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Capillarity

Rise of the water-air meniscus in glass capillaries with different radii

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Due to the surface tension, the water in a glass capillary rises up to a certain height above the water level. A wettable wall, e.g. glass, attracts the water molecules more strongly than the mutual attraction between water molecules. The water film tends to cover as much of the glass surface as possible. This leads to a lifting force inside the capillary. As soon as the weight of the water column in the capillary compensates this capillary lifting force, the air-water meniscus stops rising. The lifting force is proportional to the wetting line of the three phases air, water, and glass and therefore depends on 2 π r, with r being the radius of the capillary. The weight of the column of a given height depends on the cross section of the capillary, π r2 . The smaller the radius of a capillary is, the higher the water will rise because due to a decreasing radius the weight decreases faster (r2 ) than the lifting force (r).

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