Sci: The angle of rest

 

A simple gadget that explores the steepest slope formed by dry sand.

Particles of matter will develop a maximum slope that depends on local gravity, the attractive forces between the particles, their shapes, friction, and maybe a few other bits and pieces. This is the angle of rest (or the angle of repose).

Sand dunes and sand banks are controlled by this angle. So what is the angle for sand? Put some clean dry sand in a cylindrical glass jar with a lid. You will get the best results if the jar is about half full. After you have put a lid on the jar, tip it on its side and roll it along a table. You will notice the way in which the sand builds up to the angle of initial yield, then avalanches down the slope, and settles out at the angle of rest. (The pliers stop the jar rolling back.)

How are the sand dunes near where you live? Are they healthy? Are they stable? What angle does the sand lie at? How much water is available in the dunes at different places? What lives there, what is changing about the life forms in the area, and what are their prospects? What tracks can you find in the sand?

A wind-blown advancing dune in the Sahara: sand falls down the dune front.

You will need to make a number of visits at different times of day, and in different weather conditions. Try taking photographs from the same spot at regular intervals. If you are going to do this, choose a stable fixed point, like a walkway, just outside a fenced area, and centre all of your shots on distant landmarks, so that as far as possible you are taking equivalent shots each time.

A less healthy dune on the New South Wales coast.

There is probably an interesting science project for somebody here, collecting sand from various places, and carefully measuring the angles of initial yield and rest for each sample, and the difference between these, which is called the angle of dilatation. Typical published values for this measure are around 8 to 13 degrees. You would probably need to link this to the shape of the sand grains, and maybe the amount of salt, organic matter or shell grit in the sand.

You begin with the sand surface horizontal, with the jar on a sheet of blank paper on a table. Roll the jar several times to mix the sand well, and return it to the horizontal position. Mark where the jar touches the sheet of blank paper, and then roll the jar very slowly, until you see a single grain tumble down the slope. Mark the point where the jar touches the paper, and keep slowly turning the jar, and the rest of the sand will tumble down to the angle of rest.

For older mathemagicians: to get the angle of rest, measure the distance between two marks, divide that by the circumference, and multiply that by 360 to get the angle in degrees.

Continue rolling until there is a second avalanche, a third, and so on, marking the paper each time. The distances between the marks along the paper will then tell their own story. The angle of rest effect is also important to animals: read the section on ant lions in the next chapter to see why. Now here are three things to play with:

* The textbook definition of angle of rest refers to sand and stuff (usually just sand) pouring out of a funnel onto a flat surface. Obviously, if the surface has a slope greater than the angle of rest, this would make quite a difference. But does it make any difference to the angle of rest when the surface is at a small angle to the horizontal?

* What is the angle of rest of rice grains? Wheat? Macaroni of different shapes?

* How do sand with shell grit, coarse sand and fine sand compare?

* Sand dunes are not just piles of sand, and they aren’t deserts. They are living ecosystems, where there are many tough plants that shelter many more animals than you may imagine. Sometimes the angle of the sand slope on a mature dune may be greater than the angle of rest for pure sand, because the dune is held up and held together by plant roots.

The angle of rest measurer

You need well-washed sand that has been thoroughly dried. If you use a microwave to dry it, do 30-second bursts on medium. If you start out two minutes on high, exploding pockets of steam will send sand everywhere. I know this from experience…

In 1959, a Dutch geologist called Phillip H. Kuenen calculated that all through the long geological past, each second, the number of quartz grains on the planet increased by one billion.

More to look at:

Angle of rest important when we look at cross-bedding and ant lions.

To search this blog, use this link and then use the search box

Another way: use the index!