Surfactants: making the water "wetter"

[bigben2k]

I came across an article in CleanTech magazine today ( www.cleantechcentral.com ) that discusses nPB and pFB's (normal-propyl bromide, and 1,1,1,3,3-pentafluorobutane).

They're used, blended (as they work better together than individually), to improve the cleaning effect, by having an increased ability to wet a surface to be cleaned. They're also very good at removing oils, even silicone oil. They're meant as replacement to CC (ChloroCarbons), HC (HydroChlorocarbon), and CFC (ChloroFluoroCarbon), which have all been banned. They are solvents, not surfactants. They can attack some plastics.

The article mentions that water (pure) tends to "bead up", so it doesn't "spread evenly" (relevance?) but the nPB and pFB blend doesn't bead, and not only "spreads fast", it also "penetrates microscopic crevices" and "flows into blind holes" much more effectively than either component alone.

The article includes results of a drop spread experiment.

So now I'm wondering, what is it exactly that makes the water wetter, and what (readily?) available substances are out there? If microscopic crevice penetration is desirable (for better cooling?), why can't water do it, and what can I do (if anything) to make my water "wetter"?

What if I use Windshield Wiper fluid? What if it has a little soap in it?

[Alchemy]

[The "wetness" is its surface tension, in basic terms the ratio of its cohesive to adhesive properties. Water beads because it likes sticking to itself rather than sticking to the piece of metal it's placed on (when compared to something of lower surface tension, like alcohol).

(By "sticking to" I mean it in the sense of "associating with," not adhesion. Adhesion is something different.)

Surface tension is different from viscosity, but I've noticed in my personal experience that fluids with low viscosity tend to have low surface tension, and vice versa. I'd assume it's because the molecular interaction that leads to these properties are somewhat similar, but I'm not sure since I'm not an expert on surface tension.

Decreasing surface tension will lead to improved interaction with surfaces and thus better heat transfer only if the surfaces were not being wetted beforehand. Otherwise it will make no difference.

Even in the small crevices found in waterblocks and radiators I don't think the surface tension of water is nearly high enough to impede wettening.

I think if you compare two fluids in a watercooling setup, both identical except for surface tension, they would behave the same.

Alchemy

[bigben2k]

Thanks, I was hoping you'd chime in here!

I tend to agree; all things being equal, surface tension shouldn't affect the thermal properties significantly.

So what's the effect of adding a surfactant (soap?)?

[Alchemy]

Originally posted by bigben2k
So what's the effect of adding a surfactant (soap?)?

Surfactants are by definition chemicals that can be added to a fluid to reduce surface tension. Adding soap to water reduces the surface tension of water, and (I belive) increases the attraction between the water molecules and molecules of oil, further increasing cleaning ability.

Alchemy

[myv65]

Most items I've looked at that reduce water's surface tension tend to increase its viscosity. The difference is generally minor, but real. Anything that increases viscosity is likely to have an adverse affect on temperatures (again, very minor) due to the resulting flow rate decrease.

[airspirit]

All I know is some fool (a very good friend of mine) at my high school nicked some surfactants (god only knows why they had them in the first place!) from the chem lab and stuck them in the fountain at a nearby mall. The thing went from shooting four feet into the air to about twelve feet, and water was spraying everywhere. He only added about two cups of the stuff to about 100 or so gallons of water. It was funny watching mall security standing there dumbfounded trying to figure out how to turn the blasted thing off.

Moral: I think it would INCREASE the flow rate in your system and thereby increase the efficiency of it.

[Enyin]

The theoretical points seem well covered above - as far as readily available surfactants there are lots around.

I think the place where you would see the most difference is in blocks designed around turbulence like #rotor's blocks, or in blocks with very rough channels or pressure nozzles. At higher velocities the surface tension of the water in these blocks might cause less wetting of the surface.

However, I doubt in our applications that you would see any difference between an industrial chemical like nPB or pFB and a household like JetDry.

The windshield washer is probably a good idea, I've been thinking about that too for when I add a chiller into my system, both for the freezing point benefits and surfactant.

[gmat]

Originally posted by bigben2k
water (pure) tends to "bead up"

That was exactly my point when we were talking about sandblasted surfaces and feature size. I'm convince there's a minimal feature size dictated by the fluid surface tension. Under that size, features will only help thickening the laminar layer in the fluid.

[bigben2k]

It's starting to sound like an experiment is in order.

Maybe I could try a few concentrations of distilled water and JetDry, in a closed loop with a flowmeter... (of course I'd need a flowmeter!). I could just keep adding JetDry until I see a difference.

[myv65]

Originally posted by airspirit
The thing went from shooting four feet into the air to about twelve feet, and water was spraying everywhere.

Moral: I think it would INCREASE the flow rate in your system and thereby increase the efficiency of it.

These conditions would require roughly doubling the flow. Only way that happened is if some nozzles in the fountain were plugged and the chemical loosened the blockage. Would have been pretty funny to see such a change.

[airspirit]

I don't know what happened, but about 45 seconds after he poured it in, a couple of women with their kids sitting on the edge of the fountain suddenly got drenched. It was like someone slowly opened up a valve for about a minute before it was at its new full throttle ... we got the hell out of there after about 15 minutes and we were convinced we were going to get caught.

I'm guessing that the security people got the fountain off before all the water was sprayed out of it, since it was running normally the next week. I do feel sorry for the poor bastard that had to drain and refill it, though.