welcome to a mad scientist production of surface tension in which we will look at
how intermolecular forces affect the surface of a liquid. So what is going on
at the surface of a liquid? Llet's take a look by shrinking to the nano scale.
So here we are looking at the individual molecules in a liquid and let's say that
each blue circle represents a molecule of water. Due to kinetic energy the
particles that make up a liquid are in constant random motion, and so they will
have a random arrangement. You might expect the particles at the surface, at
the micro-level to form a random surface as shown here. But what we should find
out is how do intermolecular attractions influence the surface? First let's look
at intermolecular forces under the surface, Where attractions of individual
molecules pull on each other in all directions. At the surface, pull on the
molecules is lateral and downward. There is negligible attractions above the
molecules, and so the net force on surface molecules is downward. The result
of this downward force is that surface particles are pulled down until
counterbalanced by the compression resistance of the liquid.
Surface molecules are compressed more tightly together at the surface, forming a sort
of skin on the surface, with less distance between them compared to the
molecules below them. Surface molecules also form a much smoother surface than
one would expect from randomly moving molecules. What do you think surface
tension would do to a liquid not confined in a container? In this model of
a small water droplet, what is the net direction of the force on the surface molecules?
That's right it is toward the center!
And so a free-falling drop of
liquid takes on a spherical shape.
Here we see water dripping from a faucet.
If this does not look familiar to you turn on your faucet at home with very little
pressure, and see what happens. Or do an internet search for the behavior of
water and zero gravity to get a great view of surface tension.
Water in particular has a very high surface tension.
Do you know the reason for this? What creates the attraction between water molecules?
In looking at an individual water molecule, we can see the particularly
strong attraction between the oxygen and hydrogen due to their opposite charges
hydrogen has a strong partial positive charge and oxygen a strong partial
negative charge. This strong attraction occurs between
many water molecules close by. The strong partial charges result in a
strong attraction between water molecules called hydrogen bonding.
Interestingly there are creatures that have evolved to take advantage of water's
high surface tension. The water Strider is a common insect that you may have
encountered. What adaptation in their feet has occurred to take advantage of
water surface tension? Think about it.