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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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