Capillary action

Capillarity or capillary (Latin capillaris, the hair on ) is the behavior of fluids, they show in contact with capillaries, such as narrow tubes, crevices or cavities in solids.

Example: If you dip a glass tube vertically in water, the water a piece against the gravitational force rises upward in the narrow glass tube.

These effects are due to the surface tension of liquids itself and the interfacial tension between the liquids and the solid surface (in the example of the glass) caused.

Effects

The capillary rise occurs in fluids which wet the material of the capillary, such as water glass. The water rises in a glass tube and forms a concave surface ( meniscus ). This behavior is due to the adhesive force ( force that acts between the two fluids ) caused.

The capillary depression occurs when the material of the vessel, the liquid surface is not wetted. Examples are mercury on glass or glass with water on greased surface. Such fluids have a tube in a lower level than in the area and a convex surface.

The smaller the diameter, the larger the capillary pressure and the height of rise. In a capillary of 2 microns in diameter ( 1 micron radius ) amount corresponding to the formula below, the height of rise of dampening water 14 meters and the capillary pressure 1.4 bar.

The liquid increases due to adhesion forces to the wall of the tube and thus only to the end thereof, even if the capillarity allowing a larger height of rise.

Formula

The riser height h of a liquid column is given by:

Where:

For a water-filled glass tube that is open to the air at sea level ( 1013.25 hPa), is:

The result for the height of rise:

The Washburn equation describes capillary flow in porous materials without considering the gravity.

Molecular Perspective

In principle the effect of capillarity of the molecular forces within the material ( cohesion forces ) and at the interface between a liquid, a solid body ( vessel wall ), and a gas- based (e.g., air) occur ( adhesion ). Often the capillary has the significance of surface tension.

In the interior of a body is the force acting on a particular molecule forces cancel each other out from its surroundings. At the edges, however, there is a resultant force which is directed depending on the material either into the liquid or out. The effect of the vessel wall against the cohesive forces in the fluid small, then displays the resultant force inside of the liquid. The surface of which is curved at the point of contact to the wall down and wet the vessel wall does not (for example, mercury in the glass vessel ). However, the effect of the vessel wall towards the cohesion forces within the liquid bulk, then displays the resultant force in the vessel wall inside and the liquid is bent upward at the edge. The liquid wets the wall ( eg, water or petroleum in the glass vessel ).

Practical Applications

Filler: An example of an application is the pen or fountain pen, or his pen. It has usually at half the length a small, round hole in which the ink gathers in order to be transported from there by capillary action through a very fine slit at the top.

Paper: It is not possible to write to a non-wetting surface with liquid ink. Therefore, writing with a fountain pen on glass is hardly possible because the ink does not wet the glass surface and thus does not adhere to it. Wetting on media such as paper, the ink can adhere. Paper draws also by capillary ink on, it is even possible to write on head.

Plants: In trees and other plants, the water is absorbed by the roots and then transported to the crown, where it is through the stomata of the leaves (or needles) evaporates or needed for photosynthesis. When moving against gravity, evaporation in the upper part of the plant acts as a suction ( transpiration ), cohesive forces of the water in the plant prevent tearing of the liquid flow and the capillary favored with the osmotic effect ( root pressure ) the rise. According to new findings trees can be up to 130 meters high, since then the osmotic pressure is no longer sufficient, together with the capillary forces to overcome gravity.

Chemistry: In the paper chromatography one uses the capillary by applying a solution is dropped onto a special paper and ascends in this, constituents of the solution are supported. Due to the different walking distance of the substances can be separated.

Medicine: To siphon off small amounts of blood, one can make a small puncture in the blood vessels of the fingers or on the earlobe and hold on the emerging blood collection tube is a thin, rises in the blood due to the capillary effect and thus can be collected.

Textiles: A similar suction as in the paper can also be observed at cloths or fabrics. The same applies also for sponges. For this paper, rags and sponges: the larger the internal surface area ( per unit volume ), the greater the suction.

Soldering: Even when soldering the effect occurs: the liquid solder flows through the capillary, for example, in the gap of copper pipe fittings. For de-soldering of electronic components from printed circuit boards is often a wire mesh, the unsoldering used.

In the form of the quality of the Lötkegels Lötergebnisses is immediately apparent. Should not this be concave and on the board edges tapered, it is most likely a cold solder joint. Because of capillarity are also soldering "overhead" possible.

Construction: In houses and buildings without proper protective measures in masonry often - to observe capillary effect, because let the construction materials used, depending on the void content in varying degrees of moisture against gravity (mostly from the soil in direct contact with the ground ) ascend - here, however, undesirable. The thermal conductivity of the structural material increases with increasing moisture absorption, so that the heat loss of the building increases. A large absorbency, for example, in walls of brick (slightly burned ), autoclaved aerated concrete (so-called " Concrete " ) and calcareous sandstones given a significantly lower capillary is in hard-baked brick (clinker ) and concrete observed. To interrupt the capillary flow in buildings, are in the foundation area waterproof barrier layers, eg a bitumen membrane installed.

Oenology: In oenology be used to measure the ethanol content of wines Vinometer, based on the dependent capillarity ethanol content of the wine.

Literature and links

• H. Schubert: capillarity in porous solids systems. Springer, Berlin 1982, ISBN 3-540-11835-7
• Medical Physics Lecture: Surface tension and capillarity, University of Veterinary Medicine Vienna
• Valentin L. Popov: Contact mechanics and friction. A teaching and application book by nanotribology to numerical simulation. Springer, 2009, 328 pp., ISBN 978-3-540-88836-9.