Where Grey Matter meets Dark Matter

Episode 17 - 16 August 2009

Friction - often discussed, but not well understood. It may surprise a lot of people to learn that the physical basis of friction is still hazy. In each particular case, the friction depends on temperature, shape, roughness, chemical composition, the presence of other materials, the relative speed of the two surfaces, etc. Describing all this for a material is impossible, so physicists and engineers have a 'coefficient of friction' which tries to capture the effect of all the things in one convenient number. For a lot of purposes it's useful, but it's not very enlightening. Here's one way in which friction occurs:

When two surfaces are touching each other, they're not really touching. "What does this bizarre statement mean?" you ask. Well the answer lies in what makes up a surface - atoms. But atoms are just a really, REALLY small nucleus with a haze or cloud of electrons buzzing around it. If you've got a whole row of these electron puff-ball atoms then what does it mean to say that it is 'in contact' with another line of puff-balls? Well, since electrons are all negatively charged, and like charges repel, the electrons in the two surfaces push each other apart. When they are close enough, you can't force them any closer. At that point, you would say that they are 'in contact'. This distance is called the 'van der Waals distance. (There is also the 'Pauli exclusion principle', but we'll save that for another time.)

Consider the frictional forces in this situation.
Source: www.istockphoto.com

So now that they are 'touching', this is when friction comes into play. It might sound like a contradiction, but the two repelling surfaces are attracted to each other. "Okay", you say, "You've totally lost me." Well, the electrons that whizz around the atom are not exactly evenly spread out all the time. And so, sometimes there will be more electrons on one side of the atom than the other. This partially exposes the positive charge of the nucleus, giving something like a small bar magnet, called an 'electric dipole'.

On the left is a situation where the electrons are even spread around the atom, giving no dipole.
On the right, the electrons happen to be bunched on the right of atom, giving a slightly more negative charge
on that side (that's what δ- means) and a slightly more positive charge on the left side.
Source: http://www.chem.purdue.edu/gchelp/liquids/disperse.html

This little electric dipole now attracts electrons to its negative side - so the nearby surface will be momentarily attracted to the other surface. The is called a 'dispersion' or 'London' force (which was described by Fritz London). Now one atom that has a fleeting positive and negative charge is not going to create anything like the sort of attraction needed for friction, but if you have two surfaces, each of many trillions of atoms, and each of them randomly attracting the atoms of the other surface, then you get one big effect.

Here's a Youtube vid that talks about the types of bonding you get in molecules and between them:

So, to recap, you have repulsive forces between the atoms that don't let them get too close, but you have dispersion forces that don't want them to leave. It's very much like a pathological love affair. And in the same way, when they've realised they can't go on like this, the bonds are separated (by sliding one surface over the other) yet some atoms will stay with their partner anyway, and gradually the surface wears away. And the circle of life continues - or something.

Refs:

• Silberberg, Martin (2000), Chemistry: The molecular nature of matter and change, McGraw Hill, Boston.
• Halliday, David, Resnick, Robert & Walker, Jearl (1997), Fundamentals of physics, John Wiley & Sons, New York.
• This page provides a good introductory description of the London force.

BETA:

To live in two dimensions - besides the obvious physical and biological impossibilities - is an interesting thought. Edwin Abbott Abbott's novel of political and social satire tells us all about what it would be like.

Flatland, the book.

For more, read the book, it's flat out awesome.

Or maybe see this film. It looks good (but note the more politically correct treatment of women than in the book - ahhh... the joys of talking down to an audience):

Refs:

• There's heaps of editions of the book but here's the one in Melbourne Uni's library: Abbott, Edwin Abbott (1963), Flatland: A romance of many dimensions, Barnes & Noble, New York.
• This sequel is excellent: Burger, Dionys (1965), Sphereland: A fantasy about curved spaces and an expanding universe, Crowell, New York.

Mistakes we shouldn't have made but did anyway:

• The name of the Captain Ahab's ship in Moby Dick is the Pequod, named for an exterminated Native American tribe.