Capillary Action

Capillary action, also known as capillarity or capillary rise, is the phenomenon by which a liquid spontaneously rises or falls in a narrow tube, porous material, or small spaces due to the intermolecular forces between the liquid and the surrounding solid surfaces. It occurs without external assistance (like gravity or pressure) and is a direct consequence of cohesive and adhesive forces within the liquid and between the liquid and the solid surface.

Definition

Capillary action is defined as the ability of a liquid to flow in narrow spaces or thin tubes without the assistance of external forces, and sometimes even in opposition to gravity.
For example, when a narrow glass tube (a capillary tube) is dipped into water, the liquid rises inside the tube. However, if the same tube is dipped into mercury, the liquid level falls.

Scientific Explanation

Capillary action results from the combined effects of adhesion, cohesion, and surface tension.

1. Adhesive Force:
   • The attractive force between the molecules of the liquid and those of the solid surface (e.g., water and glass).
   • When the adhesive force is stronger than the cohesive force, the liquid tends to rise in the tube.
2. Cohesive Force:
   • The attractive force between molecules of the same liquid.
   • When cohesive forces dominate, the liquid molecules tend to stick together, causing the liquid to fall or show a depression in the tube.
3. Surface Tension:
   • The tendency of a liquid’s surface to minimise its area due to cohesive forces.
   • Surface tension acts along the surface of the liquid and contributes to the curvature (meniscus) observed in capillary tubes.

Capillary Rise and Depression

• When the adhesive forces > cohesive forces, the liquid wets the tube walls and rises inside it (e.g., water in a glass capillary).
• When the cohesive forces > adhesive forces, the liquid does not wet the tube walls and falls inside it (e.g., mercury in a glass capillary).

The curved surface of the liquid inside the tube is called the meniscus.

• Concave meniscus: Observed in water due to adhesion (surface curves upward at edges).
• Convex meniscus: Observed in mercury due to cohesion (surface curves downward at edges).

Mathematical Expression

The height (h) to which a liquid rises or falls in a capillary tube is given by Jurin’s Law:
h=2Tcos⁡θrρgh = \frac{2T \cos \theta}{r \rho g}h=rρg2Tcosθ​
Where:

• h = height of rise (or depression) of the liquid,
• T = surface tension of the liquid,
• θ = angle of contact between the liquid and the tube wall,
• r = radius of the capillary tube,
• ρ = density of the liquid,
• g = acceleration due to gravity.

Interpretation:

• The height of capillary rise is inversely proportional to the radius of the tube. Narrower tubes show higher rise.
• The rise is directly proportional to surface tension and cosine of the contact angle.

Factors Affecting Capillary Action

1. Nature of the Liquid:
   • Liquids with higher surface tension (e.g., water) show greater capillary rise than those with lower surface tension (e.g., alcohol).
2. Radius of the Tube:
   • Narrower capillaries result in higher rise or fall due to greater relative surface area.
3. Contact Angle (θ):
   • Smaller contact angle (good wetting) → higher rise.
   • Larger contact angle (poor wetting) → depression.
4. Temperature:
   • As temperature increases, surface tension decreases, thus reducing capillary rise.
5. Density of the Liquid:
   • Capillary rise is inversely proportional to density; lighter liquids rise higher.

Examples of Capillary Action in Daily Life

1. Rise of Water in Plant Stems:
   • Capillary action helps water move upward through the narrow xylem vessels from roots to leaves.
2. Absorption of Ink by Blotting Paper:
   • Ink rises through the pores of blotting paper by capillary action.
3. Soil Moisture Movement:
   • Water moves upward through small pores in the soil, nourishing plant roots.
4. Lamp Wick:
   • Oil rises in the wick of a lamp due to capillarity, allowing continuous burning.
5. Towel or Sponge Absorption:
   • Liquids are drawn into the small pores of fabric or sponge through capillary attraction.
6. Chromatography:
   • In paper chromatography, solvent rises up the paper by capillary action, carrying solute molecules with it.

Capillary Action in Nature and Science

• In Plants:
  • Capillary action, combined with transpiration pull and root pressure, enables water to move to great heights in tall trees.
• In Construction:
  • Capillary rise of groundwater can cause dampness in walls, known as capillary rise of moisture. Hence, waterproofing and damp-proof courses are used in buildings.
• In Laboratories:
  • Capillary tubes are used in instruments like thermometers, viscometers, and manometers, where fluid rise must be precise.

Importance of Capillary Action

• Facilitates transport of nutrients and water in plants.
• Crucial for moisture distribution in soils and building materials.
• Enables scientific techniques such as paper chromatography and thin-layer separation.
• Essential in medical applications such as blood sampling in fine tubes.