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:
* sand
* water
* 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. Back
to Krampf Index Including permission to post
these
experiments on my web site.
