Ask A Scientist© General Science Archive

name         Michael G.
status       student
age          17

Question -   Hello, I have read many science books and publications
and have yet to find a good way to measure that point at which a material
is "wet" to the touch and how to measure how much water or liquid is in
the material.  any information that you could provide me with would be
greatly appreciated. 
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Michael,

Your search has been difficult because there is no standardized way to
determine something that is subject to opinion. On person's assessment of what
feels wet might differ from that of another.

Wetness often is a function of a cooling effect. A cold wet washcloth held in
the hand feels wetter than one that's warm -- even though both may contain the
same amount of water. The cooler cloth withdraws heat from the hand and
therefore it feels wetter. As you already know, a breeze flowing over wet skin
cools the skin. Thus under such circumstances we tend to equate coolness with
wetness.

Wetness (whatever that might mean) is also related to contact between the wet
object and the skin. The same washcloths handled while one was wearing rubber
gloves would feel very different from the bare-handed experience. Try it and
see for yourself.

In gloved hands, the heat must flow through the rubber to the nerve endings
without the benefit of a layer of wetted skin as the transfer medium.

I wish you well in your search for an answer to your question. However, things
get difficult when one tries to quantitate anything that is subject to the
opinion of the subject doing the testing.

Regards,
ProfHoff
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This is a tricky thing to do. It is a problem that occurs in the coatings
industry daily. In short their question is: "When is the paint dry to the
touch?" This is often measured by tapping the coating with your finger [or a
Q-tip] and measuring the time it takes before any of the paint sticks to
your finger or the
Q-tip -- admittedly not very scientific or very controlled.

If you are measuring a film such as a paint or something kin to that, one
can follow the weight loss as a function of time. When the film is "wet" and
"open", the weight loss will be approximately linear. When the film "skins
over" the evaporation of solvent or water slows down considerably because it
becomes a diffusion controlled process. So one observes a "break" in the
weight loss vs. time. Some people use this criterion, but whether this
relates to some behavior you are trying to measure may not be clear.

It's not a problem with a really good "scientific" answer.

Vince Calder
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