Paint Can Physics 101

Here's a deep scientific thread for those so inclined.

To better understand paint can behavior - spraying, mixing paint, pressure loss - you need a basic understanding of some aspects of physics.

MATTER

Matter comes in three forms - solid, liquid, and gas. (There's a fourth form called plasma, a superheated ionized gas, but we won't bother with that because it's rare.)

Matter is just atoms or molecules of different types, usually found in big groups of all one type - a block of lead, a pool of water, a helium-filled balloon. Or you'll find it in mixtures, which are usually a few different kinds of atoms or molecules in the same state (all solid, or all liquid, or all gas) closely mixed in the same area - a block of steel (several different metals are in there), a jug of paint solvents (xylene, toluene, acetone, etc. all mixed), or the plain old air you breathe (which is a mixture of nitrogen, oxygen, carbon dioxide, and very small amounts of argon and some other rare gases).

Different types of matter have different physical properties. Now I don't want to go into this or I'll have to write a whole science book, but take my word for it on this: different substances have different "personalities" which means different melting and boiling points.

All matter can exist in all states - solid, liquid, and gas. Now that sounds weird because you're used to thinking of lead as a solid, water as a liquid, oxygen as a gas. Water is easiest to understand for flexibility of state: you know it freezes into solid ice, and can be boiled into a gas (water vapor). Same is true for lead though - if you heat it up enough, it will melt into a liquid; and if you heat the liquid hot enough, it will boil into a gas. It would be a pain in the ass to make lead into a gas - you'd need an expensive pile of machinery and a whole lot of heat, but it can be done. Same with oxygen - if you cool it down enough (way, way down), it will turn into a liquid (you've heard of liquid oxygen as a rocket fuel, right?), and again if you had some super-expensive, super-powerful machinery you can cool it further into a solid.

So what's up with these different "states" of matter? What's really the difference between a solid, a liquid, and a gas, other than your basic experience with how they look and feel different? To answer that we have to discuss energy.

ENERGY

All matter - at least all of it that you can see and feel around you right now - has energy. Matter VIBRATES: atoms and molecules don't just sit there. They vibrate, rattle, spin, knock against each other. Right now, in front of your face, nitrogen and oxygen and CO2 molecules are flying around at a high speed, crashing into one another and your face. They're way too small to see or feel, but they are full of energy. The water in your toilet bowl is vibrating - again, this is at the molecular level and you can't see or feel it happening. The molecules that make up the minerals which make up the porcelain of the toilet bowl itself, they're vibrating too, all packed together tightly but still chattering a bit.

So all matter is vibrating, moving, charged with energy. In most solids it's kind of a packed crowd, where nobody can move very far but they're vibrating against their neighbors. In a gas, it's more like a 3-D mosh pit where gravity has been drastically reduced - everybody's flying through the air, bouncing off each other.

If you add heat to matter, it starts to vibrate faster, the mosh pit collisions get more violent. If you heat a solid enough, the molecules will be rattling so hard that they'll come loose, spread out - and become a liquid. That's what melting is. If you heat a liquid enough, the already loose molecules will be crashing into each other so hard that they too will move farther apart and spread into a gas. That is the definition of boiling. To get from gas back to liquid, cool it down till it liquefies. To get a liquid to solidify, freeze it.

What happens if you could cool matter down so low that it's not moving or vibrating at all? That's called ABSOLUTE ZERO - matter with no energy. It's almost impossible to get stuff down to absolute zero - even in space, where there's so much radiation flying around that everything gets to warm up a couple degrees above absolute zero.

ENERGY TRANSFER

Once you become used to the idea that heat is just a measure of how hard matter is vibrating/moving at the molecular level, other everyday things and occurences make more sense. There really is no such thing as "cold", only more heat or less heat, higher molecular energy or lower molecular energy.

