Forces in Capillary Action

The capillary effect is the ability of a liquid to flow in narrow spaces without the assistance of, or even in opposition to, external forces like gravity. We can see this effect when paint is pulled up between the hairs of a paint-brush, in thin tubes, in porous objects like paper, plaster, or wood and cork.

Capillary action results from the difference in energy between a liquid-air interface and a liquid-solid interface. The higher the energy of the solid-liquid interface, the higher the capillary force. 

This is known as Laplace’s law and can be used to calculate capillary rise when given the contact angle. 

The contact angle is a measure of wettability, defined as the angle at which a fluid comes into contact with a solid or a liquid. A high contact angle means that it will take more energy to pull water up than a low contact angle would. For example, it takes more energy to pull water up a waxed paper towel than it would up an untreated one.

Factors affecting capillary action

Many factors affect capillary action:

• Surface tension: It is the property that causes liquids to behave as though their surface were covered with elastic skin. This elastic ‘skin’ is due to the attraction between the molecules on the surface and those below it.
• Wetting: Wetting is defined as the ability of a liquid to flow across a solid surface. A liquid that does not spread out but forms drops (like oil) will not wet, whereas if it spreads out on the surface, then it wets the surface (like water).
• Complete wetting: If the liquid fills all of the pores of a porous solid, then we say that capillary action has occurred due to complete wetting.

Forces acting during capillary action

There are three forces involved during capillary action.

• Adhesive forces between the liquid and solid material pull the liquid up the sides of the tube. Adhesion is stronger than cohesion in most cases so that the liquid remains outside of the tube, but there is still some attraction between molecules of the liquid.
• Cohesive forces between molecules of liquid push the molecules together to form a meniscus. The cohesive forces are stronger than the adhesive forces so that molecules at the bottom of the tube are pulled upward. This makes a concave meniscus that curves upward from both sides of the tube.
• The third factor that affects capillary action is surface tension. This is a phenomenon that occurs when liquid molecules arrange themselves into a flat film to minimise their contact with other materials or air.

How do these forces affect capillary action?

The basic mechanism of capillary action can be understood on a macroscopic level as follows: when the surface of a liquid comes into contact with a solid boundary, it is attracted to that boundary. As an example, if you took a glass tube and placed it in water with one end above the waterline and the other below, water would tend to cling to the walls of the glass tube. This is because cohesive forces in water are greater than adhesive forces between water and glass. 

This effect works for any type of liquid, with different types of liquids exhibiting different maximum heights of capillary action. For example, mercury has a higher maximum height than water does because it has a weaker adhesion to materials like glass than water does but stronger cohesion between its own particles (due to its higher density).

Capillary action happens because of differences in cohesion and adhesion between two substances. Water, for example, has strong cohesive properties—that is, it sticks to itself well—but it also has strong adhesive properties with other substances like glass. Cohesion is less when water meets air at the top of a tube; adhesion is less when water meets another liquid. This difference creates tension at the surface of a liquid like water, and that tension allows it to move against gravity in narrow spaces.

Conclusion

Capillary action is due to surface tension. Surface tension is present in all liquids; it is caused by attractive forces between molecules at the liquid’s surface. Molecules at the interface have no molecules above them and have weaker attractions to molecules immediately adjacent to them.

Capillary action occurs when molecules at the surface of a liquid are attracted to those in an adjacent material. Water molecules are attracted to other water molecules; this cohesive force is stronger than the adhesive force that binds water to glass or plastic surfaces. But when water encounters a narrow channel with solid walls—like a thin tube—cohesion and adhesion work together to draw water upward through capillary action.