Kapillaraufstiegsprüfungen

Kapillaraufstieg, also known as capillary action, is the ability of a liquid to flow in narrow spaces without the assistance of, or even in opposition to, external forces like gravity. This phenomenon is driven by the interplay of adhesive and cohesive forces. Adhesive forces are the attractions between molecules of different substances, in this case, the liquid and the walls of the narrow tube or porous material. Cohesive forces are the attractions between molecules of the same substance, referring to the liquid itself.

When a liquid is in contact with a surface that attracts it more strongly than it attracts itself (e.g., water in a glass tube), the adhesive forces cause the liquid to "wet" the surface. This wetting action pulls the liquid molecules upwards along the sides of the tube. Simultaneously, cohesive forces within the liquid create a surface tension, which acts like a stretched membrane. This surface tension, pulled upwards by the adhesive forces at the edges, draws the bulk of the liquid up the tube until the upward pull is balanced by the downward force of gravity acting on the elevated liquid column.

The height to which a liquid will rise in a capillary tube is inversely proportional to the radius of the tube. Narrower tubes result in a higher rise because the adhesive forces acting along the circumference have a greater relative influence compared to the weight of the liquid column. This principle is fundamental to many natural processes, such as the movement of water from soil into plant roots and the absorption of liquids by porous materials like paper or sponges.