Definition
The spontaneous movement of a liquid upward through narrow tubes or microscopic gaps, defying gravity. It happens when the attraction between the liquid and the wall teams up with the liquid's tendency to stick to itself. The narrower the gap, the higher the liquid climbs.
Dip Just the Tip, and Water Spreads Upward
Dip the corner of a folded paper towel into a glass of water. Within seconds, you will see moisture creep upward, well above the water level—without anyone pushing it.
The secret lies in two traits of water molecules. First, water molecules love to stick to each other. Second, they also love to cling to other surfaces. Scientists call the first force cohesion and the second adhesion.
A paper towel is made of countless tangled plant fibers, creating a maze of microscopic gaps. When water clings to the walls of these narrow gaps, it creeps up a tiny bit. As it does, it drags neighboring water molecules along with it like a chain. As the water climbs higher, it meets fresh fiber walls, and the climb continues.
In this way, adhesion pulling on the walls and cohesion holding the water together lift the entire column upward. Water does not magically cancel gravity; it keeps climbing only as long as this upward pulling force outweighs the column's weight.
The Narrower the Gap, the Higher It Climbs
Place two drinking straws of different widths into a glass of water, and you will spot an immediate difference: water rises noticeably higher inside the narrower straw.
The lifting force comes from the perimeter where water touches the inside wall. On the other hand, the weight of the water column—the load that needs to be lifted—depends on the cross-sectional area of the tube.
If you cut the tube's diameter in half, the perimeter shrinks by half. But its cross-sectional area shrinks by three quarters, down to one-fourth. The upward pull is halved, but the heavy load drops to a fourth. As a result, the water can climb twice as high.
That is why liquids climb much higher in narrower spaces. You see this everywhere there are tight microscopic tunnels: inside paper towels, woven fabrics, tiny gaps between soil grains, and the microscopic vessels within plant stems.
Looking Closer: A Tug-of-War Between Two Forces
Not every liquid climbs up narrow tubes. Liquid mercury, for example, clings to itself far more strongly than it sticks to glass. Put a narrow glass tube into mercury, and the liquid level inside will actually drop below the outside surface, forming a dome-shaped bulge.
The winner of the battle between cohesion and adhesion determines what happens. If adhesion to the wall wins (like water on glass), the liquid climbs. If cohesion wins (like mercury on glass), it sinks.
By the way, towering trees reaching hundreds of feet in the air cannot rely on capillary action alone. They also depend on transpiration—water evaporating from the leaves pulls the water column upward like an unbroken rope.
Back to our paper towel: it does not absorb spills through magic. It works through a delicate tug-of-war between adhesion and cohesion.
Common misconceptions
Capillary action happens because water has a special power to cancel out gravity.
There is no special antigravity force. It is simply the combined teamwork of adhesion and cohesion. The water stops rising once the weight of the water column perfectly balances the upward pulling force.
Any liquid will spontaneously climb up any narrow tube.
It depends on the liquid and the material of the tube. For liquids like mercury, where cohesion between molecules is stronger than adhesion to the tube walls, the liquid level inside a narrow glass tube actually drops lower.
Where you meet it
A paper towel quickly wicking up a spilled drink against gravity.
A candle wick drawing liquid wax up toward the flame to keep burning.
Plants and trees drawing moisture and nutrients from the soil through roots and stems.
In one sentence
Capillary action is the phenomenon where liquids spontaneously climb through narrow gaps, driven by a partnership between adhesive forces pulling against walls and cohesive forces holding the liquid together.