Name: Pauline Y.
If I have two towels which are soaked with different amounts of water, will
the rate of evaporation differ?
Please allow me to narrow the focus a bit. Let us assume that you spread two identical
towels out under identical environmental conditions and then poured one cup of water
into the middle of one towel and two cups of water into the middle of the second towel.
Also let's assume that none of the water dripped away -- all was absorbed by the towels.
The "wicking effect" of the towels' fibrils will determine how far the water spreads. The
greater the exposed surface, the faster the evaporation. In general. the overall rate of
evaporation will depend on the surface area exposed to the drying effects of the
environmental conditions -- temperature, humidity, presence or absence of adjacent air
This sounds like a perfect opportunity for an experiment.
NEWTON BBS receives many inquiries about "evaporation rates" of water.
While it seems to be so simple, the rate of evaporation is a very complex
phenomenon. It depends upon so many parameters that it is virtually
impossible to give an unqualified satisfactory answer. A partial list of
variables that affect the evaporation rate is: temperature, relative
humidity, surface area, air speed, air direction, presence of solutes in the
water, cooling produced by the evaporation, ... And this is a short list.
Yes, MOST ABSOLUTELY they will be DIFFERENT. Let us imagine two pieces of fabric. Both
are identical in size, shape, chemical makeup etc. THESE TOWELS ARE IDENTICAL IN EVERY
Case A: This towel is drenched to the point to where there is NOTHING BUT WATER
encompassing the towel. By this, I mean that surrounding air only "sees" the water on
not even a square inch of towel. This CASE A is the fastest of the evaporation rates
and is CONSTANT, with the assumption that ambient conditions (air around the towel /
water "system") remain constant. This means that the RELATIVE HUMIDITY, moving air,
TEMPERATURE etc... REMAIN CONSTANT ... By the way, this constant ambient condition
assumption must be made for both CASE A & CASE B, RIGHT? Otherwise, we are just
comparing apples to oranges.
Why? When the air is trying to "drag off" or evaporate water from the towel, it can only
see 100% water and will therefore dry off at a much quicker rate, IN THE BEGINNING.
This leads us into CASE B: When sufficient water has been evaporated to where the air
can "see" a little bit of the towel. Now, things have changed, right? Why? Well,
the same amount of air has access to a decreasing amount of exposed water embedded
in the towel.
Case B: OK NOW. The towel is dampened just to the point to where there is ONLY ENOUGH
WATER to cover the towel (barely). But the air is in direct contact with an increasing
amount of area of the towel. Now, if you were to look at it this way:
Now imagine your HAND IS THE AIR and there are now 1000 MARBLES ( marbles represent the
water molecules ) laying on the ground. How many marbles per time would you be able to
grab if these marbles were placed in the following arrangement
a.) marbles are placed 1 inch apart?
b.) marbles are placed 2 inches apart?
z.) marbles are placed 26 inches apart?
You know just from common sense that in [ a.) ] you will be able to pick up the most
since they are the closest together and your hands and shoulders will only allow you to
pick up the marbles at a given rate. You also know that in [ z.) ] you will be able to
pick up the least because they are so far apart. This, in a very crude manner,
represents the decreasing availability of water ready to be evaporated.
Do you see how this analogy compares to the statistics associated with how water
molecules become decreasingly available for evaporation with every passing moment of
time of evaporation? This point is the key.
So this towel is soaking wet and was just taken out of the washing machine and placed in
the laundry dryer. In the beginning, of course, the evaporation RATE will be constant
and at its HIGHEST. However, in time, as the amount of "sites" that water molecules
are residing at is DECREASING, so is the evaporation rate. At the risk of repeating
myself, this explanation shows the following evaporation rate vs. time plot:
I hope this has helped some.
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Update: June 2012