Capillary action

 

Principle

 

Capillary action described in the Cohesion-tension theory is considered a mix of cohesion and adhesion. Capillary action or capillarity (also known as capillary motion) is the ability of a substance (the standard reference is a tube in plants but can be seen readily with porous paper) to draw a liquid upwards against the force of gravity. It occurs when the adhesive intermolecular forces between the liquid and a solid are stronger than the cohesive intermolecular forces within the liquid. The effect causes a concave meniscus to form where the liquid is in contact with a vertical surface. The same effect is what causes porous materials to soak up liquids.

A common object used to demonstrate capillary action is the capillary tube. When the lower end of a vertical glass tube is placed in a liquid such as water, a concave meniscus forms. Surface tension pulls the liquid column up until there is a sufficient weight of liquid for gravitational forces to come in equilibrium with intermolecular adhesive forces. The weight of the liquid column is proportional to the square of the tube's diameter, but the contact area between the liquid and the tube is proportional only to the diameter of the tube, so a narrow tube will draw a liquid column higher than a wide tube. For example, a glass capillary tube 0.5 mm in diameter will lift a theoretical 28 mm column of water. (Actual observations show shorter total distances.) In addition, the angle of incidence (contact angle) can be calculated exactly (see the textbooks of physics).

With some materials, such as mercury in a glass capillary, the interatomic forces within the liquid exceed those between the solid and the liquid, so a convex meniscus forms and capillary action works in reverse. Now the liquid level in the tube is lower.

 

 

Application

 

In medicine   Capillary action is also essential for the drainage of constantly produced tear fluid from the eye. Two cannaliculi of tiny diameter are present in the inner corner of the eyelid; their openings can be visualized with the naked eye when the eyelids are everted.

In botany   A plant makes use of capillary action to draw water into its system (although larger plants also require transpiration to move a sufficient quantity of water to where it is required).

In hydrology    capillary action describes the attraction of water molecules to soil particles.

In chemistry   Chromatography utilizes capillary action to move a solvent vertically up in a plate or paper. Dissolved solutes travel with the solvent at various speeds depending on their polarity. Paper sticks for urine and pH tests are also applications.

In daily life    Towels (fluid transfer from a surface) and sponges (the small pores) absorb liquid through capillary action. Some modern sport and exercise fabrics use capillary action to "wick" sweat away from the skin.

 

 

More info

 

With notes on the dimension in SI units, the height h of a liquid column (m) is given by:

h = 2γcosθ/ρgr,                    (1)

where:

γ = surface tension (J/m² or N/m)

θ = contact angle

ρ = density of liquid (kg/m3)

g = acceleration due to gravity (m/s²)

r = radius of tube (m)

For a water-filled glass tube in air at sea level,

T = 0.0728 J/m² at 20 °C, θ = 20° (0.35 rad), ρ = 1000 kg/m3 and g = 9.8 m/s². And so the height of the water column is given by:

h ≈ 1.4∙10−5/r

Thus in a 1 m wide capillary tube, the water would rise an unnoticeable 0.014 mm. However, for a 1 cm wide tube, the water would rise 14 mm, and for a tube with radius 0.1 mm, the water would rise 14 cm.