Surface tension is a property of liquids that causes their free surfaces to behave like a stretched elastic membrane, minimizing their surface area. This phenomenon is explained by molecular interactions.
Inter-molecular Forces
- Cohesive Forces: The attractive forces acting between molecules of the same substance (e.g., the force between two water molecules). Cohesion holds the liquid together.
- Adhesive Forces: The attractive forces acting between molecules of different substances (e.g., the force between water molecules and a glass container wall).
Sphere of Influence and Liquid Layers
Every liquid molecule exerts an attractive pull on surrounding molecules within a small spherical region known as its sphere of influence (radius ≈10−9 m).
- Bulk Molecules: A molecule deep inside the liquid is completely surrounded by other liquid molecules. Its sphere of influence is entirely filled, meaning it experiences equal attractive forces from all directions. The net cohesive force acting on a bulk molecule is zero.
- Surface Molecules: A molecule situated in the surface film (the top layer of thickness equal to the molecular sphere of influence) is only partially surrounded by liquid. The upper half of its sphere of influence contains air molecules, which exert negligible attraction compared to the dense liquid below. Consequently, surface molecules experience a net downward cohesive force pulling them toward the interior.
Mathematical Definitions
- Surface Tension (T or γ): Quantified as the perpendicular force acting per unit length along an imaginary line drawn anywhere on the free surface of the liquid.
T=LF
- Units and Dimensions: The SI unit is Newton per meter (N/m) or Joule per square meter (J/m2). Its dimensional formula is [M1L0T−2].
- Surface Energy: To bring a molecule from the interior to the surface against the net downward cohesive pull, work must be done. This work is stored as potential energy in the surface film. Surface energy is defined as the work done per unit area to increase the surface area of a liquid under isothermal conditions.
Surface Energy=ΔAreaWork Done
Physical Variables Modifying Surface Tension
Temperature Dependence
Surface tension is inversely proportional to temperature. As a liquid is heated, the kinetic energy of its molecules increases, weakening the inter-molecular cohesive forces. Consequently, surface tension decreases linearly with rising temperature and becomes zero at a specific temperature known as the Critical Temperature.
Presence of Solutes and Impurities
- Highly Soluble Impurities: When highly soluble substances like common salt (NaCl) or sugar are dissolved in water, the inter-molecular cohesive forces increase, raising the surface tension of the solution.
- Sparingly Soluble Impurities: When poorly soluble or amphiphilic substances like soap, detergents, phenol, or oil are added, they concentrate at the surface layer and disrupt the water-water cohesive bonds. This drops the surface tension of the liquid significantly.
Contact Angle and Meniscus Profile
When a liquid surface comes into contact with a solid boundary, the surface curves near the solid edge. The profile of this curve (meniscus) depends on the balance between cohesive and adhesive forces.
Angle of Contact (θ)
The angle of contact is defined as the angle enclosed between the tangent to the liquid surface at the point of contact and the solid surface inside the liquid.
Structural Variations of Meniscus
| Force Balance | Angle of Contact (θ) | Meniscus Shape | Physical Behavior | Key Examples |
|---|---|---|---|---|
| Adhesive Force > Cohesive Force | Acute (θ<90∘) | Concave | Wetts the solid surface; rises up the container wall. | Pure water in a glass tube (θ≈0∘). |
| Adhesive Force = Cohesive Force | Right Angle (θ=90∘) | Flat / Plane | Neither wetts nor repels; flat surface boundary. | Water in a highly specific silver container. |
| Adhesive Force < Cohesive Force | Obtuse (θ>90∘) | Convex | Does not wett the surface; depresses down the wall. | Mercury inside a glass tube (θ≈135∘). |
Concept of Capillarity
Capillarity, or capillary action, is the phenomenon of the rise or fall of a liquid surface inside a sufficiently narrow tube (capillary tube) when dipped vertically into a liquid reservoir.
Mechanics of Ascent and Descent
- Capillary Rise: Occurs when the liquid wetts the tube wall (acute contact angle, e.g., water in glass). The concave meniscus creates a pressure deficit (ΔP=R2T) immediately below the curved surface compared to the flat exterior. To equalize this pressure differential, the liquid climbs up the tube until the hydrostatic pressure of the elevated liquid column balances the surface tension forces.
- Capillary Depression: Occurs when the liquid does not wett the tube wall (obtuse contact angle, e.g., mercury in glass). The convex meniscus increases the pressure beneath it, pushing the liquid column downward below the external level.
Ascent Formula (Jurin’s Law)
The vertical height h to which a liquid rises or falls within a circular capillary tube is mathematically determined by Jurin’s Law: h=rρg2Tcosθ Where:
- T represents the surface tension of the liquid.
- θ represents the angle of contact between the liquid and the tube wall.
- r represents the internal radius of the capillary tube core.
- ρ represents the absolute density of the liquid.
- g represents the acceleration due to gravity.
Critical Inference
According to Jurin’s Law, the height h is inversely proportional to the tube radius (h∝r1). A narrower capillary tube will yield a higher ascent or deeper descent of the fluid column.
UPSC High-Yield Scientific Trivia
Spherical Morphology of Raindrops
A falling raindrop, a tiny oil droplet suspended in water, or a small bead of liquid mercury assumes a spherical shape. According to geometric principles, for a given volume, a sphere possesses the absolute minimum surface area. Because surface tension forces a liquid surface to minimize its area to achieve the lowest possible potential energy state, it pulls the liquid mass into a spherical shape.
Action of Chemical Detergents in Hot Water
Detergents clean soiled clothes because they contain surfactants that lower the surface tension of water. Pure water has a high surface tension, causing it to form large droplets that cannot penetrate the microscopic gaps between fabric fibers. Lowering the surface tension allows the water to spread, wetting the fabric deeply to dislodge dirt particles. Doing laundry in hot water enhances this effect because the higher temperature further lowers the surface tension.
Agricultural Plowing and Moisture Preservation
Following a spell of rain, farmers plow their agricultural fields to break up the topsoil layer. If left unplowed, the drying soil forms continuous, fine microscopic capillary pathways extending to the surface. Soil moisture from deeper layers then climbs these capillaries via capillary action and evaporates into the air. Plowing disrupts these capillary pathways, trapping and preserving moisture within the subsoil to benefit future crops.
Mechanism of Blotting Paper and Oil Lamps
- Blotting Paper: Contains a porous network of cellulose fibers that act as a system of microscopic capillary tubes, drawing in ink or liquid spills.
- Oil Lamps (Diyas): The cotton wicks used in traditional oil lamps possess small spaces between the interwoven threads. These spaces act as capillaries, continuously drawing oil upward from the reservoir to sustain the flame at the top.
Antiseptic Spreading Efficiency
Antiseptics and medicinal ointments are formulated to have a lower surface tension than water or human blood. This low surface tension allows the liquid medicine to spread quickly and flow into tiny open wounds, cuts, and microscopic skin pores rather than forming beads on top of the skin.
Last Modified: May 27, 2026