Surface Tension
Surface tension is the cohesive force per unit length acting along the surface of a liquid, arising because molecules at the surface experience a net inward attractive force from neighbouring molecules and therefore resist increases in surface area. It causes liquids to form droplets, allows certain insects to walk on water, and governs the rise of liquids in narrow tubes through capillary action. Surface tension decreases with temperature and is reduced by surfactants (detergents), which disrupt intermolecular cohesion.
Key Formula
γ = F / L
LaTeX: \gamma = \frac{F}{L}
| Symbol | Meaning | Unit |
|---|---|---|
| γ | Surface tension | N/m (or J/m²) |
| F | Force acting along the surface | N |
| L | Length along which the force acts | m |
Worked Example
Problem
A thin wire frame with a moveable wire is used to form a liquid film. The moveable wire has length 0.04 m. A force of 1.2 × 10⁻³ N is required to hold the wire in place. Calculate the surface tension of the liquid. (Note: the film has two surfaces.)
Solution
Step 1 — The film has two surfaces, so the effective length is 2L = 2 × 0.04 = 0.08 m. Step 2 — Surface tension: γ = F / (2L) = 1.2 × 10⁻³ / 0.08 = 0.015 N/m.
Answer
γ = 0.015 N/m = 15 mN/m
Surface Tension of Common Liquids at 20 °C
| Liquid | Surface Tension (mN/m) | Effect of Surfactant | Notable Phenomenon |
|---|---|---|---|
| Mercury | 485 | Minor reduction | Meniscus curves downward (convex) |
| Water | 72.8 | Drops to ~30 with soap | Water strider support, capillary rise |
| Ethanol | 22.1 | Further reduction | Spreading on surfaces |
| Acetone | 23.7 | Moderate | Fast evaporation, low wetting |
| Blood serum | ~47 | Reduced by proteins | Alveolar fluid films in lungs |
| Olive oil | 32 | Slight | Cooking emulsification |
Interactive Tools
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Related Terms
Capillary Action
Capillary action is the spontaneous rise (or depression) of a liquid in a narrow tube or porous medium against or with gravity, driven by the interplay between adhesive forces (liquid to solid) and cohesive forces (liquid to liquid), as characterised by surface tension. When adhesion exceeds cohesion — as in water in glass — the liquid rises and forms a concave meniscus; when cohesion exceeds adhesion — as in mercury in glass — the liquid is depressed and forms a convex meniscus. Capillary action is vital in plant water transport, paper chromatography, inkjet printing, and soil hydrology.
Fluid Pressure
Fluid pressure is the force exerted per unit area by a fluid on any surface in contact with it, arising from the continuous collisions of fluid molecules. In a static fluid, pressure at a given depth depends on the fluid's density, gravitational acceleration, and the depth below the free surface. It is fundamental to hydraulics, hydrostatics, and the design of dams, pipelines, and pressure vessels.
Viscosity
Viscosity is a measure of a fluid's resistance to deformation or flow under an applied shear stress, arising from internal friction between adjacent fluid layers moving at different velocities. Dynamic (absolute) viscosity quantifies the shear stress needed to produce a unit velocity gradient, while kinematic viscosity is the ratio of dynamic viscosity to fluid density. Viscosity governs flow behaviour in lubrication, blood circulation, polymer processing, and aerodynamics.
From Latin "superficies" (surface) and "tensio" (a stretching), from "tendere" (to stretch). The physical concept was analysed quantitatively by Thomas Young and Pierre-Simon Laplace in the early 19th century; their combined Young–Laplace equation describes pressure across curved interfaces.