The meniscus (plural: ''menisci'', from the

Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...

for "crescent") is the curve in the upper surface of a liquid close to the surface of the container or another object, produced by surface tension. A concave meniscus occurs when the attraction between the particles of the liquid and the container (

adhesion Adhesion is the tendency of dissimilar particles or surfaces to cling to one another ( cohesion refers to the tendency of similar or identical particles/surfaces to cling to one another). The forces that cause adhesion and cohesion can b ...

) is more than half the attraction of the particles of the liquid to each other ( cohesion), causing the liquid to climb the walls of the container (see surface tension#Causes). This occurs between water and glass. Water-based fluids like sap, honey, and milk also have a concave meniscus in glass or other wettable containers. Conversely, a convex meniscus occurs when the adhesion energy is less than half the cohesion energy. Convex menisci occur, for example, between mercury and

glass Glass is a non-crystalline, often transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most often formed by rapid cooling ( quenching ...

in barometers and thermometers. Sigma tensiometer

In general, the shape of the surface of a liquid can be complex. For a sufficiently small circular tube, the shape of the meniscus will approximate a section of a spherical surface, while for a large container, most of the upper surface of the liquid will be almost flat, only curving up (if concave) or down (if convex) near the edges.

Contact angle and surface tension

The formation of menisci is commonly used in surface science to measure

contact angle The contact angle is the angle, conventionally measured through the liquid, where a liquid–vapor interface meets a solid surface. It quantifies the wettability of a solid surface by a liquid via the Young equation. A given system of solid, liq ...

s and surface tension. In a contact angle measurement, the shape of the menisci is measured with a balance or optically with a digital camera. In a surface tension measurement, the measurement probe has a contact angle of zero and the surface tension can be obtained by measuring the mass of the menisci. This is typically done with a Wilhelmy plate.

Measurement of volumes

When reading a depth scale on the side of an instrument filled with liquid, such as a water level device, 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 Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight and is measured by the angle or semi-angle of inclination between those two lines. Due to foreshortening, nearby objects ...

) 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 Capillary action (sometimes called capillarity, capillary motion, capillary rise, capillary effect, or wicking) is the process of a liquid flowing in a narrow space without the assistance of, or even in opposition to, any external forces li ...

, by which either 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. When a tube of a narrow bore, often called a capillary tube, is dipped into a liquid and the liquid

wets During the 1980s, members of the moderate wing of the British Conservative Party who opposed some of the more hard-line policies of Prime Minister Margaret Thatcher were often referred to by their opponents as "wets". Thatcher coined the usage in ...

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 2πrσ, where σ is the surface tension of the liquid.

References

{{Use dmy dates, date=April 2017

External links

Using menisci to measure surface tension
Fluid mechanics Greek words and phrases