CAPILLARITY: CHARACTERISTICS AND EXAMPLE IN WATER - CHEMISTRY - 2025

The capillarity is a property of liquids which allows them to move tubular holes or porous surfaces even against gravity. For this, there must be a balance and coordination of two forces related to the molecules of the liquid: cohesion and adhesion; these two have a physical reflection called surface tension.

The liquid needs to be able to wet the internal walls of the tube or the pores of the material through which it travels. This occurs when the adhesion force (liquid-capillary tube wall) is greater than the intermolecular cohesion force. Consequently, the molecules of the liquid create stronger interactions with the atoms of the material (glass, paper, etc.) than with each other.

Source: MesserWoland via Wikipedia

The classic example of capillarity is illustrated in the comparison of this property for two very different liquids: water and mercury.

In the image above it can be seen that the water does rise up the walls of the tube, which means that it has greater adhesion forces; while with mercury the opposite occurs, because its cohesion forces, of metallic bond, prevent it from wetting the glass.

For this reason, water forms a concave meniscus, and mercury a convex (dome-shaped) meniscus. It should also be noted that the smaller the radius of the tube or the section through which the liquid travels, the greater the height or distance traveled (compare the heights of the water columns for both tubes).

Capillarity characteristics

-Surface of the liquid

The surface of the liquid, to say water, in a capillary is concave; that is, the meniscus is concave. This situation occurs because the resultant of the forces exerted on the water molecules near the wall of the tube is directed towards it.

In every meniscus there is a contact angle (θ), which is the angle formed by the wall of the capillary tube with a line tangent to the surface of the liquid at the point of contact.

Adhesion and cohesion forces

If the adhesion force of the liquid to the capillary wall prevails over the intermolecular cohesion force, then the angle is θ <90º; the liquid wets the capillary wall and the water rises through the capillary, observing the phenomenon known as capillarity.

When a drop of water is placed on the surface of a clean glass, the water spreads over the glass, so θ = 0 and cos θ = 1.

If the intermolecular cohesion force prevails over the liquid-capillary wall adhesion force, for example in mercury, the meniscus will be convex and the angle θ will have a value> 90º; mercury does not wet the capillary wall and therefore runs down its internal wall.

When a drop of mercury is placed on the surface of a clean glass, the drop maintains its shape and the angle θ = 140º.

-Height

The water rises through the capillary tube until reaching a height (h), in which the weight of the water column compensates the vertical component of the intermolecular cohesion force.

As more water rises, there will come a point where gravity will stop its ascent, even with surface tension working in your favor.

When this happens, the molecules cannot continue to “climb” the inner walls, and all physical forces equalize. On the one hand you have the forces that promote the rise of the water, and on the other your own weight pushing it down.

Jurin's Law

This can be written mathematically as follows:

2 π rϒcosθ = ρgπr ² h

Where the left side of the equation depends on the surface tension, whose magnitude is also related to the cohesion or intermolecular forces; Cosθ represents the contact angle, and r the radius of the hole through which the liquid rises.

And on the right side of the equation we have the height h, the force of gravity g, and the density of the liquid; which would be the water.

Solving then for h we have

h = (2ϒcosθ / ρgr)

This formulation is known as Jurin's Law, which defines the height reached by the column of liquid, in the capillary tube, when the weight of the column of liquid is balanced with the force of ascension by capillary action.

-Surface tension

Water is a dipole molecule, due to the electronegativity of the oxygen atom and its molecular geometry. This causes the portion of the water molecule where the oxygen is located to become negatively charged, while the portion of the water molecule, containing the 2 hydrogen atoms, becomes positively charged.

The molecules in the liquid interact thanks to this through multiple hydrogen bonds, holding them together. However, the water molecules that are in the water: air interface (surface), are subjected to a net attraction by the molecules of the sinus of the liquid, not compensated by the weak attraction with the air molecules.

Therefore, the water molecules at the interface are subjected to an attractive force that tends to remove water molecules from the interface; in other words, the hydrogen bonds formed with the molecules at the bottom drag those that are on the surface. Thus, surface tension seeks to reduce the surface of the water: air interface.

