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Capillary Action

Last updated date: 17th Jul 2024
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What is Capillary Action?

Capillary action is also sometimes called capillarity, capillary motion, or wicking. Capillary action definition is summed up as the impromptu flow of a liquid into a narrow tube or porous material.


If you put a fine straw into a glass of water, You can observe that the water has mounted the straw and is more than the level of water in the glass. It appears to have defied gravity by lifting up the straw. This is called capillary action. This movement does not need the force of gravity to take place. Indeed, it often acts in opposition to gravity.


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Capillary Action Formula

The rise of a column of liquid within a narrow capillary tube is also because of the surface tension. The formula for capillary rise \[(h) = \frac{2T}{rρg}\]


Capillary Action Formula Derivation

Assuming the radius of the glass capillary tube in which the liquid is presented is r, the coefficient of surface tension of the liquid he T, the density of the liquid in a representative column be ρ, the degree of contact between the liquid and tube walls be θ, g is the acceleration caused by the force of gravity and the height to which the liquid climbs up in the tube be h.


Take into account the circumference of the liquid surface where it meets the glass. The vertical component of the surface tension force along this segment will be 2πr cosθ T.


This will sweep the liquid up the tube. Thus,

\[ 2πr cosθ T = πr2ρgh\] which renders,

Capillary rise \[(h) = \frac{2Tcosθ}{rρg}\]

Which for an angle of contact of 0°  becomes:-

Capillary rise \[(h) = \frac{2T}{rρg}\]


Occurrence of Capillary Action

Capillary action is induced by the combination of cohesive forces of the liquid and the adhesive forces between the liquid and tube material. Adhesion and cohesion are two forms of intermolecular forces. These forces pull in the liquid into the tube. For the capillary to arise, a tube needs to be adequately small in diameter.


Convex Meniscus

In some pairs of materials, there can be a unique phenomenon of convex meniscus, for example in mercury and glass. In them, the intermolecular forces within the liquid exceed those between the solid and the liquid,  making the capillary action work in reverse (hence the convex shape of the meniscus). Here capillary fall takes place.


Capillary Action Examples

Examples of capillary action include the action of wicking in your eyes. The tear duct in the corner of each eye uses capillary action to evacuate excess tears into the nasal passage. A less striking but more familiar example of capillary action is the wicking action of a paper tissue used to wipe up a spill.


If you have ever donated blood, you may have observed that before you can donate, the attendant will prick into your finger to obtain a sample of your blood for testing iron levels. A narrow glass tube is placed, called a capillary tube, where your finger was pricked. Your blood rapidly rushes up the tube. It seems as if the attendant has sucked your blood out with that tube, but it's only capillary action. This illustrates the capillary action of water because your blood is by & large made of water.


Another example of capillary action is how do plants, including the tallest shrubs, obtain water from their roots all the way out to leaves and branches against the force of gravity? It’s through Capillary action.


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Key Takeaways of Capillary Action Study

  • Capillary action was first recorded by the Italian greatest polymath Leonardo da Vinci.

  • Robert Boyle conducted trials on capillary action in 1660, documenting a partial vacuum had no impact on the height a liquid could attain via wicking.

  • A mathematical model of the wicking phenomenon was presented by Pierre-Simon Laplace and Thomas Young in 1805.

  • Albert Einstein wrote his first scientific paper in 1900 on the subject of capillarity.

Solved Examples for Capillary Action Formula

Calculate the radius of a capillary tube if water climbs up to a height of 12.5 cm within it, supposing the angle of contact between the glass and the water to be 0°.


Using the capillary action formula: \[h = \frac{2T}{rρg}\]

Radius of tube  \[r = \frac{2T}{hρg}\]

= \[ \frac{2 \times 72.7 \times 10^{-3}}{ 0.125 \times 1000 \times 9.8}\]

= \[1.2 \times 10^{-4} m \]

=0.12 mm

FAQs on Capillary Action

1. What are the Daily Life Applications of Capillary Action?

Practical use of capillary action is visible in all kinds of our daily lives. Capillary action is a significant ability of water to move through other materials. It not only makes water do this, but its properties make it better at wicking than most other substances. Thus, allowing performing tasks efficiently and effectively.

Some applications of this unique phenomenon include:

  1. Capillary action helps to separate mixtures from substances. A technique called chromatography employs capillary action in which a layer of liquid is used for separation.

  2. It helps us naturally draw tear fluid in the eye. This process cleanses the eye and ejects out dust and dirt particles that are around the ducts of the eye.

  3. The principal property (cohesive and adhesive) of wicking to absorb water by paper towels allows withdrawing the fluid into the paper towel.

  4. The phenomenon can be used as a source of renewable energy. It can produce electricity. By enabling water to rise through capillaries, vaporize once it reaches the top, condensate and dives down back to the bottom spinning a turbine to generate energy.

2. What are the Properties of Capillary Action?

Capillary action is a property most pronounced in water, though is observed in many liquids. It is most evident in water due to water's unique properties and that water is the basis of most liquids that we use every day. When you place cut flowers in a vase of water, you are using capillary action that keeps them fresh.

Capillary action takes place when adhesive forces override cohesive forces. How well a liquid can conduct the coup of capillary action depends on cohesion and adhesion. Cohesion is the mechanism of attracting particles of the same type.

Water has a strong cohesion. Adhesion is the attraction between two distinct particles. The adhesion between water and a plastic straw is also quite strong

3. Does oil demonstrate capillary action and does viscosity have any effect on it?

Yes, oil demonstrates capillary action just like water and some other fluids. Yes, viscosity does have a direct effect on the ability of a fluid to rise in a capillary tube. The higher the viscosity of the fluid, the lower it rises in the capillary tube . Oil also has a lower contact angle and Capillary Action thereby decreasing its surface tension. Therefore must show a low rise in column.

 However, contrary to what one might expect, despite the high viscosity of oil, it tends to rise higher than water in a capillary tube under the same laboratory setup and conditions. In order to understand this we must carefully glance at the formula below:

\[(h) = \frac{2T}{rρg}\], where h is the height of the liquid in the capillary tube.

Here ρ (liquid density) is inversely proportional to h. Since oil is lighter (less dense) than water, it rises higher than water.

4. How is capillary action useful in plants?

Capillary action serves as a useful phenomenon in vascular plants. Along with root pressure, capillary action contributes in the absorption of water from the soil by the root system up in the xylem vessels from where it is transposed to the other parts of the plant such as the leaves for photosynthesis. Without capillary action, the plants may not be able to uptake water from the soil. The root system consists of fine root hairs that act as capillary tubes, owing to their small diameter.

5. Can capillary action function in absence of gravity (e.g. space)?

The rise of the column in the capillary tube is denoted by ‘h’, which is indirectly proportional to the force of gravity ‘g’. In case ‘g’ becomes zero (such as in space), then for a finite length of capillary tube, the liquid will rise and fill the capillary tube completely.