Search This Blog


Monday, 13 February 2012

Ant lions and the angle of rest

An ant lion, shot with a very shallow
depth of field

I like ant lions, and I have a fair amount about them in my upcoming Australian Backyard Naturalist, but one of the things I wanted to do was, we decided, a bit too complex for younger readers. It also has a small element of danger about it, but with common sense, there is no risk.

The book has a lot of information about managing ant lions and I will be in trouble with the publishers if I give away too much of that here, but I will add some brief hints at the end to help home experimenters. This is mainly about slopes on sand.

Ant lion pit
Ant lions make really neat pits in sand, by burrowing into trhe sand and then throwing sand out until a conical pit forms.  The sand on this slope is barely stable.

When an ant (or any other sort of insect) goes over the edge, it slides down. If it begins to scramble out, the ant lion is alerted and starts throwing sand out from the bottom. This makes the side start to slide down as it is undermined, and some of the sand tumbles down.

Edge of the Sahara, near Merzouga, Morocco: dune face,
about 1.5 metres high showing the angle of rest.
Once the victim is in reach, the ant lion seizes it and sucks out all its juices, before tossing the empty husk out.

Wheat, gravel or sugar, all sorts of matter will develop a maximum slope. The angle depends on local gravity, the attractive forces between the particles, their shapes, friction, and maybe a few other things.

Sand dunes and sand banks are also shaped by this angle.  It is called the angle of rest, or sometimes, the angle of repose.

You can also see this effect sometimes in sand banks at the surf or in a creek that is silting up, but it is more convenient to study it on a table top.  To do this, you need a cylindrical glass jar, some clean dry sand and a device for measuring angles.

When I did this to take photos, I was in a hurry, so I used a microwave oven to dry some beach sand that I had washed in lots of fresh water to get rid of the salt.

That was when I discovered that wet sand behaves a bit like volcanic mud when it is really wet—and heated. Bubbles of steam blew up through the wet sand and sand blasted out of the 'Vegemite' jar, onto the 'roof' of the microwave.

So don't try this at home! Solar drying is safer. Spread your wet sand out on a shallow tray in the sun and keep turning it. In my case, the only adult who got annoyed was me, but it was messy. Once your sand is dry, half-fill a cylindrical jar, put a lid on, and you are ready to measure the angle of rest in your sample of sand.

Incidentally, the jar also got extremely hot in the microwave, and I came close to burning myself—and if you look carefully at the first photo of my apparatus, you may be able to see where the glass jar cracked at the base.

(And here is a behind-the-scenes shot of how a bit of coloured cardboard can tidy up a messy work bench for photography.)

Once you have dry sand in a jar, you are ready to start. Lay the jar on its side, with the sand surface level. Mark the point where the jar touches the paper, and roll the jar until the sand tumbles down to the angle of rest. Continue rolling, very gently until there is a second avalanche and mark the paper again. Then do a third, a fourth and so on, marking the paper each time. The distances between the marks along the paper will then tell their own story.
Current bedding in Hawkesbury sandstone,
near Sydney, Australia

So does measuring the two angles.

Most undisturbed sand dunes lie at this angle, because sand is blown over the top of the dune. If the slope is less than the angle of rest, the sand grain stays at the top, if not, it rolls down the slope until it comes to rest.

A slope like this is unstable. If you dig a small hole in the side of a dune, or walk up it, a whole load of sand comes sliding down at you. The same thing happens in inland Australia when you dig a well in a dry, sandy riverbed.

Hickson Steps, central Sydney, an old and weathered
sandstone surface, showing current bedding.
Sand in water has a different angle of rest from dry sand in air. Sandstone formations often show current bedding, where a sand bank has been built up by sand pushing out over a front, and tumbling down into deeper water.

Current bedding has little to do with ant lions or even backyards, but it has everything to do with angle of rest, and it shows how what we learn in one part of science can be relevant to something quite different.

The angle of rest stuff sounds a bit like a science project, collecting sand from various places, and measuring the angles of initial yield and rest for each sand. Technically (you can ignore this if you like), the difference between these is called the angle of dilatation. Typical published values for this are around 8 to 13 degrees. You would probably need to relate this to the shape of the sand grains, and maybe the amount of salt, organic matter or shell grit in the sand. It's your project, not mine!

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

Remember Rule 1 of being a naturalist: the most interesting questions are your own questions!

If your questions were more about ant lions, here is the basic kit that I use. The small bucket in a tub full of water is useful to stop the ants, intended as food, from escaping.

NOTE: I don't use the pooter to pick up ants, I only use it on small ant lions. Large ant lions would be injured, and ants would injure my throat by releasing formic acid,

No comments:

Post a Comment