Menisci


The meniscus (plural: menisci, from the Greek for "crescent") is the curve in the upper surface of a liquid close to the surface of the container or another object, caused by surface tension. It can be either convex or concave, depending on the liquid and the surface.

A convex meniscus occurs when the particles in the liquid have a stronger attraction to each other (cohesion) than to the material of the container (adhesion).[1] This occurs, for example, between mercury and glass in barometers[1] and thermometers. Conversely, a concave meniscus occurs when the particles of the liquid are more strongly attracted to the container than to each other, causing the liquid to climb the walss of the container. This occurs between water and glass.

The meniscus and measurement

When reading a depth scale on the side of an instrument filled with liquid, the meniscus must be taken into account in order to obtain an accurate measurement. Depth must be measured with the meniscus at eye level (to eliminate parallax error) and at the center of the meniscus, i.e. the top of a convex meniscus or the bottom of a concave meniscus.

Manufacturers of glassware and other tools calibrate their measurement marks to account for the meniscus. This means that any instrument is calibrated for a specific liquid, usually water.

Capillary action

Menisci are a manifestation of capillary action, by which surface adhesion pulls a liquid up to form a concave meniscus or internal cohesion pulls the liquid down to form a convex meniscus. This phenomenon is important in transpirational pull in plants. Honey, water, milk etc. have a lower meniscus. When a tube of a narrow bore, often called a capillary tube, is dipped into a liquid and the liquid wets the tube (with zero contact angle), the liquid surface inside the tube forms a concave meniscus, which is a virtually spherical surface having the same radius, r, as the inside of the tube. The tube experiences a downward force of magnitude 2πrdσ.

References

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