12 junio, 2024

Capillarity: what it is, definition, characteristics, examples

What is capillarity?

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; having these two 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.

water capillarity

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.

in the plants

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

Transpiration 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 capillaries (xylem).

The water molecules do not act independently of each other, but rather interact by Van der Waals forces, which causes them to ascend linked to each other through the capillaries of the 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.

Examples of capillarity

surface tension in insects

Some insects can walk through the water, this is because the weight of the insect is compensated by the resistance of the water when it is deformed.

glass capillary tube

If we introduce a glass tube into a container with water, the water level will rise through the tube.

If we introduce a tube with a larger diameter, the water will remain at a lower level. The surface of the liquid will be left with a concave shape called a meniscus.

mercury capillary tube

If we introduce a capillary tube into the mercury, the level of this will rise through the tube, but to a lesser extent than the water.

In addition, its surface will present a convex inverted meniscus curvature.

surface tension in sheets

As with insects, the surface tension that is created makes the leaf or some flowers float on the water without sinking, despite the fact that their weight is greater than that of the water.

plant feeding

Through the phenomenon of capillarity, plants extract water from the soil and transport it to their leaves.

Nutrients ascend through the capillaries of the plants until they reach all parts of the plant.

Ascent of the sap in the trees

The sap rises along the tree thanks to the capillarity process. The ascent is due to the fact that in the leaves an evaporation of the liquid is generated that causes a negative pressure to be produced in the xylem, allowing the sap to ascend by the action of capillarity. It can reach a height of 3 km of ascent.

With a paper napkin

If we place a paper napkin that touches the surface of the water and that comes out of the container, through the process of capillarity the water can move through the napkin, coming out of the container.

water transfer

Just as we can make the liquid come out of the container, as in the previous example, if we connect two containers through an absorbent material, such as a paper napkin, the water from one container will flow into the other.

Detergents and soaps on water

There are some detergents and soaps that have chemical compounds that cause them to settle on top of the water and the surface tension prevents them from sinking.

Rise of water on the ground

The capillarity of some soils causes the water to rise through the ground until it exceeds the water table, despite the fact that it is a movement contrary to gravity.

Moisture on the walls

The capillarity that some walls present causes the water to seep through them and enter the houses.

This causes houses to have a higher concentration of water molecules in the air, which is known as humidity.

dipping cookies

When we dip the biscuits in the milk for breakfast, the action of capillarity causes the milk to enter the biscuit, thus increasing its liquid capacity.

As the milk moves up the biscuit, it undoes the cohesive forces of the solid and therefore the biscuit breaks.

butter candles

If we take a piece of butter and stick a wick in it and light it with a match, it will burn.

However, butter that is in contact with oxygen in the air does not burn. This happens because the capillarity of the candle allows the melted butter to rise up the wick and function as combustion fuel.

Sugar cubes

The capillarity of the sugar cubes means that if we put them in contact with a liquid, such as water, the cubes absorb it in such a way that they retain the liquid inside them.

If the liquid is in a higher concentration than the sugar cube, it can cause the cohesive forces of the sugar cube to break.

capillarity with flowers

To observe the phenomenon of capillarity that occurs in plants, we can submerge the stem of a flower in a container with a dye.

Through the flower’s capillarity, the water will rise to its petals and change their color.

Land capillarity

For water to rise to the surface of a ground, the ground has to be porous. The more porous the ground, the lower the adhesion forces of the water, so the water will filter more.

For example, soils with sand and gravel, being more porous, drain water quickly, while in clayey soils, the water does not drain and forms puddles, since the pores are much smaller.

ink for fountain pens

Capillarity is responsible for transporting the ink from the tank to the tip of the fountain pen.

Tears

Capillarity is essential for the drainage of tear fluid, as it causes this fluid to go up through the tear ducts and out.

References

Surface phenomena: surface tension and capillarity. [PDF]. Retrieved from: ugr.es
Risvhan T. (sf). Capillarity in plants. Retrieved from: academia.edu

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