Relationship with h

If we look at Jurin's law equation, we will find that h is directly proportional to ϒ; therefore, the higher the surface tension of the liquid, the greater the height that can be raised by a capillary or pore of a material.

In this way, it is expected that for two liquids, A and B, with different surface tensions, the one with greater surface tension will rise to a higher height.

It can be concluded with respect to this point that a high surface tension is the most important characteristic that defines the capillary property of a liquid.

-Radius of the capillary or pore through which the liquid rises

The observation of Jurin's Law indicates that the height that a liquid reaches in a capillary or pore is inversely proportional to the radius of the same.

Therefore, the smaller the radius, the greater the height that the liquid column reaches by capillary action. This can be seen directly in the image where water is compared to mercury.

In a glass tube with a radius of 0.05 mm radius, the water column per capillarity will reach a height of 30 cm. In capillary tubes with a radius of 1 µm with a suction pressure of 1.5 x 10 ³ hPa (which is equal to 1.5 atm) corresponds to a calculation of the height of the water column of 14 to 15 m.

This is very similar to what happens with those straws that turn on themselves several times. Sipping the liquid creates a pressure difference that causes the liquid to rise up to the mouth.

The maximum height value of the column reached by capillarity is theoretical, since the radius of the capillaries cannot be reduced beyond a certain limit.

Poiseuille's Law

This establishes that the flow of a real liquid is given by the following expression:

Q = (πr ⁴ / 8ηl) ΔP

Where Q is the flow of the liquid, η is its viscosity, l is the length of the tube, and ΔP is the pressure difference.

As the radius of a capillary decreases, the height of the liquid column reached by capillarity should increase indefinitely. However, Poiseuille points out that as the radius decreases, the fluid flow through that capillary also decreases.

Also, viscosity, which is a measure of resistance to the flow of a real liquid, would further decrease the flow of the liquid.

-Contact angle (θ)

The greater the value of cosθ, the greater the height of the water column per capillarity, as indicated by Jurin's Law.

If θ is small and approaches zero (0), the cosθ is = 1, so the value h will be maximum. On the contrary, if θ is equal to 90º, the cosθ = 0 and the value of h = 0.

When the value of θ is greater than 90º, which is the case of the convex meniscus, the liquid does not rise by capillarity and its tendency is to descend (as occurs with mercury).

Capillarity of water

Water has a surface tension value of 72.75 N / m, relatively high compared to the values ​​for surface tension of the following liquids:

-Acetone: 22.75 N / m

-Ethyl alcohol: 22.75 N / m

-Hexan: 18.43 N / m

-Methanol: 22.61 N / m.

Therefore, water has an exceptional surface tension, which favors the development of the capillarity phenomenon so necessary for the absorption of water and nutrients by plants.

In plants

Source: Pixabay

Capillarity is an important mechanism for the ascent of sap through the xylem of plants, but it is insufficient by itself to get the sap to the leaves of trees.

Transpiration or evaporation is an important mechanism in the ascent of sap through the xylem of plants. The leaves lose water through its evaporation, generating a decrease in the amount of water molecules, which causes an attraction of the water molecules present in the capillaries (xylem).

Water molecules do not act independently of each other, but rather interact by Van der Waals forces, which causes them to ascend bound together through the capillaries of plants towards the leaves.

In addition to these mechanisms, it should be noted that plants absorb water from the soil by osmosis and that a positive pressure generated in the root, drives the start of the rise of water through the capillaries of the plant.

References

1. García Franco A. (2010). Superficial phenomena. Recovered from: sc.ehu.es
2. Surface phenomena: surface tension and capillarity.. Recovered from: ugr.es
3. Wikipedia. (2018). Capillarity. Recovered from: es.wikipedia.org
4. Risvhan T. (nd) Capillarity in plants. Recovered from: academia.edu
5. Helmenstine, Anne Marie, Ph.D. (December 22, 2018). Capillary Action: Definition and Examples. Recovered from: thoughtco.com
6. Ellen Ellis M. (2018). Capillary Action of Water: Definition & Examples. Study. Recovered from: study.com
7. ScienceStruck Staff. (July 16, 2017). Examples That Explain the Concept and Meaning of Capillary Action. Recovered from: sciencestruck.com