Can you help me understand aerosols (see details)?

Well, I don’t really understand it either, but the small droplets or particles, or whatever, aren’t suspended indefinitely; they do fall out eventually. I guess they fall reeaallly slowly?? :-s

I may be way off base, but isn’t it the shape of the droplets that allows them to float for a time, kind of like a leaf falling to the ground? Wow…what if I am semi correct…that means I answerwed a Harpie Question!!!...I wonder if there is an award for that?

When you get down to such a small size the actual molecules of air have to be considered. When the particle is that small, gravity gets very weak, and the collisions with air molecules gets relatively important. These collisions can keep it afloat.

Remember that 2 objects of different mass will fall the same speed in a vacuum, but not in air? This is that famous experiment taken to its extreme. The terminal velocity of those small particles is so low that they effectively just float.

@ccrow was right, the do fall eventually though.

Take the water molecule as an example: 2 hydrogen atoms plus one oxygen atom – total atomic mass (weight) approx. is 18. Now take the oxygen molecule which consists of two oxygen atoms – total atomic mass is approx. 32. This also explains the evaporation effect. The whole story changes when too many water molecules agglomerate. This is when the dipole moment of the water molecule comes into play which makes water very dense and heavy. This is good for us. We get rain.

I guess I don’t get why, with two solids of the same density but different volume, the interactions with air molecules would be different. Wouldn’t the larger solid have proportionally more interactions/collisions by virtue of its larger surface area?

I’ll admit that gravity is a big mystery to me. Intuitively, it seems that density should be all that matters here. Sorry to be so obtuse.

My head keeps trying to make an analogy with emulsions here. In an oil-in-water emulsion, the droplets of oil are segregated by the attraction of the water molecules for each other. If the oil droplets are fine enough, their buoyancy isn’t sufficient to counteract that group hug of the water, so it can’t break through the membranes of water surrounding each droplet of oil.

Is there anything remotely similar to that attractive force going on with the air in an aerosol? Does the air have a weak cohesion (total ignorance of gas laws)?

Let’s take a small feather. When there’s no wind, eventually gravity wins. But the electrons of the air below the feather and the electrons of the feather push each other apart. For the same reason your feet (normally) don’t sink into the ground. When you trip and hit your head blame the electrons.

Soot from a truck without a particle filter. Flying in the air. Same thing.

Why would that scenario play out differently with a very small feather verses a larger one having a proportionally larger surface area (relative to mass)? Wouldn’t they both encounter a repulsive force proportional to their respective surface areas?

@Harp so, to start, you have a problem in your reasoning here: “Wouldn’t the larger solid have proportionally more interactions/collisions by virtue of its larger surface area?” That’s actually not true, it’s the other way around. A large particle has more surface area then a small one, but it’s a much smaller surface area/volume ratio (or surface area/mass), which is much more important here. The smaller particles have less mass, and more surface area exposed to the air, because the smaller the particle, the less that are inside and totally surrounded by other particles of the same type. So, as we get smaller, the total impacts per mass increases, which is the important part.

In general, it is indeed similar to the oil problem. As the mass reduces and the particle shrinks, gravity starts having a much smaller effect. Sure, it still exists, and in the same degree, but the total force is tiny, since the mass is tiny (Force=G*m1*m2/(r^2). m1 gets tiny, force gets tiny). But the attraction of the aerosol particles for the air particles, and the kinetic energy imparted by collisions, starts to increase in importance, since there’s more of those interactions per aerosol particle. So, the ratio flips, and those attractive forces are enough to keep the particle in air, and away from the force of gravity.

That make any more sense?

@BhacSsylan Are you saying that if I have a large sphere of water and a tiny sphere of water, that the ratio of surface to volume will be different?

@Harp: Yes, I can help with this. Give me a couple minutes… I have to get some things done, but I will try to respond…

@Harp Yes, precisely.Think about it this way: say you take that large sphere of water, and you break it up into a million tiny spheres. You have the same volume as before, but now you don’t just have one surface, you have a million spheres, so it’s much more.

Another way to think about it, is in terms of atoms. When you have a big sphere, you have many atoms, and a huge amount of them are not on the surface, they’re trapped inside and never see air. What if you break that drop down to component atoms and put those in air? Now every one is seeing and interacting in the air.

Yes, I understand that breaking a large sphere of up into several smaller spheres would increase total surface area. But that doesn’t mean that the larger sphere would have fewer surface atoms relative to interior atoms than any one of the smaller spheres, does it?

But it does! Think it terms of atoms, again. One atom’s surface to volume ratio is infinitely large, since it’s all surface. But a big sphere is large. Let’s see if i can find the equation for a sphere’s volume and surface area. One moment.

