Table of Contents
What Is Surface Tension?
Surface tension is the tendency of a liquid surface to contract to the minimum possible area, acting like an elastic membrane. It arises from the cohesive forces between liquid molecules. Molecules at the surface experience a net inward force because they have fewer neighbors on the air side, creating a tension along the surface.
Surface tension is measured in newtons per meter (N/m) or equivalently joules per square meter (J/m2). Water at 20 degrees C has a surface tension of about 0.0728 N/m. This is what allows insects to walk on water, needles to float, and capillary action to draw water up narrow tubes.
Key Formulas
Surface Tension of Common Liquids (20°C)
| Liquid | γ (N/m) | Notes |
|---|---|---|
| Water | 0.0728 | Decreases with temperature |
| Mercury | 0.465 | Very high surface tension |
| Ethanol | 0.0223 | Used as solvent |
| Acetone | 0.0237 | Low surface tension |
| Blood | 0.058 | Slightly less than water |
Applications
- Capillary action: Water rises in narrow tubes, plant xylem vessels, and soil pores due to surface tension and adhesion.
- Detergents: Surfactants lower water's surface tension from 0.073 to about 0.025 N/m, improving wetting and cleaning.
- Inkjet printing: Droplet size and placement depend on controlling surface tension and viscosity.
- Medical devices: Lung surfactant reduces surface tension in alveoli, preventing collapse during exhalation.
Frequently Asked Questions
How does temperature affect surface tension?
Surface tension decreases as temperature increases because thermal energy weakens intermolecular cohesive forces. Water's surface tension drops from 0.0756 N/m at 0 degrees C to 0.0589 N/m at 100 degrees C. At the critical temperature, surface tension becomes zero as liquid and vapor become indistinguishable.
What is the contact angle?
The contact angle is the angle at which a liquid-gas interface meets a solid surface. A contact angle less than 90 degrees indicates a wetting (hydrophilic) surface; greater than 90 degrees indicates a non-wetting (hydrophobic) surface. Water on clean glass has a contact angle near 0 degrees, while on Teflon it is about 110 degrees.
Why can insects walk on water?
Small insects like water striders have hydrophobic hairs on their legs. The surface tension force along the contact line can support the insect's weight because the force scales with length (perimeter of feet) while weight scales with volume. At small sizes, surface tension dominates gravity.