The capillarity It is a property of liquids that allows them to move through tubular holes or porous surfaces even against the force of 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..
The classic example of capillarity is illustrated by comparing 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).
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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 this.
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.
If the adhesion force of the liquid to the capillary wall prevails over the intermolecular cohesion force, then the angle is θ < 90º; el líquido moja la pared capilar y el agua asciende por el capilar, observándose el fenómeno conocido como capilaridad.
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.
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.
This can be written mathematically as follows:
2 π rϒcosθ = ρgπrtwoh
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; what 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..
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, is positively charged..
The molecules within 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; that is, 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..
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 rise through a capillary or pore of a material.
In this way, it can be 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..
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 column of liquid reaches by capillarity. This can be seen directly in the image where water is compared with mercury.
In a glass tube with a radius of 0.05 mm radius, the column of water 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 103 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. By sipping the liquid, a pressure difference is created that causes the liquid to rise to the mouth.
The maximum height of the column reached by capillarity is theoretical, since the radius of the capillaries cannot be reduced beyond a certain limit..
This establishes that the flow of a real liquid is given by the following expression:
Q = (πr4/ 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 flow of the liquid through this 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..
The greater the value of the 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).
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
-Hexane: 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..
Capillarity is an important mechanism for the ascent of the sap through the xylem of the plants, but it is insufficient by itself to get the sap to the leaves of the trees..
Perspiration or evaporation is an important mechanism in the ascent of sap through the xylem of plants. The leaves lose water through evaporation, generating a decrease in the amount of water molecules, which causes an attraction of the water molecules present in the capillary tubes (xylem).
Water molecules do not act independently of each other, but rather interact by Van der Waals forces, which causes them to rise linked 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 capillary tubes of the plant..
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