A capillary tube works by exploiting the natural attraction between a liquid and the walls of a narrow channel. In a tube thin enough, liquid will rise (or fall) on its own, without any pump or outside pressure, because molecular forces at the tube’s inner surface pull the liquid along. The narrower the tube, the higher the liquid climbs. This same principle is also used in refrigeration, where a long, thin tube creates a pressure drop that cools refrigerant, though that application works by a different mechanism entirely.
The Two Forces Behind Capillary Action
Capillary action depends on a tug-of-war between two molecular forces: adhesion and cohesion. Adhesion is the attraction between the liquid and the tube wall. Cohesion is the attraction between the liquid’s own molecules, which makes them cling together. When adhesion to the wall is stronger than cohesion within the liquid, the liquid creeps upward along the tube’s inner surface.
Water in a glass tube is the classic example. Water molecules are strongly attracted to glass, so they climb the walls and drag neighboring water molecules along with them. Surface tension, the elastic-like film at the water’s surface, holds everything together so the liquid rises as a column rather than just wetting the walls individually. The result is a curved surface at the top of the liquid column called a meniscus. For water in glass, this meniscus is concave, dipping down in the center like a shallow bowl.
Mercury behaves in the opposite way. Mercury molecules are far more attracted to each other than they are to glass. Cohesion wins the tug-of-war, so mercury actually sinks below the surrounding surface level when placed in a glass tube, forming a convex (dome-shaped) meniscus. This is sometimes called capillary repulsion, or capillary depression.
Why Narrower Tubes Pull Liquid Higher
The height a liquid reaches inside a capillary tube follows a relationship known as Jurin’s law. The key insight is simple: the rise is inversely proportional to the tube’s radius. Cut the radius in half, and the liquid climbs twice as high.
This happens because the upward pull comes from the contact line where liquid meets the tube wall, which scales with the tube’s circumference. The weight of the liquid column that must be lifted, on the other hand, scales with the tube’s cross-sectional area. In a thinner tube, there’s relatively more “pull” per unit of liquid, so the column goes higher before gravity balances things out.
For water at room temperature in a glass tube, the numbers are striking. A tube with a 1 mm radius will draw water up about 13.5 mm. Shrink that radius to 0.1 mm and the water rises to roughly 135 mm, about 5.3 inches. The exact height also depends on the liquid’s surface tension, its density, and the contact angle between the liquid and the tube wall. A contact angle below 90 degrees means the liquid wets the surface and rises. Above 90 degrees, as with mercury in glass, the liquid is pushed down instead.
Capillary Action in Nature and Everyday Life
Plants rely on capillary action as one of several mechanisms to move water from their roots upward. The tiny vessels in a plant’s water-transport tissue act like bundles of microscopic capillary tubes, pulling water upward through adhesion to cell walls. On its own, capillary action can only lift water about a meter or so in the narrowest plant vessels, which isn’t nearly enough to reach the top of a tall tree. The rest of the work is done by evaporation from leaves, which creates a negative pressure that pulls the water column upward like a chain.
You also see capillary action when a paper towel absorbs a spill. The tiny gaps between cellulose fibers act as capillary channels, drawing liquid in all directions. The same principle is at work when a paintbrush holds paint, when ink flows through a fountain pen nib, or when a thin crack in concrete slowly wicks moisture from the surrounding soil.
Tube Material Changes the Behavior
The tube’s material matters because adhesion depends on the surface chemistry of the wall. Glass, especially the borosilicate glass used in laboratories, has a high surface energy that strongly attracts water. This makes it ideal for applications that depend on reliable capillary rise. Plastics vary more widely. Polypropylene and polystyrene interact with liquids quite differently from each other, and both behave differently from glass.
Surface treatments can alter capillary behavior dramatically. Coating a glass tube with a water-repelling silicone layer, for example, reduces the adhesion between water and the wall, lowering the contact angle and potentially reversing the direction of the meniscus. In laboratory settings, these material differences are significant enough to affect experimental results, so the choice of tube material is rarely arbitrary.
Capillary Tubes in Refrigeration Systems
In refrigeration and air conditioning, a “capillary tube” refers to something physically similar but functionally different. It’s a long, very narrow copper tube (typically a meter or more in length) that connects the high-pressure side of a cooling system to the low-pressure side. Its job is not to draw liquid upward through molecular attraction but to restrict flow and create a pressure drop.
High-pressure liquid refrigerant enters one end of the capillary tube. As it’s forced through the narrow bore, friction and the tube’s small diameter cause the pressure to fall steadily along the tube’s length. By the time the refrigerant exits, its pressure is low enough that it rapidly evaporates, absorbing heat from its surroundings and producing the cooling effect. The tube essentially replaces a mechanical valve, metering out exactly the right amount of refrigerant to the evaporator.
One practical advantage of capillary tubes in refrigeration is that they allow pressure to equalize across the system when the compressor shuts off. This means the compressor doesn’t have to restart against a large pressure difference, which reduces the starting torque needed and allows manufacturers to use smaller, less expensive motors. You’ll find capillary tubes in most household refrigerators, window air conditioners, and small freezers for this reason. Larger commercial systems typically use adjustable expansion valves instead, since capillary tubes can’t adapt to changing cooling loads.
What Limits Capillary Rise
Capillary action has a natural ceiling. Gravity always pulls the liquid column back down, and eventually the weight of the column exactly matches the upward force from adhesion and surface tension. At that point, the liquid stops rising. For water in practical-sized tubes (radius above a few millimeters), the rise is negligible. Capillary effects only become dominant when the tube diameter drops below a few millimeters.
Temperature also plays a role. Warming a liquid generally lowers its surface tension, which reduces capillary rise. The viscosity of the liquid affects how quickly the column rises but not the final height. A more viscous liquid like honey will creep upward slowly through a capillary tube, yet given enough time, it reaches the same equilibrium height that a thinner liquid of the same surface tension and density would.