Now and then as you get to know your lake, it’s useful to
consider the properties of water itself. One of those is surface tension, a
property that will relate to the topic of my next post.
You’ve seen the rounded shape of droplets on a lakeside leaf
or pier board. You’ve noticed how it’s possible to fill a glass with water to
just very slightly above the brim – if the glass is kept very level and very
still. In both cases, surface tension is the phenomenon responsible.
Because of surface tension, water behaves as if it had a
(very thin) skin. In reality it doesn’t, of course. What we see with surface
tension is the result of molecular forces, specifically the cohesive force
between the water molecules.
As the U.S. Geological Survey explains it: “The molecules at
the surface of a glass of water do not have other water molecules on all sides
of them and consequently they cohere more strongly to those directly associated
with them (in this case, next to and below them, but not above)...The stronger
cohesion between the water molecules, as opposed to the attraction of
the water molecules to the air, makes it more difficult to move an object
through the surface than to move it when it is completely submersed...
“Within a body of a liquid, a molecule will not experience a
net force because the forces by the neighboring molecules all cancel out.
However, for a molecule on the surface of the liquid, there will be a net
inward force, since there will be no attractive force acting from above. This
inward net force causes the molecules on the surface to contract and to resist
being stretched or broken. Thus the surface is under tension...”
How might you observe or experience surface tension? Believe
it or not, it is possible to (very carefully) place a small needle on water and
have it float, even though its material, steel, is several times as dense as
the water.
When camping in the rain as a kid, did your parent’s ever
warn you, “Don't touch the tent”? Touching the fabric will in fact cause water
to leak through. Why? Surface tension in the water bridges the pores of the
material, in effect creating a rain barrier. But touching the material breaks
that surface tension. Drip, Drip, Drip.
Detergents in effect “make water wetter” (as an old
commercial claimed), reducing surface tension so that it soaks into the fabrics
more easily. Similarly, hot water is better for washing because it has lower
surface tension than cold water.
On clam days around your pier, look for visual evidence of
surface tension. You may find one in the incredibly cool insect my next post
will describe.
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