This experiment uses the similarity between soil and a sponge
to observe the interaction of water and a sponge as representative
of what occurs in the subsurface near the water table.
The experiment materials are a kitchen sponge, a wire rack
such as a cooking rack , and a container full of water that
is large enough to hold the sponge in your hand below the
water level.
Select the sponge and place it in the container of water
where it is assumed that it is squeezed to get all the air
out and have it totally saturated with water. The sponge is
then selected and taken out of the water container and placed
on the wire rack with the large sponge surface area up. It
is allowed to drain until all the water that will drain out
has done so. It is then selected and rotated so that the mid-sized
area is up. It is again allowed to drain. After the water
stops draining, it is again selected and repositioned so that
the smallest sponge area is up. Water is again seen to drain
out of the sponge. It slowly stops. The sponge is then selected
and repositioned to any orientation to see if more water will
drain out.
The experiment question is: Why does water drain out and
then stop as the sponge is rotated to each of the three orientations?
What the experiment shows: The sponge is comprised of many
small pathways that can hold water similar to soil. The water
drains out of the sponge leaving a small layer near the bottom.
This bottom layer is held there by capillary forces. A given
sponge will only hold a certain thickness layer in the bottom.
As the sponge is turned the surface are at the bottom is smaller
and since only a fixed layer thickness of water can be held,
there is a draining of the excess water. The same occurs again
as the sponge is rotated to the minimum area at the bottom.
Only the fixed thickness layer of water can be held, the excess
drains out.
Water is held in soil by capillary forces also. There is
a capillary fringe, just above the water table in an aquifer,
where capillary forces are holding the water. The height of
the fringe depends on the fineness of the channels between
the soil grains.
