explaining capillary action

Capillary Action | Capillarity

Capillarity or capillary action is often used to explain why water rises up a wall.

What is capillary action?

The draw of water up a capillary is a useful analogy for explain why and how water is absorbed into brickwork.

The rise is the perpendicular height. The capillary rise depends on the surface area and volume. The height is inversely propertion to the width: h ∝ 1 / r

For clay or plaster with pore size <0.004 the rise ≈ 1M.

The analogy is useful in that you can see how the smaller the size of the capillary, the higher the rise.

However, the analogy can give the false impression that the force is only upwards. In fact the force is in all directions,. Gravity limits the upwards rise but does not limit the downward draw.

Meniscus

There is a positive force of attraction between molecules of water and silicone in glass. Capillaries create a circle of meniscus around the circumference of the water held in a capillary. The smaller the capillary, the less volume there is for gravity to hold the water down. In zero gravity the water in the capillary will keep rising until the tube is full.

Absorption

The rise of water in a brick depends on the pore size. The speed of rise and effect of the rise depends on permeability or porosity. If there are very few joined up pores, i.e. low permeability, then there will be minimal absorption of water in a wall and negligible surface dampness.

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