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Fabric and Water Tension
Name: Elizabeth
Status: student
Grade: 4-5
Location: CT
Country: USA
Date: N/A
Question:
Why is water not absorbed in some fabrics? What is
happening to that water? It is grouped together in drops - and
they move away when you touch them.
Replies:
Hi Elizabeth,
Water is not absorbed into some fabrics because they are
often treated to be what is termed "hydrophobic". Hydrophobic
surfaces have what is called "low surface energy". This
results in the molecular attraction between the water
molecules and the molecules of the surface of fabric the
water drop sits on to be much lower than the attraction
between the water molecules themselves. Thus in the
absence of strong attraction between fabric and water, the
attraction between water molecules pulls the water into a
near spherical drop that does not "wet" the fabric.
An extreme example of a low surface energy material, is
Teflon. Place a drop of water on a sheet of Teflon, and it
will not flow out over the Teflon's surface, but instead the
drop will remain as a near-spherical drop on the Teflon's
surface. The attraction between water molecules in the drop
pulls the water into the characteristic drop-shape. The
attraction between the water and the Teflon is so weak that
there is no force to pull the water toward the Teflon.
There are many treatments for fabrics that, when applied to
the fabric, reduce the surface energy of the fabric and
cause water to bead up and not be absorbed. Some of these
treatments such as ScotchGard contain chemicals similar to
Teflon that make the fabric hydrophobic (that is, reduce the
fabric's surface energy) when applied.
Perhaps the ultimate in water repellency is GoreTex. This is
simply a thin film of porous Teflon that is placed in the
inner layers of a jacket. It is thin enough that air can pass
through, but because of Teflon's extremely low surface
energy, water droplets simply bead up and cannot pass through
the pores.
There are also some types of fabrics, such as polyolefins
(one example of this class is polypropylene) that are
naturally somewhat hydrophobic and without additional
treatments will do a reasonable job of repelling water.
Regards,
Bob Wilson.
Water has a high surface tension, which means it is strongly
attracted to itself compared to surrounding surfaces. The term
"surface tension" is not as instructive as the equivalent term
"surface energy". The surface energy is the amount of energy (in
ergs) required to increase the surface area of a liquid 1 cm^2 (cgs
units are used because that is how you will find most of the data
tabulated. For pure water at 25 C. the surface energy (tension) is
about 72 ergs/cm^2, which in terms of force is 72 dyne-cm. So all
the water molecules on most fabrics tend to "huddle up" together.
The same is true of your fingers, because of the natural skin oils.
Soaps (or the more general term, surfactants) absorb
preferentially on the surface of a water droplet and reduce its
surface energy. Then the water droplet can "spread out" and "wet"
the fabric (or your finger). A web-search on the terms "soap" or
"surfactant" will provide dozens, or hundreds of hits that will
allow you to dig as deeply into that aspect of the behavior of
water as you might care to go.
Vince Calder
Elizabeth,
Two major things have to happen in order for any liquid to be absorbed
into any material: (1) there must be sufficient holes in the material in
order for the water to get in, (2) water must have some attraction at
the molecular level with the other material.
Fabrics that are stain resistant or water repellant are woven with a
tight weave, the threads are very close together so that a water droplet
does not penetrate into the fabric - this satisfies condition 1 above.
Also, the threads are often either made of a substance or coated with a
substance that does not interact well with water. You notice for example
that oil does not mix with water. This is because water and oil do not
have the same type of interaction at the molecular level. This is the
same principle in fabrics. The threads are either coated with a
substance that does not interact at the molecular level with water, or
the thread itself is made of a substance that does not interact with
water. This satisfies condition 2 above.
Since water does not penetrate or interact with the fabric, it interacts
with itself instead and forms a bead. This bead of water can then be
brushed off with ease and without making the water go into the fabric.
Greg (Roberto Gregorius)
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Update: June 2012
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