Capillary Action

Liquids (such as water) that wet glass climb upward on the surfaces of their containers to form a concave meniscus. This occurs when adhesive solid-liquid intermolecular forces are stronger than liquid forces. Such liquids will rise in a narrow capillary tube until a balance is established between the effects of surface tension and gravity. The capillary rise increases sharply as the tube is made narrower. For example, water in a glass capillary of radius 0.1 mm will rise by about 140 mm. The capillary rise is given by , where is the solid-liquid surface tension in N/m, is the contact angle for the meniscus (measured upward from the vertical wall), is the density of the liquid, is the gravitational acceleration (9.81 m/s), and is the radius of the capillary. In this Demonstration, is expressed as a specific gravity ( corresponding to 1000 kg/m), while and are given in mm. The default values are those for water in glass at 20°C.
Capillary action provides a mechanism for carrying nutrients upward from the roots of plants. Towels soak up liquids because of capillary action by their fibers. Likewise, a candle depends on the capillary rise of melted wax into the wick.
For nonwetting liquids, such as mercury, the contact angle is greater than 90°. The formula given above still applies, so the meniscus is convex and fluid moves downward in the capillary tube. (This is not shown in this Demonstration since the relevant parameters are outside the displayed ranges of values.)



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Snapshots 1 and 2: narrower tubes show higher capillary rises
Snapshot 3: water at 100°C has a surface tension reduced to 0.0599 N/m, hence a lower capillary rise
Reference: G. K. Batchelor, An Introduction to Fluid Dynamics, New York: Cambridge University Press, 2000.
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