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We often find bathroom mirrors get fogged while hot water is used. Looking this up on the internet, we find several easy solutions:

  • use a hot air blower
  • use a heater behind the mirror
  • vinegar + water mixture
  • thin film shaving cream/foam lather
  • thin film of regular(any) bath soap lather

I would like to know:

How does a thin film of soap lather prevent condensation on the mirror surface during a hot shower?

A thread on DIY(StackExchange): technique to make a shower mirror fog-free.

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  • $\begingroup$ Is it suggesting you put a thin film all over the mirror? $\endgroup$
    – jerk_dadt
    Mar 8, 2014 at 20:35
  • $\begingroup$ yes, using your soapy hands. $\endgroup$ Mar 8, 2014 at 20:38
  • $\begingroup$ It seems soap would be more of a problem to clean off than condensation $\endgroup$
    – jerk_dadt
    Mar 9, 2014 at 10:51

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If you look at condensation fog through a strong magnifier, you'll notice that the fog is actually composed of a large number of hemispherical water droplets. The optical effect is caused by the fact that the tiny droplets act like lenses, scattering the light. On a vertical glass panel like a mirror, the maximum stable droplet radius is strongly affected by the surface tension of the liquid; with a high surface tension, large hemispherical droplets are stable.

If you coat the mirror with soap, however, the surface tension is vastly reduced, and thus there is a reduction in the maximum stable condensation droplet radius. As a result, instead of condensing as a large number of hemispherical droplets which scatter light, you get a uniform thin film of soap water, which has no optical scattering effect.

So it's not so much that you're eliminating condensation (the water will still condense), but rather you're changing the geometric (and thus optical) properties of the water droplets that condense on the surface.

Admittedly, this is a guess, so if anyone has any corrections feel free to point them out.

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  • $\begingroup$ Can you elaborate on which part is a guess? $\endgroup$
    – BMS
    Mar 8, 2014 at 23:54
  • $\begingroup$ @BMS: Actually, all of it is a guess. While I've observed condensation processes before with a variety of liquids, I've never specifically looked at a soap-covered mirror while steam was condensing on it to see what exactly was happening to give a different scattering profile than a non-soap-covered mirror, but the surface-tension explanation seemed the most sensible as far as explaining the difference in optical properties. $\endgroup$ Mar 8, 2014 at 23:58
  • $\begingroup$ In addition, I've noticed that during fractional distillation of low-viscosity organic compounds, the distillate condenses on the wall as a uniform thin film, in contrast to water condensation, which initially form as individual droplets on a dry glass surface. So I suspected that a similar process was occurring on the soaped-mirror surface. $\endgroup$ Mar 8, 2014 at 23:59

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