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Dirt and water streaks of car bonnet

  1. Dec 28, 2007 #1
    Dirt and water streaks on car bonnet

    I noticed something today on a friend's shiny black Mercedes. He'd driven it from Cambridge to Oxford last night and the bonnet (hood) had streaks of fine white (pale really) dirt on it. The streaks followed a fairly regular pattern and were only on the top or side facing panels of the car, near the front (i.e. the top of the bonnet and on the sides as they lead away from the lights). Each streak started where the front grill curved to meet the top of the bonnet and they were about 1 cm apart all the way along the front. They then flowed all the way up the bonnet, but curved slightly outwards towards the sides of the car, until a little further than halfway up the bonnet (as it approached the windscreen) when they curved much more sharply outwards. Also, the closer each streak was to the side of the bonnet, the more it curved and the bonnet behind the hood ornament was clear. So basically there were streaks forming a kind of fan shape with the base of the fan at the front grill and the top of the fan at the windshield. This was all quite visible because it was white(ish) dust on shiny black paint.

    Then later in the day it had rained when I went back to check out the pattern and mull over why it had formed. The whole bonnet was clean of any dirt. It was covered randomly with tiny water droplets (it had been light rain) but as well as these tiny droplets there were larger elongated or oblong shaped drops, exactly where the streaks had been. The long axis of those drops was aligned with the lines the streaks had formed, so the net effect was that long water drop chains had replaced the streaks of dirt

    I'm thinking there must be some physical explanation for the dirt and then water drop pattern so I thought I'd pop in here to see if anyone had one

    Does anyone have a model of a) why the dirt formed such a distinct and regular pattern and b) why the drops of water followed that same pattern?

    My theory.
    Why it's fan shaped? While driving the air is forming a fan shaped vector field, due to the shape of the front of the car (and the bonnet), the speed and the windshield (and the hood ornament).
    Why the dust forms streaks along that fan shape? Not sure. I suppose it could be turbulent flow as it comes off the leading edge (the front of the bonnet) that produces "ribbons" or lower pressure in this vector field. The dust settles in those lower pressure ribbons
    Why the water drops followed the shape of the streaks. The water clings to the dirt and kind of "sticks" (higher surface tension or something) to the bonnet where the dirt is.

    My friends theory. When he washes the car at the automatic car wash it's sprayed with polish. Then it's blow dried by a blower running along the length of the bonnet. As it does this is forms streaks (on the same area that the dirt was) so those areas (or lengths or strips) are more highly polished (or maybe even less polished) so the dirt sticks to them. The water drops cling to the dirt (i.e. the explanation of the water drops is the same as mine)

    Anyone have any thoughts?
    Last edited: Dec 28, 2007
  2. jcsd
  3. Dec 28, 2007 #2


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    I'm going to guess that it's not polish that's attracting the water since polish is usually some type of a fat derivative that's trying to make a surface waterproof.

    Perhaps the soap or detergent used was initially dissolved in water and it was not properly rinsed off. The blower will increase the rate of water evaporation by removing vapor pressure above the car surface. If there is some detergent left, the excess water that doesn't directly form a solvation shell may be evaporated, leaving a sticky viscous liquid. The detergent itself is resistant to evaporation due to its molecular size.

    Through driving, this small amount of detergent is then reapplied to the post-polished car. Although water is unreactive to the polish, the soap or detergent can emulsify it and in such a way the elongated water drops replace the positions of the noted dirt. The cleaning agent may still be there underneath the water although invisible to the naked eye.

    Anyways, just a guess. =)
  4. Dec 28, 2007 #3
    Check for the presence of a wetting agent. Here in the colonies, where we call it a hood instead of a bonnet, a popular one is Rain-X which is generally used for the glass but washing soaps can have some of the same effect where surface tension is rather whacked and the water doesn't bead.
  5. Dec 28, 2007 #4
    OK - so I'm getting that polish/washing/blow-drying rather than turbulent /vector flow makes more sense to people. Good news for my friend, bad news of me (we discussed this at length and got pretty entrenched and defensive of our little theories :-)

    Those car wash blowers are very powerful though - and they're only a few inches above the car surface as they dry. I'm sure they push water/whatever liquid mixture is present in all directions as they go along, so whatever wasn't washed off kind of gets smeared in all directions as it's drying (maybe a little bit more down the bonnet as there's a slope leading down). Also I think the way they work is the car is sprayed with detergent, then washed with cloth rollers, then fine sprayed with wax then blow dried. I wash mine by hand thee days so I can't remember precisely - I'm gonna insist on going along with him next time he washes it though, to check it out and get a few more "experimental" results to revise my theory with.

    I like the description of the difference between polish and detergent though. Not entirely sure why I was thinking the dirt would be sticking to the polish....
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