Krampf Experiment of the Week -
#176 Sand Castles
Since we spent a lot of our time today building sand castles,
I decided to turn it into this week's experiment. It was not as tasty
as the chocolate experiment, but it was a LOT of fun. To see some of
the science involved in building a sand castle, you will need:
* a large bowl
* a large plate
Unless you live near the beach or have a sand pile nearby, you will
have to build your sand castle on a smaller scale. Start with a handful
of dry sand. Make a tall pile of sand in the center of the plate. Make
it as tall and thin as you can. Not very tall or thin, is it? The dry
sand grains do not stick together, so you tend to get a low, wide hill
instead of a castle. As you pile the sand, when the slope reaches a
certain point the sand will shift and slide. The steepest slope that
you can get will depend on how rounded the sand grains are. Rough
grains let you make steeper slopes than smooth grains.
As you can see, making sand castles with dry sand does not give much
room for creative work. If you want more decorative castles instead of
just mounds, the sand needs to be wet. Put the dry sand into the bowl.
Pour a little water into the sand and mix it up. Keep adding water, a
little at a time, until all of the sand is moist.
Take a handful of the wet sand and put it in the center of the plate.
Try to shape it into a tall, thin tower. Do you see a difference? The
sand stick together much more than it did when it was dry. Why?
Water is sticky. When you stick your finger into water, some of the
water clings to your skin. Hold two fingers together and stick them
into the water. When you pull them out, you will see water sticking in
the space between them. The water also sticks to the grains of sand,
acting as a weak glue to hold them together. As long as there is a thin
film of water in between the sand grains, they tend to stick together,
making it easy to build a nice, tall, sand castle.
What if there is more water between the grains? To find out, pour some
more water into the bowl with some sand. This time, put in enough water
to cover the sand with about an inch or two of water. Use your hand to
stir the sand and water together. Notice that once you get it started
moving, the sand and water flow very easily. Give the mixture a good
stir and then quickly scoop up a handful of the wet sand. Hold your
hand over the sand castle you made from the wet sand and let the
mixture in your hands dribble out onto your sand castle. As it flows
from your hand, it pours just like a liquid. As soon as it hits the
sand, it seems to harden into a solid, letting you make tall, thin,
"dribble towers" that have marvelous shapes. The smaller your sand
grains are, the better the dribble technique will work. With a little
practice, you should be able to make nice, thin towers.
Why does the sand/water mixture start like a liquid and wind up as a
solid? The combination of sand and water is known as a thixatropic
mixture. Thixatropic means that it becomes more fluid when it is
agitated or stirred and becomes less fluid when it sits still. When the
sand and water are stirred together, there is enough water in between
the grains of sand to let them slide easily past each other. This
causes it to flow easily. As soon as the mixture hits your sand castle,
most of the water flows into the sand below. Now there is only a thin
film of water between the sand grains. It is enough to make them stick
together, but not enough to let them flow.
Other examples of thixatropic mixtures include: Quicksand. This is just
sand that has water flowing upwards through it, usually from a spring.
In spite of what you see in Tarzan movies, it is actually denser than
plain water and you tend to float in it. In fact, it is exactly like
what you have in your bowl when you are stirring the sand and water.
Cornstarch and water. Mix one part cornstarch and one part water. Scoop
some into your hand and squeeze. It seems like a solid. Stop squeezing
and it flows. Squeeze and it turns solid again.
Catsup. (Tomato sauce for those of you outside the US.) At first it is
very thick and hard to get out of the bottle, but once you get it
started, it flows quickly and you often wind up with too much on your
plate. Give the bottle a good, hard shake before you open it and the
catsup will flow much easier. Just don't try to build a catsup castle.
to Krampf Index
Including permission from Robert
Krampf to post his
experiments on my web site