Harp:
After reading some of these responses, I think you have a good explanation here to work with. I think you are getting your head wrapped around something that I think you need to forget about. Yes, if you took a tiny mist droplet and increased it to the size of a grape, it would fall. That’s why rain falls. But cloud droplets, and the water vapor in the air you breathe are tiny. Cloud droplets have diameters less than that of a human hair, and water vapor, even smaller than that (significantly smaller). Don’t think about if these things were bigger, because yes, they would fall out of suspension. But that’s the key: they are smaller. The smaller the diameter of a (let’s stick with water droplets), the easier it is for that particle to be suspended in air. It has less mass, and is less affected by forces of gravity to help pull it down. All a small particle needs to stay afloat is air.

If you’ve ever been in a movie theatre and looked at the beam created by the projector, you’ll see all sorts of tiny particles floating around in that beam. Remember that air is always moving. It doesn’t take much to suspend these tiny particles, and air currents, even the weakest of them, are sufficient to do this. And one last thing. Yes, aerosol particles do “fall out” of suspension. Many of them are “rained out” by being captured in rain or snow droplets, and then falling to the earth.

Okay, so the volume of a sphere is V=(4/3)pi*r^3, and the surface area is A=4*pi*r^2. So, let’s try this with a sphere of radius 2”, and a sphere with radius .05” (which is still too large for an aerosol, but it should work. So:

For 2”:
V=(4/3)pi*2^3=33.5
A=4*pi*2^2=50
So, the S/V ratio is: 1.49

For .05”
V=(4/3)pi*.05^3=.00052
A=4*pi*.05^2=.0314
So the ratio is: 60.4

So, that ratio is huge! much larger then before. and keep in mind aerosol droplets are usually several times smaller then .05”.

Yes, that totally explains it. Thanks so much for your patience, all!

Excellent! Glad to hear we helped.

I’m fairly certain it has to do with size and weight as well as turbulence. A less dense aerosol can have less mass and thus be forced around by air currents and turbulence more easily. The surface area to volume ratio is a fairly good explanation actually. Since a smaller particle has a larger surface area and a smaller mass, the small perturbations in air current (turbulence) keeps the particles moving. A larger size particle has more mass and less surface area so the force required to accelerate it the same amount is less. I believe viscosity and drag properties have something to do with this as well. A more viscous droplet or a particle with higher drag (flat particles like smoke are extremely draggy due to separation and high surface area) allows for more force to be applied to the particles by air currents.

I’ve experienced trying to create an aerosol with both smoke and oil droplets. Smoke is rather hard not to get to become an aerosol. The particles in the smoke are very small naturally and weigh very little. The visible part of smoke is made of carbon atoms which form into flat crystal structures as in graphite. You can’t really control the size of smoke particles except by how you burn your fuel. I used small oil droplets to produce the smoke in my experiment so there was little fuel to create large carbon molecules and the smoke was very light. With oil droplets it was a bit different. I had to adjust the nozzle for the oil dispenser and the pressure to produce a fine mist of oil particles. Oil is not as dense as water (as you know from the oil and water emulsion characteristics) so a larger oil droplet can lend itself to aerosol more than a water droplet of the same size would. Basically I was trying to get a Laser Doppler Interferometer to be able to pick up reflections off the droplets. This requires that the droplets be big enough to reflect the laser, but also must have a small mass and high drag properties in order to give a good indication of what the air currents were doing. It took a little tweaking but generally any high velocity spray will atomize a liquid small enough to become an aerosol. It just took a bit to make them large enough to get good laser returns off the particles.

Oil is used quite a lot in laser interferometry because it is very low density and is highly viscous. This gives high drag properties and low mass at the same time, which is at the heart of how aerosols work. With high drag a particle will stay pretty much exactly where a flow of air is going and with low mass the force gravity exerts on the particle is far less than the forces from the surrounding air. Also, if you’re interested in mixing as well since this is related, the primary mixing agent in a fluid flow is turbulence. Turbulence creates flows that move particles around much better. You can imagine stirring sugar into a cup of coffee. It takes a long time for the sugar to mix without stirring but adding turbulence allows for the particles to mix over a wider volume.

I’m actually fairly interested in engine design and aerosols and mixing are a very important part of efficient engine design.

I still think I was right…: )

There’s always a repulsive force (except in a vacuum). Electrons push each other apart and there are two reasons for this: electromagnetic force and the Pauli exclusion principle. Regarding surface relative to mass, the answers given above are great. I’ve got nothing to add.