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I've noticed this before and still don't have a picture, but...

Today it was lightly raining in the morning. As always, I turned on the rear defogger. I do this because I have noticed it clears the water from the rear window.

It takes maybe a minute to start, and then you can see the water being repelled from the areas where the resistor stripes are, and eventually, the entire area covered by the stripes is clear. The area above and below remains covered in water.

If you turn it off again, the water fills back in and the rear window is covered again, more rapidly, perhaps a minute.

I can't imagine why this might happen. Any ideas?

UPDATE: yes, it's clearing water on the outside of the window, not frost on the inside. It does that quite well too, but that one I completely understand.

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2 Answers 2

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Without more quantitative data regarding the ambient temperature and the temperature of the external window surface it would be difficult to know for sure if this can explain your observation, but my hypothesis is that the heating is decreasing the surface tension of water in the area of the heater - enough to cause the water to form a film as opposed to a bead that's happening at the lower temperature. I have seen this also on the rear window of my Mitsubishi!

Decreasing surface tension generally decreases contact angle, the projected angle at the liquid solid interface. You can learn more detail by googling surface tension and/or contact angle, but the following link provides a good explanation as well as data of contact angle vs temperature of water on silicon (not glass - but close)

Thesis titled: Temperature-Dependence of the Contact Angle of Water on Graphite, Silicon, and Gold

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    $\begingroup$ That is very interesting. I had totally forgotten the temperature curve here. $\endgroup$
    – Maury Markowitz
    Commented Jan 13, 2020 at 19:40
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I don't have anything to back this up beyond the assumptions I've always made regarding this phenomenon (I often run the rear window heater during rain).

I've always just assumed it's due to the evaporation in that area. It's the same reason it gets rid of condensation; just the energy barrier would be a bit higher due to the increased mass of rain compared to condensation. Given that the glass seems to stay clear; I just always assumed it was still warm enough to cause the rain to evaporate in those areas.

As soon as you turn off the rear defrost/defogger, the rain starts to cool the window panel back down (through evaporation and convection), so the effect is short lived once the power is taken away.

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  • $\begingroup$ That seems like a LOT of heat. Its on the outside of the window so it has to heat the entire glass, as opposed to the frost which is on the inside. And unlike frost, which disappears, the rain keeps falling. $\endgroup$
    – Maury Markowitz
    Commented Jun 13, 2019 at 18:29
  • $\begingroup$ @MauryMarkowitz Depending on your window, the elements can be right inside the glass as well. Also, I've had my rear window defoggers evaporate frost on the outside as well. Obviously the comparison is a bit off due to outside temperature differences, but it's something. Anyways, given that you didn't even mention temperature considerations in your question it seemed worth mentioning. $\endgroup$
    – JMac
    Commented Jun 13, 2019 at 18:40
  • $\begingroup$ @MauryMarkowitz, is the water being repelled (balling up as if the surface were waxed), or is it instead forming into a thin and nearly-flat sheet in the warm zone (still wetted but very uniformly, without beads of water)? $\endgroup$
    – niels nielsen
    Commented Jun 13, 2019 at 20:57
  • $\begingroup$ @nielsnielsen- hmmm, thinking about it now it does seem to be balling up. There is a line on either side of the element's "clear zone" that is definitely thicker. It seems it is being pushed out of the area. $\endgroup$
    – Maury Markowitz
    Commented Jun 15, 2019 at 13:08

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