1. Let's start with the drop of water on the wax paper:
It is easy to see that the drop seems to have a "skin" holding it into a sort of flattened
sphere. It turns out that this surface tension is the result of the tendency of water
molecules to attract one another (called cohesion). The lowest energy state for this drop
occurs when the maximum number of water molecules are surrounded on all sides by
other water molecules -- meaning that the drop should have the minimum possible
surface area, which is a sphere. The effect of gravity flattens this ideal sphere into the
shape we see.
Surface tension can be shown with many other demonstrations, including one in which
you add coins to a full glass of water and note how the surface rises well above the rim
of the glass:
2.
3.
4. Though teachers often talk about surface tension being like a "skin", this is not really an
accurate model, as there is no other compound other than water in the glass. In addition,
the surface does not react to being poked with a stick as a skin would. In fact, the water
not only sticks to itself, it sticks to the wood as you can see by looking at the arrows in
the picture. This is called adhesion because the attraction is to a different substance.
In all systems within which water interacts with another surface, both adhesion and
cohesion are factors. When cohesion is more of a factor, the water forms spherical
5. droplets; when adhesion is more of a factor, we get sheets of water.
water drop on oiled plastic wrap
(cohesion of water and only a little adhesion of water water drop on untreated plastic wrap
to plastic) (cohesion of water molecules to one another as well
as strong adhesion of water to plastic)
The wax on the wax paper, it would seem, acts like the oil. You might have read
somewhere that wax and oil are hydrophobic and might think that the wax and water are
repelling one another. However, this is not quite true, as you will see if you turn a piece
of wax paper with droplet on it upside down:
Since the water drops stay on the wax paper, there must be some sort of attraction
between these materials. It turns out the force of attraction between the water and the
wax is actually quite strong, but it is less strong than the water-water (cohesive) force.
6. For a good short discussion of a common misconception about hydrophobic materials,
check out the Bad Chemistry page at
http://www.princeton.edu/~lehmann/BadChemistry.html#hydrophobic.
On clean glass, the forces of adhesion between water and the surface are stronger than
they are on oil or wax. The following image is of a mirror half of which I treated with
butter (right) and half of which I left alone (left). I then put the mirror in the freezer to
cool it and harden the butter, removed it, and dropped some water on it. Notice that the
water on the left side (where cohesion and adhesion were both major factors) formed a
sheet. On the right, where cohesion won out over adhesion, the water formed drops:
So now we have a mental model of what is going on with a drop of water on a surface,
and this should help us think about the original problem. The water drop is close to
spherical because of the cohesion of the water molecules. (Gravity squashes it,
however.) This ball of water, of course, is also somewhat attracted to the wax paper
(adhesion); thus, when it is tilted, the surface of the water "sticks" to the wax, and the
drop rolls. For more discussion of intermolecular forces, a good webpage is
Intermolecular Bonding -- VAn Der Waals Forces, at
http://www.chemguide.co.uk/atoms/bonding/vdw.html
Now let's look at the glass again:
7. Like the water drop, the surface tension on the water on this glass will be lost if you add
some soap to the water. One end of each soap molecule is attracted to the water
molecules and comes between them. If one floats a paper clip on the water (bend a bit
of the clip up so it is easy to put it on the surface carefully), the effect of soap can be
observed easily.
This principle is behind the trick of putting soap on your bathroom mirror to keep it
from fogging. The fog on a mirror is caused by many tiny drops of water condensing
on the mirror and reflecting light in a variety of directions. The soap disrupts the
cohesion of the water molecules, causing the water to form sheets (through which the
image can be seen relatively easily) rather than drops.
By the way, if we come back to our original drop of water on the wax paper, we can
break the surface tension by adding soap, and we'll see that the drop changes into a flat
puddle. Notice the relatively flat profile:
When we reduce the force of cohesion (by introducing soap), the drop changes shape as
now the forces of adhesion and cohesion are more nearly the same.
