I have been incredibly remiss on just about everything for the past couple of months due to a rather unpleasant personal situation, but I think things are on the upswing because I'm back to being really bothered by things I don't understand.
That particular trait — not being able to sleep when there's something you can't figure out — is in my mind the fundamental requirement for being a scientist. I was groggily reading my email Friday and found a request from one of my NASCAR contacts to explain something "simple": why tires make white smoke, even though the tire is black and the skid marks are black.
The question, which was posed by Mike Bagley of The Morning Drive on NASCAR Sirius 128, is a really good one, and it followed right on the heels of a question from Danica Patrick after her Daytona debut as to why they can't make tires that don't generate so doggone much smoke when you skid. Danica was taken out of (aka "crashed and couldn't get back on the track for") the Nationwide race when the smoke prevented her from making her way around an accident ahead of her.
The two questions are linked: They both have to do with how and what you see. And they both made me think I knew the answer, realize I didn't, and then figure it out myself. I posted a companion video blog on this topic on the Stockcar Science Blog. The two pieces are for slightly different audiences, but most people should have no problem understanding both.
Let's start with electromagnetic waves. Waves are characterized by their wavelength, which is their repeat distance. The electromagnetic spectrum includes everything from gamma rays, which are very small (10-12 m, or a millionth of a millionth of a meter) all the way through radio waves, which can be kilometers long.
Our eyes detect visible light, and we need special instrumentation to detect other types of electromagnetic waves, like infrared waves. Visible light is a very small part of the electromagnetic spectrum, having wavelengths from 400 nm to 700 nm. Red, on one side of the rainbow, has longer wavelengths and blue/violet have shorter wavelengths. White light is the superposition of all visible wavelengths at once, and black is the absence of any color light. Natural sunlight contains all of the wavelengths of visible light, in addition to a lot of infrared radiation that makes the Earth warm enough to be habitable.
Just to give you an idea of how small the wavelength of visible light is: If the wavelength of light were equal to the length of a NASCAR Sprint Cup car, California Speedway would stretch from the Earth halfway to Venus. (From Earth's orbit halfway to Venus's orbit, technically.)
I'm not sure how many angels can dance on the head of a pin, but I know that you could fit 5000 wavelengths of blue light across the diameter of a 2mm pin. (Light dances. It likes nu-wave music.)
The wavelength of light is important because it provides us with information. We see things in one of two ways: either the object emits light (e.g. the Sun, a light bulb), or the object reflects (or scatters) light from another source. When you look at me outdoors, sunlight strikes my hair, my hair absorbs some wavelengths and reflects others. The reflected wavelengths make it to your eyes where they register as 'reddish-brown'.
Most things we look at are much, much larger than the wavelength of the light we use to look at it. But optics gets a little trickier when you start looking at things that are comparable to (or smaller than) the wavelength of light. Imagine trying to pick up sesame seeds with a pair of kitchen tongs. The tongs are just so large that they don't "see" the sesame seeds.
The gas molecules that form the atmosphere are smaller than the wavelength of light. (In fact the size of a typical molecule is comparable to UV light. When light hits these molecules, it doesn't reflect back so that you can see the molecules. The molecules absorb the incoming light, which increases their energy. To get rid of the extra energy, the molecules emit their own light. When the scatterer (the molecule) is much smaller than the wavelength of light, shorter wavelengths scatter much more strongly than longer wavelengths. (The intensity is proportional to the inverse wavelength to the fourth power, so if you double the wavelength, you decrease the intensity of the scattered light by 64 times.) This wavelength-dependent scattering is called Rayleigh scattering and explains is why the daytime sky is blue. I've illustrated this schematically by showing white light coming in from the left and blue light being re-emitted from the atom or molecule that is doing the scattering.
What color is steam? Boil a pot of water and look close to the surface. You will see… nothing. Water vapor is colorless. So why do you see white steam above the surface of the water? (BTW, a beaker and hot plate works much better than a pot). There is a decreasing temperature gradient from the surface of the water up. The hottest water molecules are nearest the water surface. As the molecules rise and move further from the heat source, they collide with other water molecules and condense into water droplets that are close to the same size as the wavelengths of visible light. Instead of Rayleigh scattering, we get nonspecific or Mie scattering. When the scatterers are comparable to the wavelength of light, the scattered light doesn't change wavelength. Whatever color light comes in is the color light that is scattered. As with Rayleigh scattering, you're not actually seeing the object that's causing the scattering, you're just seeing the effects the scatter causes.
