Capillary Action
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The diagram at right shows how water acts when placed in a glass or plastic cylinder.  It happens with some other substances as well.  The adhesion of the water for the glass allows the water to move up the wall of the cylinder just a little above the level of the water in the center of the cylinder.  Cohesion of water molecules pulls some water up the wall, working against gravity, which is pulling down, causing the gradual downward curve observed. There is a negative pressure along the curve surface.
The meniscus in the graduated cylinder can be observed in the photo at right.  When read at the bottom of the meniscus, there are 24 ml of colored water in the cylinder.  But adhesion of water for glass has pulled the water up around the sides to about the 24.5 ml line.  Always read the bottom of the meniscus.  This can be seen more easily in the larger image.
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The height water will rise in a small cylinder, or capillary tube, is dependent on the diameter of the tube, and is affected by the density of the water and the gas (which are functions of temperature), the acceleration of gravity, the contact angle (water with the glass), and the surface tension of the liquid.
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But with water in air, the formula simplifies to h = 0.3 / d, where h is the height of rise, and d is the diameter of the capillary tube, when both are measured in centimeters.

<>In the larger image, it can be seen that the height of water in the dish is 5 mm.  The height of rise in the capillary tubes above the water level in the dish ranges from 1 mm in the straw on the left to 42 mm in the small capillary on the right.  So the smallest capillary diameter is 0.07 cm, or 0.7 mm.  The height of rise in the straws is less than would be expected in a glass capillary because of lower adhesion between water and plastic.
The effect of capillarity in soil pores can be seen between the glass beads on the left.  Water was added to the beads when they were in contact.  Then the beads were separated.  The water is adhering to the beads, but cohesion keeps a strand of water between the beads.  As the gap widens, the strand of water gets narrower until it eventually breaks because cohesion of the water is not strong enough to pull the water together over wide distances. (A brief video clip is available in Quicktime (QT) or RealPlayer (RP) formats. If neither is available on your computer, click the name above to download them. See clip: QT, RP.)
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This is essentially why water moves up farther into smaller capillary tubes.  There is less mass of water in a small capillary and the cohesion of water can hold a longer water column.  

A student asked a great question about what happens if the capillary tubes are not vertical (right angles to the water surface). We used capillary tubes of different lengths, taken from the same piece of capillary glass tubing. Even though they were taken from the same piece, they did not have the same exact diameter, and so there were small differences in the height of rise in the tubes (0.2 cm difference).  This is important to note since there are also differences in height in the measured rise when the tubes were placed at angles.

The water rose in the capillary tubes 4.1 to 4.3 cm above the free water surface. Apparently the inside diameter of the tubes was not consistent throughout the entire 3 foot length (which was broken into four smaller pieces).
 4 capillary tubes of "same" stock
Angled capillary tubes
 In this passive system at equlibrium, when the tubes were angled at 90, 30, 26, and 7.5 degrees from horizontal, the water rose vertically the same distance as it did in the picture above.

To picture this, make a right triangle down from each tube at the point where the water stopped. The height above the water at this point is the same as when the tube was held vertically in the picture above.

The difference in the height of rise for the two middle angles is the same as for the middle two tubes in the picture above.

So, angle does not affect the height of rise.
Updated 01-27-2009
Copyright 2005. Clay Robinson, Ph.D., as to all resources: Materials may not be reproduced without Dr. Robinson's written consent. Students are prohibited from selling (or being paid for taking) notes or webpages during this course to or by any person or commercial firm without the express written permission of the developer of these pages.
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