Water beading?

[Rich Z]

Despite checking the weather radar maps, one of those typical Florida *surprise* thunderstorms caught me on the way back home from my drive into Tallahassee today. And in typical fashion, the rain seemed to be limited to the road I was traveling on, and even just a half mile from the heaviest downpour where my house is located (which has LOTS of plants which would sorely appreciate the precipitation) there was only just a sprinkling of rain. Obviously that storm was specifically laying in wait to pounce on ME.....

Anyway, the first question that popped into my mind when I turned on the windshield wipers was "I wonder how many Corvette owners don't even know if their windshield wipers work or not?"

Secondly, as I was watching the rain water bead up on the hood, I got to thinking about this "slick surface" hype stuff we hear all the time about the "beading" qualities that the polishes and waxes afford us when we use them on our cars.

Well folks, I'll admit I don't understand the physics of it. I would think that a perfectly frictionless slick surface would just have water run off it it if the surface is not perfectly parallel with the ground (or the exact alignment with the downward pull of gravity if you are not on a perfectly flat and parallel surface). So why do these beads of water, even with air pressure pressing against them as I drive down the road, NOT roll off of the surface completely? To be honest, when I would wash my Jeep, which went YEARS without wax or polish, it dried much easier simply because the water did NOT bead up on it and instead just ran off quickly. The sides basically never even looked wet for more then a few minutes. Water spots from beading? Nah, not on that vehicle!

So what am I missing here? Beading, in my opinion, is BAD. If your car gets wet in a sudden shower, and then the sun pops out while you are parked somewhere, each of those beads becomes a little magnifying glass to concentrate the sun's rays on the surface of your car. Which can actually BURN etches in the surface. Further, even without that neat little magnifying glass trick, as the water evaporates, it will leave rings of minerals and whatever else is in the rainwater as a further ringing effect on your car's surface. This is supposed to be GOOD?

How many of you rush like mad to try to dry off your car after you wash it in the heat of a hot afternoon so you don't get those evaporation rings? I know I do! But this is crazy! The manufacturers of these polish and wax products are specifically designing their products to PRODUCE these beads of water! When in fact, what I would prefer is a slick protective coating that would NOT bead up and the water would just run off in sheets immediately. A blower of any type would then dry your car completely in scant minutes.

So what am I missing here? Obviously something, because I just haven't heard anyone else do anything other then proclaim proudly how well their car's surface beads up water after they have applied their favorite polish or wax? So obviously I am not looking at this correctly......

[MADN3SS]

Fortunately for me, because I work at night, my car is outside in a gated parking lot during the night and in the garage during the day. So I usually don't run into that problem.

It's funny you mention driving down the road while it's raining and what effect that has on the beading water. When I first got the Z, I was driving in the rain and noticed the water on the hood having a cyclone like effect. It was really interesting and I haven't really been able to produce it again. I must have been going the right speed and had the right amount of water on there. Strange.

[Rich Z]

Well now that we are getting more people posting here, I want to bring this up again...

Why does a slick surface "bead" water? Wouldn't a PERFECLY slick frictionless surface just allow water to run off immediately as a sheet?

Inquiring minds want to know.....

[MADN3SS]

Here is the answer. Found on the web.

Why does water bead up better on a well waxed car?
(Lansing State Journal, August 10, 1994)

Water is a polar molecule, composed of two hydrogen atoms bonded to a single oxygen atom. Water molecules like to stick to one another, like small magnets. This is called cohesion.

Water molecules also can be attracted to other substances, such as metal or dirt, especially if they have some static charge on them. This is called adhesion.

Lastly, some substances are not at all attracted to water, and even repel it. These include oils, fats and waxes; all of which are called non-polar substances.

When water falls on an unwaxed car, the forces of adhesion are almost as strong as the forces of cohesion, and the water spreads out. Furthermore, if the painted surface is not perfectly smooth, water can be channeled for some distance along tiny ridges and valleys. This is particularly true if there is dirt on the car. The dirt itself may be charged, and attract water even more. These tiny flaws may not be readily visible without a magnifying glass, but you can sense a rough surface when you run your hand over it. On such a rough surface, drops of water appear flat and wide and often uneven.

Car wax, when applied properly to a clean car, fills in the larger scratches and layers the whole car. The chemical structure of the wax prevents water from penetrating to the surface of the car. Because the wax itself is hydrophobic (literally "fears water"), the forces of adhesion are much less than the forces of cohesion. So, water is more likely to stick to itself, and water beads up higher and rounder than on an unwaxed car.

[Mark Dalton]

Now that is some very cool info. Thanks!

[Rich Z]

Thanks for the info. But it still puzzles me why a bead of water on a slick surface of a waxed car will not run off immediately when at an angle not exactly perpendicular to the ground. Is the surface just not really SMOOTH enough? Or is there enough adhesion on those beads to overcome the force of gravity if the incline is slight enough?

I normally use a leaf blower to dry off the car after washing it, and although Rejex (which I often use as my slicker-upper) seems to be pretty danged slick, those blasted beads will STILL resist that 200 mph air force I am applying to chase them away. And it seems that the smaller the bead of water, the more it resists just being blown off of the surface.

[Mark Dalton]

This part of MADN3SS reply might explain some of that Rich.

"Water molecules also can be attracted to other substances, such as metal or dirt, especially if they have some static charge on them. This is called adhesion"

Perhaps the force of gravity isn't quite enough to overcome the water molecules adhesion to the cars surface. I also doubt that any surface on the car is perfectly smooth. This might play a part in preventing the water beads from running off as well.

[Rich Z]

Yes, I believe you are right. Which, of course, brings up the question of how smooth is smooth ENOUGH to break that adhesion? And is it practically attainable on a car's painted surfaces?

[Mekanic]

There was a car wax a few years ago that had an infomercial describing just this Rich, they were saying how horrible beading was, and showed how their wax sheets water off the car...I'm gonna try to remember wtf it was.

Maybe Flash Man or Adams can chime in here.

[Mekanic]

Durashine!

Thats what it was, wipe on, hose off, no drying. Made water sheet like crazy.