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Why do liquids rise or fall in narrow capillary tubes?
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fluid mechanics
Figure 1: Schematic representations of (A) a differential manometer, (B) a Torricellian barometer, and (C) a siphon.
The rise or fall of liquids in narrow capillary tubes, a phenomenon known as capillarity, is driven by surface tension and the interplay between cohesion and adhesion forces.12
When a liquid comes into contact with a narrow tube, the behavior depends on whether the liquid "wets" the material:
- Rising (Wetting): If the adhesive forces between the liquid and the tube walls are stronger than the cohesive forces within the liquid, the liquid wets the surface. This creates a concave meniscus, and the liquid is drawn upward until the weight of the column is balanced by the upward force of surface tension.12
- Falling (Non-wetting): If cohesive forces dominate, the liquid does not wet the tube. This results in a convex meniscus, and the liquid is depressed below the surrounding surface level.12
According to Jurin's law, the height of this capillary column is inversely proportional to the diameter of the tube; the narrower the bore, the more significant the rise or depression.31