Stroking Molecules
Have
you ever tried stroking a cat’s fur from its tail toward
its head? That’s against the natural angle of the fur,
and it feels more difficult to do. Most cats don’t
like the extra friction between your hand and their fur.
Friction is the force between two things that opposes their motion. Sometimes we want a lot of it—as when the brakes on your bicycle push against the wheels and slow you down. Sometimes we want very little friction, as in skating or skiing—the faster the better!
Scientists want to know all they can about friction. We are learning a little more about it in my laboratory at the Swiss Federal Institute of Technology in Lausanne, Switzerland.
Friction depends on what happens between the surfaces of two objects that slide against each other. So we need to understand a surface down to the smallest bumps and wiggles of its surface molecules.
Molecules are the smallest bits of matter that make up any chemical compound. Some kinds of molecules are round and fat, others long and thin. One compound that was fun to study was made up of long molecules that packed together like cat's fur—though of course about ten million times smaller. I will tell you about them.
Long
Molecules
We use some special chemicals that have molecules shaped
like the long molecules of soap. These molecules also have
special properties like those of soap molecules. One end
of each molecule is attracted to water. The other end pushes
away from water. When these molecules are placed in water,
they stay on the surface. Each molecule naturally takes
a position with one end sticking into the water and the
other end sticking out.
When we put a little of this compound on a bathtub full of water, it acts like soap: it spreads out to make a surface film one molecule thick.
By dipping in paddles, we can squeeze the film into big islands in which the molecules are closely packed together. Then we slip a piece of smooth, flat mica under the surface and lift off the film. Now we can take a picture of the molecules in that surface film.
Take a picture? You might ask, “Aren’t molecules so small that we can’t see them even with really strong microscopes?”
You’re
right. We cheat a little. We take our “picture”
with a scanning-force microscope. It has a very sharp finger
that we can move as it touches a surface.
We can sense the very small force between the finger and
surface molecules. It’s like learning about a tabletop
or the side of a tree by running your fingers across it.
Then you are scanning. You make a picture in your head of
the forces you feel against your fingers.
A scanning-force microscope works with the same idea but on a much smaller scale. It works by feeling the molecules at the surface of an object. You can see in Figure 1 below, a picture we made of our special surface. It shows an area about 1/10,000 the size of the period at the end of this sentence.
The lighter parts show where molecules are taller. The lighter parts (taller molecules) occur in islands of film that take shapes like the petals of flowers. (Other kinds of molecules give different patterns, and we don't yet know why.)
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| Figure 1: Lighter areas are taller. | Figure 2: Lighter areas are harder to rub across. | |
| Here are two pictures of one group of molecules, all packed together side by side. No one knows why they make a flower shape. Each petal of the flower is made up of molecules that are leaning in one direction, and that direction is different for each petal. The scientists discovered that the tallest molecules did not create the most friction. Instead, the amount of friction depended on which way the molecules were leaning. | ||
Microscopic
Rubbing
We can also use the finger of our scanning-force microscope
to sense the friction in moving over a surface. You can
do that with your finger, too. It takes less force to slide
your finger over a greasy dinner plate than over a tablecloth.
Figure 2 (above) is a friction map of the same patterns of film shown in Figure l. In Figure 2, lighter color does not mean taller, it means more friction. The finger of our microscope senses a different amount of friction over each petal of the flower pattern.
Using other instruments, we’ve learned that the molecules of each petal are all lined up together and pointed in one direction, like a cat’s fur. Different shades show that different tilts of the surface molecules can affect friction. We can even tell that stroking molecules one way gives more friction than stroking them the opposite way—just as stroking your cat from head to tail is different from stroking it from tail to head.
The surprising result is that stroking the molecules backward gives less friction than stroking them forward. It is as if stroking a cat from tail to head were easier than stroking it from head to tail. We don’t understand this yet. It is just one of our results that we will have a lot of fun figuring out.
