Surface Tension of Water: Why It Beads and Climbs

Water's surface tension is about 72.7 mN/m (0.0727 N/m, or 72.7 dyn/cm) at 20 °C (68 °F). It is why a drop pulls itself into a bead and why water creeps up a narrow tube: molecules at the surface cling to each other more strongly than to the air above, so the surface behaves as if it were under tension. Handbooks often round the value to 72.8, and either is fine for everyday work. Here is what the number means, why heat and soap change it, and how to use it to work out how high water will climb or how much pressure sits inside a droplet.

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Surface tension of water at a glance

Surface tension can be written as a force per unit length or as an energy per unit area. The two are the same quantity, which is why you will meet it in several units. At 20 °C, pure water reads:

Millinewtons per meter and dynes per centimeter are numerically identical, so 72.7 mN/m is also 72.7 dyn/cm. To convert dynes per centimeter to pounds-force per foot, multiply by 0.0000685.

Tables round the 20 °C value slightly differently. The international IAPWS table gives 72.74 mN/m, while many handbooks list 72.8. The gap is well inside the measurement uncertainty, which IAPWS puts at about ±0.36 mN/m at this temperature.

What surface tension actually is

A water molecule deep in a glass is pulled equally in every direction by its neighbors, so the forces cancel. A molecule at the surface has neighbors beside and below it but only air above, so it feels a net pull inward. That pull draws the surface molecules together and makes the surface resist being stretched or broken, much like a stretched elastic sheet.

Hydrogen bonding makes water's inward pull unusually strong, which is why its surface tension sits far above most liquids you will find around the house:

Mercury, held together by metallic bonding, is in a different league. Most everyday liquids sit well below water.

Energy gives the same answer from another angle. Enlarging a surface means pulling molecules up from the bulk, which costs energy, so liquids shrink their surface area wherever they can. A sphere has the least surface for a given volume, which is why free-floating drops and bubbles are round.

Why water beads up

Beading is a contest between two attractions. Cohesion is water sticking to water. Adhesion is water sticking to whatever it is sitting on. When adhesion is weak, as on a waxed hood, a feather or a leaf with a waxy coating, cohesion wins and the drop pulls into a tight bead that meets the surface at a steep angle (a contact angle above 90°). On clean glass or bare metal, adhesion is strong, so the water spreads into a thin film instead.

Gravity gets a say too. Surface tension wants a sphere, while gravity wants to flatten the drop into a puddle. The crossover is set by the capillary length:

Drops much smaller than that stay close to spherical, which is why dew and mist droplets look like tiny beads. Larger ones sag and spread, which is why a spoonful of water flattens on a countertop.

Surface tension also squeezes whatever is inside a drop. The extra pressure in a spherical droplet of radius r is:

A soap bubble has two surfaces, inside and outside, so its excess pressure is roughly double, at 4γ ÷ r.

Why water climbs: capillary action

Put a thin glass tube in water and the water inside rises above the level outside. Adhesion pulls the liquid up along the wall, which curves the surface into an upward-turning meniscus. Surface tension keeps that surface intact, so the whole column is dragged up with the rim. The climb stops when the weight of the lifted water balances the pull. Capillary action happens whenever adhesion to the walls is stronger than the cohesion within the liquid, which is exactly the case for water on clean glass.

The maximum height is given by Jurin's law:

Take a clean glass tube with a 1 mm inner diameter (r = 0.5 mm) and water at 20 °C: h = 2 × 0.0727 × 1 ÷ (998 × 9.81 × 0.0005) ≈ 0.030 m. The water climbs about 30 mm (1.2 in).

Radius sits in the denominator, so halving the tube's radius doubles the rise. For clean glass at 20 °C the constants combine into a shortcut: rise in millimeters ≈ 14.9 ÷ radius in millimeters.

These are ideal figures. A dirty or waxy tube has a larger contact angle and lifts water less. Past 90° the sign flips, and the liquid is pushed down instead, which is what mercury does in glass.

The same physics works wherever water meets tiny channels: the fibers of a paper towel, cloth, the pores in soil, a wick. Dip a paper towel in a glass and the water climbs until gravity wins. Plants rely on it too, but capillary action alone only lifts water a short distance. Getting it to the top of a tree takes adhesion and cohesion working together inside the plant's xylem.

If you would rather skip the arithmetic, the surface tension calculator handles these calculations for you.

How temperature, soap and salt change it

Temperature. Warmer water has more thermal motion, which loosens the hydrogen-bond network at the surface, so surface tension falls steadily as water heats:

Between 20 °C and 100 °C it drops by about 19%. Across the whole 0–100 °C range that averages roughly 0.17 mN/m per degree Celsius. Surface tension reaches zero at the critical point, about 374 °C, where liquid and vapor become indistinguishable. Lower tension is a main reason hot water wets and cleans better, although a good detergent can do the same job at lower temperatures.

Soap and other surfactants. Surfactant molecules collect at the surface and break up the water-to-water attraction there. A small amount is enough: soapy water at 20 °C comes in at roughly 25–45 mN/m, against 72.7 for pure water. Lower tension lets water spread into fabric and pores, which is how soap cleans. It also ends the paper-clip trick, since a clip that floated on clean water sinks once soap is added.

Dissolved salt. Salt nudges surface tension up, not down. Seawater runs roughly 1% higher than fresh water. Ordinary tap water carries so little dissolved material that the effect is negligible.

Contamination. A trace of oil or detergent film can pull a measurement well below the clean-water value, which is why lab work demands spotless glassware. All values here are for pure water against air.

Real-world examples

Once you know where to look, surface tension is easy to spot:

Where it matters around water systems

Cleaning and wetting. Detergent and heat both lower surface tension so water reaches into fabric, crevices and soiled surfaces. Dishwasher rinse aids work the same way, letting water sheet off glassware instead of beading into spots.

Soil and irrigation. Water rises through soil pores by capillary action, and finer soils lift it higher than coarse sand, just as Jurin's law predicts. Very dry or water-repellent soil can bead water and shed it, which is why a light sprinkle sometimes runs off instead of soaking in.

Masonry and foundations. Concrete, brick and mortar are full of tiny pores that draw moisture upward from wet ground. Builders break that path with a layer of coarse gravel or a moisture barrier under slabs, because gaps that large have almost no capillary lift.

Faucets and drips. A drop hangs from a spout until its weight overcomes the surface tension holding it at the rim, so a wider spout holds a bigger drop.

The bottom line

Water's surface tension is about 72.7 mN/m at 20 °C, falling to roughly 59 mN/m at boiling and to about 25–45 mN/m once soap is added. Cohesion pulls surface molecules together, which rounds drops into beads. Adhesion to a narrow wall pulls the liquid upward, which is why water climbs tubes, towels and soil. Size decides the outcome: the smaller the drop, the higher the pressure inside it, and the narrower the tube, the higher the rise. When you need an exact figure for a specific temperature or geometry, the calculator below has it.