Burgess Laughlin

Wicking -- how does it work?

5 posts in this topic

1. An alcohol lamp has a wick that winds down into the alcohol. Why does the alcohol rise up to the top of the wick?

2. I can see a possibly similar phenomenon if I dip one edge of a towel into a tub of water and hold it there. The water rises a little way up the towel and stops. Why does the water rise, and why does it stop where it does? Why doesn't the water rise up the sides of the tub the same distance?

3. I once saw a chemist dip a very thin tube -- open at both ends -- into water. The water rose a little way up the tube.

These three situations seem to have at least one factor in common: They all provide very narrow pathways -- solid fibers in the first two cases and a hollow tube in the third case.

In each case, why does the rising liquid seemingly "defy gravity," relative to the liquid from which it is drawn?

I doubt that my life depends on knowing the answers, but these problems have been nagging at me for a long time.

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I know that this phenomenon is called capillarity, but I don't know the details of how it works. There's a remarkably uninformative page on Wikipedia here.

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Capillarity results out of the adhesion of the particles of the liquid to the particles of the capillary structure. This is the same adhesive force that would cause the liquid to make a plane surface wet. If the adhesive force is strong enough to overcome both the cohesive force between the liquid's molecules and the gravitational force, the liquid's particles can continue to rise up through the capillaries.

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1. An alcohol lamp has a wick that winds down into the alcohol. Why does the alcohol rise up to the top of the wick?

The alcohol rises up the wick because of surface tension and then by evaparation. The wick serves to increase the surface area of the alcohol that is exposed to the atmosphere, increasing the amount of evaporated alcohol in the wick. When the wick is lit, what is burning are the alcohol vapors.

2. I can see a possibly similar phenomenon if I dip one edge of a towel into a tub of water and hold it there. The water rises a little way up the towel and stops. Why does the water rise, and why does it stop where it does? Why doesn't the water rise up the sides of the tub the same distance?

It stops when the force of gravity equals the force of the surface tension of the water. Try the same thing with the towel and alcohol, and you'll see that the alcohol rises higher than the water.

3. I once saw a chemist dip a very thin tube -- open at both ends -- into water. The water rose a little way up the tube.

These three situations seem to have at least one factor in common: They all provide very narrow pathways -- solid fibers in the first two cases and a hollow tube in the third case.

In each case, why does the rising liquid seemingly "defy gravity," relative to the liquid from which it is drawn?

I doubt that my life depends on knowing the answers, but these problems have been nagging at me for a long time.

Any two materials when in physical contact with each other (solids also) will have a surface tension (a force between the atoms on the surface of each material). Put a drop of mercury, water, and alcohol on a surface and you'll notice that each will have different shapes (circular, elliptical, and fairly flat or spread out, respectively). The specific shape for each liquid will change when you put it on different types of surfaces. That is due to the surface tension.

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Surface tension is a measure of the energy of attraction between the molecules of a liquid. Wicking -- which is the penetration of a liquid into the pores or openings in a solid body -- does not occur because of surface tension, but in spite of it.

As CF indicated, when a liquid is brought into contact with a solid, the resulting behavior of the liquid vis-à-vis the solid depends (among other things) on how the energy of attraction between the molecules of the liquid compares to the energy of attraction (if any) between the molecules of the liquid and the molecules of the solid. If the energy of attraction between the molecules of the liquid exceeds the energy of attraction between the molecules of the liquid and the solid, wicking will not occur; in fact, the fluid may well be repelled and move away from the solid surface.

For instance, liquid mercury has the highest surface tension of any liquid at room temperature. Its surface tension is about 470 dyne/cm. As a result, it shows no net attraction for any solid; it wicks into nothing. If a drinking straw is inserted into a pool of mercury, the column of mercury inside the straw will be forced down.

(This unique property of mercury is what makes the whole science of mercury intrusion porosity testing possible. That, plus the Washburn equation.)

Compare this to water, which at 20°C has a surface tension of only 72.8 dynes/cm. This value is sufficiently low that water will show a net attraction to some solids, such as glass. This is why water will rise slightly in a glass “straw” known as a capillary tube. The function of soap, by the way, is to decrease the surface tension of water even further, increasing its ability to penetrate and dissolve things like the oil on your skin. Ethyl alcohol has a surface tension of only about 22.3 dynes/cm and thus shows a net attraction to an even greater number of solids.

The phenomena of wicking is also strongly affected by the geometry of the materials in question. For instance, materials such as paper towels and the wick of your alcohol lamp have a large number of very small openings, which serves to maximize the total attractive force by maximizing the surface area in contact between the liquid and the solid.

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