Capillary tubes made of plastic or glass are used to collect small liquid samples through capillary action. They rely on surface tension and adhesion to counteract gravity. The thickness of the tube opening varies based on the weight of the fluid. Researchers can create custom tools by heating and stretching glass tubes.
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 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 for analysis, such as blood samples for diabetics, or to carry them to other devices, such as Petri dishes or microscopes, for further study or experimentation.
The thickness of the capillary tube opening varies from one application to another, the determining factor being the density or weight of the fluids to be aspirated. There are many sizes of pre-made tubes for use with specific applications, such as drawing blood to test blood sugar levels. In research applications, researchers often heat the center of a glass tube to near its melting point. They then pull the ends, stretch the center into a very thin tube that suits their immediate needs, and once the glass cools, they break it down the middle to create a custom tool.
All of these tubes work through a process called capillary action. This process uses two physical forces – surface tension and adhesion – to counteract the effects of gravity and take a sample. Surface tension is the formation of a barrier on the surface of a fluid created by the force required to separate individual fluid molecules from each other. This force can be observed in a glass, which can be filled with water to a level slightly greater than the volume of the glass, allowing it to rise above the rim. The water stays above the glass because the force required to separate the water molecules at the surface is greater than the force of gravity exerting on the water above the rim of the glass.
Adhesion promotes capillary action by creating a mutual attraction force between the liquid and the tube itself. In the example of a glass of water, the water is attracted to and adheres to the rim of the glass, creating a boundary for the surface of the water. This is what keeps the water in the glass as surface tension determines how high the water can rise above it.
Capillary tubes are, by nature, very thin. Due to this fact, there is a high amount of adhesion around the inside of the tube, which creates surface area at the top and bottom of a liquid in the tube. Consequently, when a liquid meets one, it is drawn into the pipe by the forces of adhesion, creating a surface with a specific surface tension that remains intact due to the adhesion of the pipe walls. The liquid will continue to rise up the tube until the weight of the total mass of all the liquid in the tube exceeds the surface tension at the bottom.
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