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Surface Tension, Capillary Action

Cohesive and adhesive forces​

Cohesive forces are attractive forces within a liquid; adhesive forces are attractive forces between the liquid and surrounding surfaces. The following illustration shows that if cohesive forces dominate, the meniscus is convex at a vertical wall (left); if adhesive forces dominate, the meniscus is concave at a vertical wall (right). We say the former case has low wetting and the latter case has high wetting.

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Capillary action​

The effects of the cohesive and adhesive forces can be made obvious if only a small amount of liquid is considered. Here we can use a very thin pipe, called the capillary tube, to focus on a small amount of liquid. For water, adhesive forces are greater than cohesive forces, so that water goes up the pipe and forms a convex meniscus as usual. For mercury, cohesive forces are greater than adhesive forces, so that mercury goes down the pipe and forms a concave meniscus as usual.

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Surface tension​

Cohesive forces of neighboring molecules pull a molecule equally in every direction, if the molecule is far from the surface. That results in a net force of zero.

However, cohesive forces of neighboring molecules pull a molecule inward, if the molecule is at the surface. The inward-pointing net force makes the liquid surfaces behave similar to a stretched trampoline mat.

Surface tension is the tangential force at the liquid surfaces to cause them to shrink into the minimal surface area possible.

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