# Surface tension

The word surface tension is made up of two words, the first one, tension, comes from the word “tensio” , which means “action and effect of being stretched” . The other word, superficial, comes from “superficialis” , which translates as “the equivalent of the surface” . The word tension then is the situation in which a body is under the influence of opposing forces , and that exert a certain attraction on it. Superficial , meanwhile, is everything that is related to the outer surface, the boundary or the top layer of something.

• Unit: N / m
• Formula:  Y = surface force / length on which it acts

## What is surface tension?

Surface tension is a phenomenon in which the surface of a liquid which is in contact with a gas , such as air, comes to act as a type of elastic sheet .

• Definition
• Properties
• Who discovered
• History
• Surface tension coefficient
• Methods for determining surface tension
• Applications
• Examples

## Definition

The concept of surface tension is widely used within the field of physics to be able to refer to the amount of energy that is needed to be able to increase the surface that a liquid has per unit area. This energy is necessary because liquids exert resistance when increasing the surface.

The molecules of a liquid are molecules that attract each other, it is for this reason that the liquid is “cohesive” . When there is a surface , the molecules that are located just below it, feel a series of forces to the sides, horizontally , and down , but not up, and this happens because no molecule is found on top of the surface. surface . The result of this situation is that the molecules on the surface are attracted to the interior of it. This layer of surface molecules acts as aTight elastic membrane and in this way, it is the surface tension that is responsible for closing a drop and is capable of holding it against gravity while it hangs from a dropper.

## Properties

Some of the properties that surface tension has are:

• The surface tension is> 0, since in order to increase the state of the liquid in contact, more molecules must be brought to the surface , reducing the energy of the system.
• The surface tension will depend on the nature of the two phases that are in contact, generally a liquid and a solid.
• It can be interpreted as a force per unit length and is measured in N · m − 1.
• The value of surface tension will depend on the magnitude with the forces intermolecular within the liquid.
• The greater the cohesion forces that a liquid has, the greater its tension.
• For a given liquid, the value of surface tension decreases with temperature , this because of increasing thermal agitation , which exceeds the intermolecular forces by a lower effective intensity .

## Who discovered

The scientist who discovered surface tension was Agnes Pockels , originally from Germany , who was also the pioneer in the field of monolayer.

## History

The history of surface tension tells us that when Agnes Pockels was washing dishes, she discovered the influence that impurities had on the surface tension of liquids . To measure surface tension, he developed what became known as the Pockels cuvette , a precursor to the Langmuir cuvette . In 1891, together with the physicist Lord Rayleigh , he managed to publish his first work known as ” surface tension ” in the prestigious journal “Nature” .

## Surface tension coefficient

It is important to know that the surface tension depends on the nature of the liquid and on the temperature. In the international system , the coefficient of surface tension is expressed in N / m.  For a given liquid, the stress coefficient will only depend on the nature of the liquid and its temperature . There is a formula to obtain the coefficient of surface tension which is the following:

Y = surface force / length on which it acts = dynes / cm = 10 -3 N / m

## Methods for determining surface tension

Some of the most common methods that exist to determine surface tension are:

• Capillary rise: it  is done when a liquid rises through a capillary tube wetting the walls of the tube, forming a concave meniscus on the liquid-air surface causing a rise of the liquid inside the capillary that stops at the moment when the pressures are equal.
• Double capillary method : consists of introducing a second capillary of another diameter to the first, so that the heights are different as they are inversely proportional to the radii of their respective capillaries.
• Ring lifting method : it is based on measuring the force necessary to separate a ring from the surface.
• Drop weight : measures the surface tension at a liquid-air interface or the liquid-liquid interfacial tension . It consists of measuring the volume of the drops of a liquid that slowly come off the tip of a narrow tube vertically. The weight of the is related to the force due to surface tension.

## Applications

Surface tension can be applied in:

• Mining
• farming
• Medicine and daily life

## Examples

Some examples that we can observe of surface tension are the following:

• Water Drops:  When using a water dropper, the water does not flow continuously, but rather in a series of drops . These droplets are caused by the surface tension of the water. The only reason why the drop of water is not completely spherical is due to the force of gravity pulling on it.
• Insects walking on water : we can observe various insects that can walk on water, such as the water strider . Its legs are shaped to distribute its weight , causing the surface of the liquid to depress , minimizing the potential energy to create a balance of forces to be able to move across the surface of the water without breaking the surface.
• Needle floating in water : even though the density of these objects is greater than water, the surface tension along the depression is sufficient to counteract the force of gravity pulling down on the metal object.