A capillary tube is a very thin tube made of a rigid material, such as plastic or glass. The tube is used to collect samples of liquids, which will flow up into the tube against the effects of gravity in a process called capillary action. Capillary tubes are commonly used in medical and research fields to extract very small samples of liquids to be analyzed, such as blood samples for diabetics, or to transport them to other devices, such as Petri dishes or microscopes, for further study or experimentation.

The thickness in the opening of capillary tubes varies from application to application, with the determining factor being the density or weight of the fluids to be drawn into the tubes. There are many sizes of pre-manufactured tubes for use with specific applications, such as drawing blood to test for blood sugar levels. In research applications, researchers will often heat the center of a glass tube to near its melting point. They then pull the ends of the tube stretching the center into a very fine tube suitable for their immediate needs and, once the glass cools, break the tube in the middle to create a custom capillary tube.

All capillary tubes function through a process called capillary action. This process uses two physical forces, surface tension and adhesion, to counter the effects of gravity and draw a sample up into a capillary tube. Surface tension is the forming of a barrier at the surface of a fluid created by the force it takes to separate individual molecules of the fluid from one another. This force can be observed in a drinking glass, which can be filled with water to a level slightly more than the volume of the glass rising above the rim. The water remains above the glass because the force required to separate the water molecules at the surface is greater than the force gravity is exerting on the water above the rim of the glass.

Adhesion aides in capillary action by creating a mutually attractive force between the liquid and the capillary tube itself. In the example of a glass of water, water is attracted to, and adheres to, the rim of the glass, creating a boundary for the surface of the water. Adhesion is what keeps the water in the glass while the surface tension determines how high the water can rise above the glass.

Capillary tubes are by nature very thin. Because of this fact, there is a high amount of adhesion around the inside of the tube, which creates a surface area at the top and bottom of a liquid in the tube. As a result, when a liquid encounters a capillary tube, the liquid is drawn up into the tube by the forces of adhesion, creating a surface with a specific surface tension that remains intact due to the adhesion of the sides of the tube. The liquid will continue to rise up the capillary tube until the weight of total mass of all the liquid in the tube exceeds the surface tension at the bottom of the tube.