A capillary tube is a very thin tube made of a rigid material, such as plastic or glass. The tubes are used to collect samples of liquids, which will flow up into the tubes against the effects of gravity in a process called capillary action.They 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 in. 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, stretching the center into a very fine tube suitable for their immediate needs and, once the glass cools, break it in the middle to create a custom tool.