Science

Capillary action

In narrow spaces, interactions between a liquid and a surface can pull the liquid upward or push it downward.

What happens

Capillary action comes from the balance between adhesion to the solid surface, cohesion within the liquid and surface tension.

Water wets clean glass well, so in a narrow glass tube it forms a concave meniscus and rises above the surrounding water level. Mercury behaves differently on glass and forms a convex meniscus.

What changes the process

The narrower the tube, the more important the surface becomes relative to the amount of liquid, so capillary height can become larger.

Why it matters

Paper towels and porous materials use the same family of effects: interconnected small spaces draw liquid through the material. In plants, water transport is more complex and cannot be reduced to capillarity alone.

Go one step further

A useful way to understand this phenomenon is to separate the driving force from the visible result. Temperature, pressure, concentration, surface area and the nature of the materials can each change the rate or the final state. In real systems, several of these factors often act at the same time.

That is why the same phenomenon can look different in a laboratory, in the kitchen or outdoors. The underlying chemistry or physics stays the same, but the conditions change the balance.

01Wetting sets the meniscus shape

02Surface tension supplies an upward or downward component

03Narrower tubes amplify the height change

Wetting sets the meniscus shape

If attraction between liquid and wall favours wetting, the contact angle is small and the meniscus is concave. Poor wetting can produce a convex meniscus instead.

Surface tension supplies an upward or downward component

The liquid–air interface pulls tangentially around the tube. The vertical component of that force can support part of a liquid column against gravity.

Narrower tubes amplify the height change

For a simple cylindrical capillary, the rise magnitude scales roughly as 1/r. Smaller radius means more contact-line force relative to the weight of the column.

Worked example

Why does a thinner wetting capillary usually show a larger rise?

The surface-tension force around the perimeter becomes large relative to the weight of the smaller cross-section liquid column.

Common traps to avoid

Capillary rise is not caused by atmospheric pressure simply pushing the liquid upward.
Capillarity alone does not explain all water transport in tall plants.