Surface Tension of Water

Surface tension is the phenomenon that occurs when the surface of a liquid comes into contact with another phase, according to the definition (it can be a liquid as well). 

“The tension in a liquid’s surface film is caused by the bulk of the liquid’s attraction to the particles in the surface layer, which seeks to minimise surface area.”

The forces of attraction between particles within a liquid, determine surface tension. The effort or energy required to remove a unit area’s surface layer of molecules is roughly equivalent to the energy responsible for surface tension.

Surface tension is usually expressed in dynes/cm, which is the force necessary to break a 1 centimetre film.

What Causes Surface Tension in the First Place?

The liquid particles are drawn together by intermolecular forces such as the Van der Waals force. The particles are drawn toward the rest of the liquid along the surface. The following is a definition of surface tension:

The surface force F divided by the length L along which the force acts.

Surface tension(T) 

T=F/L

Where,

The force per unit length is F.

L is the length on which force acts. 

T is the liquid’s surface tension.

The Newton per Metre (N/m) is the SI unit for Surface Tension.

Surface Tension Examples

Small insects called water striders can walk on water because their weight is insufficient to penetrate the water’s surface. There are numerous examples of surface tension in nature, such as this.

The following are some examples:

• Walking insects on water

• Floating a needle on the water’s surface.

Water’s surface tension will bridge the pores in the tent fabric, rendering it waterproof.

In this case, there are two basic mechanisms at work.

Due to an inward push on the surface molecules, one causes the liquid to compress.

The second is a tangential force parallel to the liquid’s surface. This tangential force is referred to as surface tension. As a result, the liquid behaves as if it were surrounded by an elastic membrane that has been stretched.

However, because the tension in an elastic membrane is dictated by the degree of deformation, whereas surface tension is a characteristic of the liquid–air or liquid–vapour interface, this analogy should be utilised with caution.

Water has a higher surface tension (72.8 millinewtons (mN) per metre at 20 °C) than most other liquids due to the unusually high attraction of water molecules to each other through a web of hydrogen bonds. 

A tube’s capillarity

Capillarity is influenced by surface tension.

When a capillary tube is dipped vertically in a liquid that wets the tube’s walls, the liquid inside the tube rises.

 The liquid gets potential energy as a result of the increase. As a result, the question of where it gets this increase in potential energy emerges. The explanation, on the other hand, is straightforward.

When a capillary tube is submerged in a liquid, there are three surfaces of separation to consider:

i) an air-liquid surface 

(ii) an air-glass surface

(iii) a glass-liquid surface, each with its unique surface tension that differs from the others and is equal to its free surface energy per unit area.

As the plane liquid surface in the tube curvatures (becomes concave), the air-liquid surface grows, and as the liquid rises in the tube, the glass-liquid surface increases, while the air-glass surface decreases by the same amount.

As a result, the surface energy of the air-liquid and glass-liquid surfaces increases while the surface energy of the air-glass surface drops.

In other words, the surface energy of the air-glass surface is used to obtain the energy required to raise the liquid in the capillary tube.

A liquid that does not soak the tube’s walls, on the other hand, becomes depressed within it, falling below its level outside the tube. In this situation, the glass-liquid surface drops, while the air-glass surface grows by the same amount, resulting in a net increase in the system’s surface energy.

This energy comes from the depressing of the liquid inside the tube, which reduces the gravitational potential energy by the same amount.

The phenomenon known as surface tension is caused by the cohesive force that exists between liquid molecules. The molecules at the surface of a glass of water are not surrounded on all sides by other water molecules.

It is not true that a “skin” forms on the water’s surface in most cases. The cohesive force that exists between molecules in a liquid is shared by all molecules in the vicinity. Those on the surface have no neighbours above them, thus they exert a force that is stronger than the attractive forces exerted on their nearest neighbours on and below the surface. Surface tension can be defined as the quality of a liquid’s surface that permits it to resist an external force due to the cohesive nature of the water molecules.

Water molecules want to stick to one another. However, because there is air above, there are fewer water molecules to cling to at the surface, thus we say there are no water molecules. This usually leads to a stronger bond between the molecules that do come into contact with one another, as well as a layer of highly bonded water.

Surface tension holds this layer of the surface together, forming a significant barrier between the atmosphere and the other water. In fact, except for mercury, we may argue that only water has the highest surface tension of any liquid.

There is a molecule in a liquid-filled human body that will not experience a net force because the forces exerted by nearby molecules are cancelled out. However, because there is no attractive force acting from above, a molecule on the liquid’s surface will experience a net inward force. The molecules on the surface contract and resist being stretched or broken due to this inward force, also known as the net force.

When you add detergent to water, it changes its surface tension.

 When detergent is mixed with water, the surface tension is reduced. Surfactants are chemicals that aid in the reduction of water’s surface tension. The reduced surface tension area may be noticed when the surfactant is mixed with water in this overall occurrence. Detergents, soaps, emulsifiers, foaming agents, and dispersants are examples of surfactants. The term “surfactant” refers to a mixture of surface-active agents.

Conclusion- 

We may therefore claim that the surface is under tension, which is most likely where the term “surface tension” comes from. Small things can now “float” on the surface of a fluid due to the phenomena of surface tension, as long as the object does not break through and does not divide the top layer of water molecules. When an object is placed on the surface of a fluid, the surface under tension behaves like an elastic membrane.