posted
I'm watching Jaws, and Richard Dreyfuss just spit on the inside of the lenses of his goggles. I remember that that prevents goggles and face masks from fogging up; does anyone know why? What a handy property of a bodily fluid.
Posts: 26077 | Registered: Mar 2000
| IP: Logged |
posted
I don't know why, but I do know you have to do it right. You spit, rub it around with your fingers, and just dip it in the water once to rinse it.
Rub too much, or rinse too much, and it doesn't work.
posted
Makes a thin film coating the lens. This means condensate will dissolve into the film instead of making tiny beads or droplets on the surface which scatter light in every direction. The film, because it has a flat rather than curvy surface, lets light through with its direction relatively unchanged.
Does that make any sense?
Posts: 6246 | Registered: Aug 2004
| IP: Logged |
posted
What I think about it: the fogging is due to the fact that the vapors inside the goggles condense when the goggles come in contact with the much colder water. So the idea is, probably, to get a layer of something that doesn't conduct the heat between the goggles and the vapors. How that is achieved by the "process" you have described, kat, I leave it as an exercise for you all... (means: dunno )
Posts: 4519 | Registered: Sep 2003
| IP: Logged |
posted
Huh, I was on the right track... But ak, what exactly makes that thin film adhere to the goggles? There's something in the spit? 'Cause trying with water won't do you any good... I'll just wait for your answer before asking more.
Posts: 4519 | Registered: Sep 2003
| IP: Logged |
posted
I'm not sure exactly how it works, but I do know that it works, and well. I used that tatic all the time on swim team to keep my goggles from fogging up. I always thought it had more to do with the salt and enzymes contained in spit, but these explinations make far more sense than that one. But you don't have to be so exact. I used to wash my goggles out two or three times after spitting, and it still worked.
Posts: 3295 | Registered: Jun 2004
| IP: Logged |
posted
I suspect that the spit has a much lower surface tension than pure water, and therefore does not tend to form droplets. Why spit would have such a property I do not know.
Posts: 10645 | Registered: Jul 2004
| IP: Logged |
posted
Yeah, spit is way more than just water. It's got totally different fluid properties. Not sure about the chemical composition. Maybe someone who knows more can tell us that. But for instance it has a whole lot higher slurp up factor, the length of a string of drool which you can slurp back up again after it drools down. There's actually a technical word for that, and it's an important fluid property. Also yeah it's a lot better lubricant than plain water. It's slipperier. Has different surface tension, and so on. Different fluid properties altogether, which makes it more easily form a flat film on a surface and not tend to bead up so much, like water does.
Posts: 6246 | Registered: Aug 2004
| IP: Logged |
posted
Well I don't know its exact composition, but I do know that it contains NaCl (salt) and many enzymes and is somewhat acidic (to help us break down food we're chewing).
Posts: 3295 | Registered: Jun 2004
| IP: Logged |
posted
It has a much higher density than water, so it stays around a lot longer than simple water does. Also, water evaporates faster....which is what causes the fogging in the first place.
posted
Higher density? Does spit sink in water, really? I can't say I've every run that test. <Goes off to drool into a glass and see what happens.>
<Realizes she needs to use a contrast die. Can she do that without affecting the density of the spit? Applies one drop of blue food coloring to her mouth. Drools into a glass. Discovers that spit is in fact less dense than water. Smiles with scientific triumph and blue teeth. >
Okie dokie. What's in saliva? The primary ionic components are sodium, potassium, chloride and carbonic acid. The most concentrated enzymes are alpha-amylase, lysozyme, lingual lipase, mucopolysaccharides and glycoproteins. Also swirled in there are trace (but functionally important) amounts of calcium, iodine, phosphoric acid, fluoride, and thiocyanate. (Note that not all of these are secreted by the salivary glands, but they all end up in solution in your mouth and would be in "saliva" if you spit).
