Capillarity, also called capillary action, is the ability of a liquid to flow through narrow spaces without the help of gravity or any external force. You see it when water climbs up a paper towel, when a thin glass tube draws liquid upward, or when soil stays moist well above the water table. The phenomenon is driven by two competing forces: the attraction between liquid molecules and a surface, and the attraction liquid molecules have for each other.
How Capillary Action Works
Two forces govern capillarity. The first is adhesion, the attraction between the liquid and the material it touches. The second is cohesion, the attraction liquid molecules have for one another, which is what gives a liquid its surface tension. Capillary action happens when adhesion to the walls of a narrow space is stronger than the cohesive pull holding the liquid together. The liquid essentially gets tugged upward by the surface faster than its own internal bonds can resist.
Water in a glass tube is a clear example. Water molecules are more attracted to glass than to each other, so the liquid creeps up the walls and forms a curved surface called a concave meniscus, dipping down in the center. Mercury behaves the opposite way. Its molecules are more strongly attracted to each other than to glass, so mercury pushes away from the walls and forms a convex meniscus, bulging upward in the center. This second behavior is sometimes called capillary repulsion, and it’s why mercury in a glass thermometer sits lower at the edges.
What Determines How High Liquid Rises
The height a liquid climbs in a tube depends on a simple relationship first described by James Jurin in the early 1700s. Jurin’s law says the rise height increases when surface tension is stronger, and decreases when the tube is wider or the liquid is denser. In practical terms, a narrower tube produces a taller column of liquid. Double the tube’s radius and the liquid rises only half as high.
The angle at which the liquid meets the tube wall, called the contact angle, also matters. A contact angle below 90 degrees means the liquid wets the surface and climbs. Above 90 degrees, the liquid resists the surface and gets pushed down. Water on clean glass has a very low contact angle, which is why it climbs so readily. On a wax-coated surface, the angle jumps above 90 degrees, and the water beads up instead of spreading.
Temperature plays a role too. Experiments measuring water’s capillary rise at temperatures from 20°C to 80°C found that warmer water climbs higher. At 20°C, water rose about 20.3 mm in the test setup, while at 80°C it reached 24.1 mm. That’s roughly an 18% increase. Heating a liquid changes both its surface tension and its density, and the net effect favors a taller rise.
Capillarity in Plants and Soil
Plants depend on capillary action to pull water from the soil into their roots. Once inside the root system, water enters extremely narrow tubes called xylem, where adhesion to the tube walls and cohesion between water molecules work together to move water upward. Capillary action alone, though, can only lift water a limited distance before gravity wins. To push water all the way to the top of a tall tree, plants rely on an additional pull created by evaporation from the leaves, which creates a continuous chain of tension through the water column.
In soil, capillarity creates what hydrologists call the capillary fringe, a zone above the water table where water is pulled upward into the gaps between soil particles. The finer the particles, the higher the water climbs. Experiments measuring this fringe found it was about 13 cm thick in silty sand, 16 cm in silt, and 37 cm in silty clay. Coarse gravel, with its large pore spaces, barely supports any capillary rise at all. This is why clay soils stay damp near the surface long after rain stops, while sandy soils dry out quickly.
Everyday Uses of Capillarity
Capillary action is at work in more products and technologies than most people realize.
- Moisture-wicking fabrics. Athletic clothing made from polyester and similar synthetics uses capillary action to pull sweat away from your skin. The fibers need to strike a balance: hydrophilic enough to attract moisture into the tiny channels between fibers, but not so absorbent that they hold onto it like cotton does. Cotton soaks up water and stays wet. Wicking fabrics move water along their surface through narrow gaps, spreading it across a larger area where it evaporates quickly.
- Paper towels and sponges. The porous structure of a paper towel acts like thousands of tiny tubes. Water climbs into them through capillary action, which is why a paper towel can soak up a spill even when only one corner touches the liquid.
- Candle wicks. Melted wax travels up the wick through capillary action, reaching the flame where it vaporizes and burns. Without this upward flow, the flame would simply melt a pool of wax and go out.
- Diagnostic test strips. Home pregnancy tests, COVID rapid tests, and glucose monitors all rely on capillary-driven flow. When you place a drop of blood or urine on the test strip, the sample moves through a narrow channel without any pump or battery. The channel’s surface is designed to pull the liquid forward using capillary forces, carrying it past reagents that produce the test result. This pump-free design is what makes these tests cheap and portable enough to use at home.
Surface Properties That Control Capillarity
Whether capillary action occurs, and how strongly, depends heavily on the surface the liquid contacts. Surfaces that attract water are called hydrophilic, and they typically have contact angles well below 90 degrees. Clean glass, untreated metals, and certain polymer coatings fall into this category, with contact angles that can drop below 5 degrees on specially treated surfaces. At these extremely low angles, water spreads into a thin film almost instantly.
Hydrophobic surfaces repel water and have contact angles above 90 degrees. Superhydrophobic surfaces, like certain nanostructured coatings, push contact angles above 150 degrees, causing water to bead up into nearly perfect spheres and roll off. On these surfaces, capillary action is effectively shut down because the liquid has almost no adhesion to work with. Engineers use this principle to design self-cleaning surfaces, non-fogging glass, and waterproof coatings that prevent unwanted wicking.
This ability to tune capillary behavior by modifying surface chemistry is central to modern microfluidics. By coating sections of a tiny channel with hydrophilic or hydrophobic materials, designers can control exactly where and how fast a liquid moves, creating miniature laboratories on a chip that sort, mix, and analyze fluid samples using nothing but capillary forces.