Back to tires. Tires are complex objects chemically and mechanically. Tires are made of rubber — often three or four kinds, some natural and some synthetic — plus some processing oils, fillers (usually carbon black and/or silica nanoparticles), vulcanizing molecules (sulfides, zinc oxides) and anti-oxidants or anti-ozonants. Rubber polymer chains are long, like spaghetti, and they don't bind to each other very well. The fillers and vulcanizing molecules tie the long rubber polymer chains together, forming a strong and resilient material that allows a stock car to corner at 195 mph without the tire coming apart. Different parts of the tire are made with different types of rubber: it seems sometimes to be more of a black art than a science.
Speaking of black, tires don't have to be black. You can buy white bike tires, but I haven't found any white car tires. Natural rubber (the sticky, gooey sap of the rubber tree) is sort of a milky off-white. Tires are black because the filler material used to modify their mechanical principles — carbon black — makes them black. (Wouldn't you think white tires would get really dirty really fast? Good thinking – let's just put the dirt in the tire at the very beginning and get it over with.)
Carbon black is like soot with very large surface area. The carbon particles are amorphous or graphitic-like in structure and from 10-50 nanometers in diameter. (A human hair is about 70,000 nm – 100,000 nm.) Carbon black nanoparticles aggregate into fractal shapes that disperse through the rubber. The large surface area helps form bonds between the rubber polymer chains and improve the mechanical properties of the tires, reinforcing the tires and helping to conduct heat away from the tread. Carbon black used to be known as lampblack – it could be produced by burning oil lamps. Today, carbon black is made from pyrolyzing (a fancy way of saying "burning") tar oil.
Tires — especially race tires — are designed to work at high temperatures. Those high temperatures are produced by friction between the tires and the track. Under normal conditions, it's not unusual for a tire to reach 250 or 275 degrees Fahrenheit. When a driver skids from 200 mph to a stop with the tires locked up, we're talking a lot more than the usual amount of friction and thus a lot more heat.
You can't really melt a tire. Rubber is long polymer molecule (think spaghetti) that is usually more like thick slime than tire. Vulcanization uses molecules like sulfur to make bridges that hold the polymer molecules together. Vulcanization process made rubber a viable material: Until it was discovered, rubber items were flimsy and tended to melt when the temperature got warm.
The fillers (carbon black and silica) also form additional bonds between the rubber chains and this is one reason that tires are such a disposal problem: you can't just melt them down and recover the original rubber. Once you've made a tire, you're stuck with it. That's why most tire recycling schemes involve shredding the rubber and using it as a filler in another material like pavement or that soft stuff they put at playgrounds now so that kids don't get hurt when they fall down.
Under normal circumstances, a tire wears. Small pieces of hot rubber get scraped off the tire by the track, mix with track grit and form 'marbles'. When subjected to high temperatures and sliding, lots of tread material comes off in small particles. Those small particles — comparable to the size of the wavelength of light — scatter whatever light is incident on it. If you shine white light on tire smoke, the smoke will look white. If you shine red light on the smoke, the smoke will look red.
Carbon has an exceptionally high melting point – 6400 degrees Fahrenheit, so much of the carbon black, and any rubber molecules that didn't let go of it, is deposited on the track. That's why the skid marks are black. Black tires, black skid marks, white smoke. It's just another example of how nature likes to surprise us when we make things small.
Drifters (the ones who go around corners transversely instead of longitudinally like the rest of us) get points for creating tire smoke, so Kumho actually manufactures a tire that creates colored smoke during burnouts. A dye embedded in the tire sublimates (goes from a solid to a vapor) when it is heated. You can buy red, blue and yellow — the latter only available for 17 inch rims for some reason. Not that it matters for me: In NASCAR, the only time you want to see tire smoke is if you're doing a burnout after winning the race.