MENISCUS (CHEMISTRY): WHAT IT CONSISTS OF AND TYPES - CHEMISTRY - 2025

The meniscus is the curvature of the surface of a liquid. Also, it is the free surface of a liquid at the liquid-air interface. Liquids are characterized by having a fixed volume, being little compressible.

However, the shape of liquids varies by adopting the shape of the container that contains them. This characteristic is due to the random movement of the molecules that form them.

Source: Jleedev via Wikipedia.

Liquids have the ability to flow, high density, and diffuse rapidly into other liquids with which they are miscible. They occupy the lowest area of ​​the container by gravity, leaving a not totally flat free surface at the top. In some circumstances they can take special forms such as drops, bubbles and bubbles.

The properties of liquids such as melting point, vapor pressure, viscosity and heat of vaporization depend on the intensity of the intermolecular forces that give the liquids cohesion.

However, liquids also interact with the container through adhesion forces. The meniscus then arises from these physical phenomena: the difference between the forces of cohesion between the particles of the liquid, and those of adhesion that allows them to wet the walls.

What is the meniscus?

As just explained, the meniscus is the result of various physical phenomena, among which the surface tension of the liquid can also be mentioned.

Cohesion forces

Cohesion forces is the physical term that explains intermolecular interactions within the liquid. In the case of water, the cohesion forces are due to the dipole-dipole interaction and hydrogen bonds.

The water molecule is bipolar in nature. This is due to the fact that the oxygen in the molecule is electronegative because it has a greater greed for electrons than hydrogens, which determines that the oxygen is negatively charged and the hydrogens are positively charged.

There is an electrostatic attraction between the negative charge of one water molecule, located on oxygen, and the positive charge of another water molecule, located on hydrogens.

This interaction is what is known as the dipole-dipole interaction or force, which contributes to the cohesion of the liquid.

Adhesion forces

On the other hand, water molecules can interact with glass walls, by partially charging the hydrogen atoms of the water molecules that bind strongly to the oxygen atoms on the surface of the glass.

This constitutes the adhesion force between the liquid and the rigid wall; colloquially it is said that the liquid wets the wall.

When a silicone solution is placed on the surface of the glass, the water does not completely impregnate the glass, but droplets form on the glass that are easily removed. Thus, it is indicated that with this treatment the adhesion force between water and glass is reduced.

A very similar case occurs when the hands are oily, and when washed in water, very defined drops can be seen on the skin instead of a damp skin.

Types of menisci

There are two types of menisci: the concave and the convex. In the image, the concave is A, and the convex is B. The dotted lines indicate the correct flush when reading a volume measurement.

Concave

Source: Cutler via Wikipedia.

The concave meniscus is characterized in that the contact angle θ formed by the glass wall with a line tangent to the meniscus and, which is inserted into the liquid, has a value less than 90º. If a quantity of the liquid is placed on the glass, it tends to spread over the surface of the glass.

The presence of a concave meniscus shows that the cohesion forces within the liquid are less than the liquid-glass wall adhesion force.

Therefore, the liquid bathes or wets the glass wall, retaining a quantity of liquid and giving the meniscus a concave shape. Water is an example of a fluid that forms concave menisci.

Convex

In the case of the convex meniscus, the contact angle θ has a value greater than 90º. Mercury is an example of a liquid that forms convex menisci. When a drop of mercury is placed on a glass surface, the contact angle θ has a value of 140º.

The observation of a convex meniscus indicates that the cohesion forces of the liquid are greater than the adhesion force between the liquid and the glass wall. The liquid is said to not wet the glass.

The surface forces of cohesion (liquid-liquid) and adhesion (liquid-solid) are responsible for many phenomena of biological interest; such is the case of surface tension and capillarity.

Surface tension

Surface tension is a net force of attraction that is exerted on the molecules of the liquid that is on the surface and that tends to introduce them into the liquid.

Therefore, surface tension tends to bind the liquid and give them more concave menisci; Or put another way: this force tends to remove the surface of the liquid from the glass wall.

The surface tension tends to decrease as the temperature increases, as for example: the surface tension of water is equal to 0.076 N / m at 0ºC and 0.059 N / m at 100ºC.

Meanwhile, the surface tension of mercury at 20ºC is 0.465 N / m. This would explain why mercury forms convex menisci.

Capillarity

If the contact angle θ is less than 90º, and the liquid wets the glass wall, the liquid inside the glass capillaries can rise to an equilibrium condition.

The weight of the liquid column is offset by the vertical component of the cohesion force due to surface tension. The adhesion force does not intervene because they are perpendicular to the surface of the tube.

This law does not explain how water can rise from the roots to the leaves through the vessels of the xylem.

Actually there are other factors that intervene in this regard, for example: as the water evaporates in the leaves, it allows the water molecules in the upper part of the capillaries to be sucked up.

This allows other molecules at the bottom of the capillaries to rise to take the place of the evaporated water molecules.

References

1. Ganong, WF (2002). Medical Physiology. 2002. 19th Edition. Editorial Manual Moderno.
2. Whitten, Davis, Peck & Stanley. (2008). Chemistry. (8th ed.). CENGAGE Learning.
3. Helmenstine, Anne Marie, Ph.D. (August 4, 2018). How to Read a Meniscus in Chemistry. Recovered from: thoughtco.com
4. Wikipedia. (2018). Meniscus (liquid). Recovered from: en.wikipedia.org
5. Friedl S. (2018). What is a Meniscus? Study. Recovered from: study.com
6. Surface Tension. Recovered from: chem.purdue.edu