Capillary action

Capillary action causes the water in the thinnest tube to rise to a higher level than in the other tubes

[edit] Introduction

Capillary action is a phenomenon associated with surface tension, whereby liquids can travel – horizontally or vertically (against the force of gravity) in small spaces within materials. It is sometimes referred to as capillary attraction, capillarity or wicking.

The movement is due to the surface tension that results when liquid or moisture is contained within very fine spaces or tubes (capillaries). Essentially, the liquid is attracted to the sides of the container; the smaller the space, the greater the attraction. Examples of capillarity include the action observed when a paper towel or blotting paper absorb water, and the way oil travels up a wick in oil lamps.

[edit] Rising damp

Rising damp in concrete and masonry is also the result of capillary action. When building materials such as most brick types, some stones, concrete blocks and plaster come into contact with moisture, the water adheres to the pores of the material’s capillaries. If the adhesive force between the water molecules and the material is greater than the cohesive force existing between the water molecules themselves, the water rises up the tube through capillary action.

Typically, damp can rise up to around 1m above its source. It is usually prevented from doing so by the installation of a damp-proof course (DPC), typically a polymerised rubber material such as bitumen polymer. Installed as the brickwork goes up and bedded both sides with mortar, the bond between DPC, mortar and walling material creates a barrier to moisture rising through capillary action.

Capillary action is also seen in many plants and trees.

Leonardo da Vinci is regarded as being the first person to observe and record capillary action.

NB

Technical paper 35: Moisture measurement in the historic environment, published by Historic Environment Scotland in 2021, defines capillarity as: ‘The tendency of a liquid in an absorbent material to move as a result of surface tension.’ It suggest that capillary rise is: ‘...controlled by capillarity - the rise of a liquid in an absorbent material above the level that would be influenced solely by atmospheric pressure.’

[edit] Related articles on Designing Buildings

• Blockwork.
• Building damp-free cavity walls.
• Building science.
• Capillary active material.
• Capillary break.
• Chemical injected DPC.
• Damp.
• Damp proof membrane.
• Defects in brickwork
• Defects in stonework.
• Penetrating damp.
• Rising damp.
• Rising damp in walls - diagnosis and treatment (DG 245).
• Tempering heating.
• Vapour barrier.