repose of about 35 degrees.

http://www.lmgc.univ-montp2.fr/MIDI/gal_alfredo/main2.html

This web site gives a geologic treatment to the talus slope mechanism.

http://darkwing.uoregon.edu/~millerm/depenv.html

Here is an index page for photographs of several different depositional environments. The alluvial fan is an example of another way that granular material might be segregated.

http://www.seed.slb.com/en/scictr/watch/cybergeo/formatio.htm

This web page within the SEED web site shows how a point bar in a meandering stream accumulates sediment segregated by size.