Surface Tension Converter

Newton per Meter (N/m)

The SI unit of surface tension, representing the force exerted per unit length. 1 N/m = 1,000 mN/m = 1,000 dyn/cm. Standard unit in modern scientific research and engineering.

Common reference: Water at 20°C has surface tension of 0.0728 N/m (72.8 mN/m or 72.8 dyn/cm).

Millinewton per Meter (mN/m)

One thousandth of a newton per meter. 1 mN/m = 0.001 N/m = 1 dyn/cm. Convenient unit for liquid-air and liquid-liquid interfaces at room temperature.

Practical use: Most practical measurements of liquids at standard conditions. Water = 72.8 mN/m @ 20°C (very convenient reference).

Dyne per Centimeter (dyn/cm)

CGS unit of surface tension. 1 dyn/cm = 0.001 N/m = 1 mN/m. Still widely used in scientific literature, especially in chemistry and physics.

Widespread in: Chemical databases, older textbooks, and certain scientific research fields where CGS units remain standard.

Erg per Square Centimeter (erg/cm²)

CGS unit representing energy per unit area. 1 erg/cm² = 0.001 N/m = 1 mN/m = 1 dyn/cm. Surface energy and surface tension are equivalent concepts.

Fundamental definition: Surface tension equals the surface energy required to create additional surface area.

Pound-Force per Inch (lbf/in)

North American unit. 1 lbf/in ≈ 175.1 N/m. Used in some US industrial applications and legacy engineering documentation.

Application: Specific US industrial standards and older equipment specifications where imperial units were standard.

Understanding Surface Tension

Surface tension is the energy required to increase the surface area of a liquid. It arises because:

• Molecular attraction: Liquid molecules attract each other (cohesive forces)
• Asymmetric forces at interface: Surface molecules experience net inward force from bulk liquid
• Energy minimization: Liquid naturally minimizes surface area (forms spheres/droplets)
• Measurable effect: Causes water droplets to bead up, allows insects to walk on water, makes menisci in tubes

Definition: Surface tension (σ) = Force per unit length = Energy per unit area

Temperature Dependence - CRITICAL

Surface tension decreases significantly with increasing temperature:

• Water @ 0°C: 75.6 mN/m
• Water @ 20°C: 72.8 mN/m
• Water @ 50°C: 67.9 mN/m
• Water @ 100°C: 58.8 mN/m

Typical relationship: Surface tension decreases roughly linearly with temperature increase. At the critical temperature, surface tension reaches zero.

Always specify measurement temperature! A value without temperature is not scientifically meaningful.

Effect of Solutes - IMPORTANT

Dissolved substances can dramatically affect surface tension:

• Salts (e.g., NaCl): Increase surface tension (salting out effect)
• Surfactants (soaps, detergents): Dramatically decrease surface tension (0.3-0.5 mN/m for solutions)
• Alcohols (ethanol, methanol): Lower surface tension
• Organic compounds: Variable effects depending on structure

Example: Pure water = 72.8 mN/m, but 1% soap solution = ~30 mN/m (60% reduction!)

Typical Surface Tension Values (at 20°C)

• Water (20°C): 72.8 mN/m (reference standard)
• Mercury (20°C): 486 mN/m (highest of common liquids)
• Olive oil (20°C): ~32 mN/m
• Ethanol (20°C): 22.3 mN/m
• Acetone (20°C): 23.7 mN/m
• Glycerin (20°C): 63.3 mN/m
• Seawater (20°C): ~73 mN/m (slightly higher than fresh water)
• Liquid nitrogen (-196°C): ~8.85 mN/m
• Soap solution (1%): ~30 mN/m (60% reduction)
• Molten iron (1600°C): ~1,940 mN/m

Applications of Surface Tension

• Capillarity: Water rises in narrow tubes due to surface tension; critical for plant water transport
• Droplet formation: Ink jet printing, spray bottles, medical inhalers
• Cleaning & detergents: Surfactants reduce surface tension to enable better wetting and emulsification
• Adhesion: Used in adhesive tape, sealants, and liquid bonding
• Biological systems: Cell membrane stability, lung surfactant function, tears and saliva
• Heat transfer: Pool boiling enhancement, condensation, heat pipe operation
• Interfacial chemistry: Adsorption, emulsions, foams, colloids

Key Conversion Factors

• 1 N/m = 1,000 mN/m = 1,000 dyn/cm = 1,000 erg/cm²
• 1 mN/m = 1 dyn/cm = 1 erg/cm² = 0.001 N/m
• 1 dyn/cm = 0.001 N/m (common in older literature)
• 1 lbf/in ≈ 175.1 N/m (North American unit)
• 1 gf/cm ≈ 0.00981 N/m (gram-force unit)
• Water @ 20°C = 72.8 mN/m (universal reference)
• Surface energy (J/m²) = Surface tension (N/m) for liquid-air interfaces