Anyway, rather than getting into why all these things are there I'll just skip ahead to the whole defogging thing. The interior of the mouth has to be relatively waterproof (think about osmotic pressures and what would happen to the cells in there if they weren't waterproof and you drank a glass of water) which they primarily achieve by the secretion of lipids (fats) that are hydrophobic. Unfortunately lipids promote the beading of water (which is undesired. Beading will isolate and localize any incoming germs and actually offer them some protection) so there are many surfactants in the saliva that break down the surface tension in the mouth and promote the formation of a film. They do this by being small, aliphatic molecules. Their hydrophilic chain dissolves in the water and their hydrophobic chains disrupt the forces acting between water molecules. These function in the same way as soaps and other detergents and a thin film of these would be just as, if not more, effective than saliva. Of course, you’d have to be careful to not get any in your eyes.
Two things to note. The first being that irregular loci on the mask are prime places for droplets to form. When you rub the spit in you're actually cleaning the mask, which further reduces the formation of droplets. Secondly, the surfactants in your lung are much more effective than those in your mouth. So the best spit to use would be a good, loud hawk from the back of your throat rather than the sissy stuff from under your tongue.
Posts: 3243 | Registered: Apr 2002
| IP: Logged |
posted
After further study of the experimental apparatus described above, and given BtL's excellent analysis of the properties of spit, I've come to a different conclusion.
The immediate reaction of the spit is to float on the surface of the water. Thus I concluded it was less dense. However, each spit gob spreads out very flat across the surface of the water, and after a short while, from the center of each gob, a blue trail sinks to the bottom of the clear glass bowl.
That makes me think that perhaps spit floats at first because of low surface tension, and perhaps bubbles introduced into the spit gob in the process of sucking it to the front of the mouth to be expectorated. Then perhaps the saliva itself (or some component of the mixture, possibly), is actually denser than water and begins to sink.
<blue teeth smilie>
Interesting experiment. If anyone wants to try to repeat this, I'd be interested in hearing their results.
posted
Huh, while I totally believe BtL, I always just assumed it was a sort of "cloud seeding" maneuver. You leave one little bit of drip in there, so that all of the fog condenses to make a larger drop rather than fogging up the lens.
posted
AJ, that theory doesn't explain why spit works great while the equivalent amount of water doesn't do the trick at all.
Posts: 6246 | Registered: Aug 2004
| IP: Logged |
posted
Um, I've had water work just as well, in fact in all the years I swam, I used water 95% of the time. Only time I'd lick them is when they were dry like if you were standing on a starting block and hadn't gotten them wet.
posted
The other reason I'd lick them is because some of the crud that accumulates in the corners of the goggles are mineral deposits from the pool water itself. Obviously rinsing the goggles in the same mineraly pool water isn't going to do any good in that case. But if you actually remember to rinse them with tap water afterwards (which people rarely do) that solves that problem too.
posted
ak, food colorings are usually acid dyes that bind via hydrogen (and the occasional ionic) bonds to proteins. There great majority of proteins in saliva are glycoproteins that make up the mucous. So that's probably what you were observing. The non protein parts would diffuse without you seeing them and the other proteins were probable either in with the mucous or you couldn't see them.
AJ, just the act of rubbing your goggles cleans them up and does away with any crud for droplets to bead to. And yeah, the seeding thing is probably true as well, but I don't think it would work nearly as well as something with surfactants.
Posts: 3243 | Registered: Apr 2002
| IP: Logged |
posted
Also, the consistency of spit makes it more likely to remain on the lens...in other words it leaves a residue that normal (or pool) water doesn't due to evaporation. That evaporation process is actually what you are fighting, because without evaporation there would be no condensation build-up in the first place.
So the water can't reach the lens, and the difference in temperature between the water in the pool and the temperature of the inside of the lens contributes to the condensation process.....
So by remaining between the lens and the moist air in the lens the saliva insulates it, inhibiting condensation. The air inside the goggle is still moist enough to condense, and eventually the saliva will dry....and the condensation process will go to work again, causing the lenses to fog.
That is why it only works for a while, then needs redone...particualrily if the goggles/mask is taken off, allowing the saliva to air dry and completely evaporate.
Printer-friendly view of this topic
![[Smile]](smile.gif)
![[Big Grin]](biggrin.gif)
)
