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While cleaning a piece of glass with 70% ethyl alcohol, I made the observation that after swabbing a frosted portion of the glass, the glass appears to lose the "frosted" property temporarily and become almost as transparent as the surrounding glass. What about the act of applying rubbing alcohol produces this effect?

My initial hunch is that since the glass gets the frost effect from creating impurities on the surface by some means such as sandblasting, the alcohol when applied creates a temporarily smooth surface by filling the imperfections to a relatively similar effective height. However, I attempted to recreate the effect with water in place of alcohol and it was significantly less effective, which is the opposite of what I expected given the overall stronger intermolecular forces.

What am I missing? Do I have it backwards and the properties of water like higher surface tension make the effect less pronounced?

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Alcohol has a smaller tension surface than water, so when applied it's less likely to form "little bubbles" that works as little lenses and tend to "blur" the light passing trough them (this is caused by the refraction of the light rays changing material). Your assumptions about "filling the holes" is correct. Infact glass and alcohol have pretty similar refractive index, so smoothing the surface gives a smaller "blur effect" because the surface in no more irregular, which when alcohol is not applied cause the scattering of light rays.

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    $\begingroup$ Ehhh?? Isn't the effect just about the fact that with a closer refractive index, so better impedance matching, you will have less scattering as there is no more medium 1 to medium 2? As this kind of scattering is function of the 2 refractive indices of the interface. That's why it works well in glass-air, but not great with glass-liquid, but would work again if the liquid had a refractive index largely mismatching that of the glass. I'm not sure that micro bubbles play as big of a part here as you made it sound. With the exception of nano-scratches where water would have a harder time filling $\endgroup$ Feb 19, 2021 at 11:44
  • $\begingroup$ but as we also see from car and normal windows, which their scratch-dig quality is not so great, those scratches do not seem to affect "transparentness" that much. Of course that I reserve the possibility that I am wrong in terms of the gap filling, but until doing an experiment, (or you showing me some reference to this) I would attribute the effect rather mostly to impedance matching. $\endgroup$ Feb 19, 2021 at 11:49
  • $\begingroup$ (one easy way would be to completely submerge the glass substrate and look through the beaker, as in a thin-film maybe surface tension might indeed have enough strength to hold the film while only touching the crests of the glass surface...although it would depend on the way one creates the thin-film, and fills the surface) $\endgroup$ Feb 19, 2021 at 11:53

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