Dip the corner of a tissue in tea and watch the tea creep up the paper, climbing against gravity. Nothing is pumping it. Where does the lift come from?
Start with something you know
Fill a glass to the brim and keep adding water drop by drop. The water bulges above the rim before it spills, as if a skin is holding it in. Insects called water striders walk on that skin. The effect is called surface tension.
Deep inside the water, each molecule is pulled equally in every direction by its neighbours. At the surface there are no water molecules above, so the surface molecules are pulled inward and sideways. The surface behaves like a stretched sheet that always tries to shrink, which is why small drops are round.
Try it
Try this
- Move the radius slider. When you halve the radius, what happens to the height?
- Switch to Water in waxed glass. Why does the water go down instead of up?
- Choose Mercury in glass and look closely at the shape of its surface inside the tube.
What is happening
Two pulls compete. Cohesion is the pull of water molecules on each other. Adhesion is the pull of water on the glass. Water is attracted to clean glass more strongly than to itself, so it creeps up the wall and the surface curves up into a dip called a meniscus. Surface tension then drags the whole column upward until the weight of the lifted water balances the pull around the rim.
Balancing those two forces gives Jurin’s law:
- h
- height the liquid rises (m)
- γ
- surface tension (N/m), about 0.073 for water
- θ
- contact angle between the liquid and the tube wall
- ρ
- density of the liquid (kg/m³)
- g
- gravity, 9.81 m/s²
- r
- inside radius of the tube (m)
The radius sits at the bottom, so a tube half as wide lifts water twice as high. In a glass tube with a 0.5 mm radius, water climbs about 3 cm. In a tube with a 0.05 mm radius, it would climb about 30 cm.
The contact angle decides up or down
The contact angle θ is the angle the liquid surface makes with the wall. Water on clean glass spreads out, so θ is close to 0°, cos θ is close to 1, and the water rises. On waxed glass water beads up, θ is more than 90°, cos θ is negative, and the level inside drops below the outside surface. Mercury does the same in glass, which is why its surface bulges upward in the tube.
Where you see it
- Kitchen towels and tissuesThe fibres leave tiny gaps that work like thousands of thin tubes, pulling liquid up and along the paper.
- Oil lamp wicksOil climbs the cotton wick to the flame, so a diya keeps burning without any pump.
- Damp wallsGround water climbs through the fine pores in bricks. Builders add a damp-proof layer to break that path.
- Soil and plantsCapillary action holds water in soil and helps it move towards roots. Tall trees also depend on water evaporating from leaves to pull the column up.
Easy to mix up
- Surface tension is not a film of some other material. It is the water itself, behaving differently at its surface.
- A thinner tube lifts water higher, but the amount of water lifted is tiny. Capillary action cannot lift a bucket of water.
Check your understanding
5 questions.Surface Tension Quiz