When you bring hot matter and cold matter together, the hot matter transfers energy to the cold matter. At the molecular level, think of what happens to the first row of people around a mosh pit who aren't moshing. The ones who are occasionally crash into that first row, and those people get shifted around a little as they absorb the hits. That is a transfer of energy.

Let's look at some everyday examples.

CUP OF COFFEE

When you pour a cup of hot coffee, the liquid is, at the molecular level, pretty agitated, moving fast. (Coffee is a mixture, and a crude one at that - let's just say that it's the water molecules in the coffee that are doing the vast majority of the heat transference to come.) It's crashing into the matter that makes up the walls of your mug, making it vibrate faster and harder. You can feel this in the form of the mug heating up in a few seconds: the coffee has transferred some of its energy to the mug, and now the mug is transferring some of that energy to your hand.

If you let the mug sit there long enough, the coffee will cool to room temperature. What that really means is that the coffee starts transferring heat to the mug and the air above the liquid, making those molecules vibrate faster. The mug and the air above it then start to transfer their newfound energy to the matter adjacent to them: the mug warms up the table under it, and some air next to it; the air over the mug passes the buck to air further away. If there is no breeze in the room, the energy transfer will take place slowly and in gradual layers: the coffee in the middle of the bottom of the mug will be the hottest, and the air furthest away from the mug will be the coolest, down to room temperature. If you could see this through infrared goggles, the center of the coffee would be very bright, and a cloud around the mug would get dimmer the further away you get.

WIND CHILL

Remember that heat is just a measure of molecular energy/movement. Our cup of coffee cools slowly because it has to transfer away its heat in a series of layers: the coffee to the air above it, the air above it to the air above that, and so on. Heat can be transferred faster when the temperature difference between adjacent matter is big: a red-hot poker in a bucket of water, or coffee spilled in the snow. But our cup of coffee, after transferring some energy to the mug and the air around it, takes a while to cool because there's kind of a traffic jam: the air above the mug has to get rid of some of its new energy to air further away before it can accept any more from the coffee. It's like a bucket brigade going outward in all directions, and each layer's inefficiency slows down the whole job.

But if you blow on your hot coffee, you just swept away those gradual, insulating layers of warm air, which are replaced by room-temperature air. The coffee can now transfer a bunch more to this cooler air, speeding up the process. Blow on it again after a few seconds, and you are on your way to cooling off the coffee.

As you're aware, though, there are layers of different energy levels within the liquid coffee itself. That's because the coffee can only transfer heat away at its edges, where it meets either the mug or the air. The stuff in the middle of the mug, near the bottom, is still moshing like crazy, waiting for the bucket brigade process to work itself out. You know this because you've blown on a cup of hot coffee enough to be able to drink it, but if you chug the whole thing you burn your face on the last gulp. Just drink half, and then do another cycle of blowing on the rest, giving that interior mosh pit somewhere to throw its energy, and you're all set.

Outside in the cold, you are like that cup of coffee: hottest at your core, but your skin is transfering your heat away from you, to the cold air. If there's no wind, and you stand still, you will develop those gradual layers around you, that bucket brigade of energy transfer which slows the rate at which you cool off. (SIDE NOTE: If you drink alcohol while in the cold, it dilates (relaxes and enlarges) your blood vessels, bringing more hot blood from your core toward your skin; you FEEL warmer, but you are actually transferring your heat away from you faster, a dangerous thing if you're going to be in the cold for a long time. In this state you will get hypothermia (core temperature loss) faster, although in the short run you're at lower risk of frostbite than the sober guy, in general, because your skin is a bit warmer.)

Now if you're standing out in the cold, and there's a wind, that's the atmosphere blowing on YOU. It strips away those insulating layers of slightly warmer air around you, and replaces them with fresh cold air, so your body can dump off heat to the air much faster. That's what wind chill really is - an enabler of energy transference that you don't want transfered.

STAY TUNED - I'll be adding the rest of the lecture in installments when I have time. Once it's done I'll unlock the thread and take questions.