Ask A Scientist Chemistry Archive

name         Cynthia M.
status       educator
age          50s

Question -   How does an anti-fogging agent work?  Breath on a mirror
with a circle of detergent on it, and the circle does not fog up but the
mirror does. The mirror and detergent are both at the same temperature
(room temperature) so it cannot be because the mirror is colder.  Is it
because droplets fail to form on the detergent which is hydrophilic?  Or
is it because the water vapor fails to condense?
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The water condenses on the surface, it just doesn't make a fog.  The reason
condensation normally makes a fog is that the condensation forms into tiny
water droplets, which scatter light.  Basically, the surface is covered by
millions of tiny lenses, each bending light this way and that.  Detergent on
the surface lowers the surface tension of water, so that it spreads out into
a thin, even film instead of many small droplets.  The thin film does not
scatter light, so it does not appear as a fog.

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois
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