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How does the contact angle affect the shape of a liquid meniscus in a tube?
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fluid mechanics
Figure 1: Schematic representations of (A) a differential manometer, (B) a Torricellian barometer, and (C) a siphon.
In fluid mechanics, the contact angleāthe angle between the liquid meniscus and the tube wallādetermines the curvature and vertical displacement of the liquid surface within a capillary tube.12
- Concave Meniscus: If a liquid "wets" the tube, the contact angle is small (approaching zero). This results in a concave meniscus, where the liquid surface curves upward at the edges and rises within the tube to balance the forces of surface tension against the weight of the liquid column.2
- Convex Meniscus: If a liquid does not wet the tube, the contact angle is large. This results in a convex meniscus, where the liquid surface curves downward at the edges and is depressed below the surrounding liquid level.2
This curvature creates a pressure difference across the meniscus, which is essential for maintaining equilibrium. The higher pressure always exists on the concave side of the liquid surface.2