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Surface tension can be defined as the tendency of a liquid surface at rest to shrink to its least surface area possible when external forces act on it.
Let’s assume there is a water bowl with water at rest. When you drop a little bit of ink on its surface, you will witness that the colour slowly starts to spread. Similarly, if you slowly drop material with more density than water, like a blade or clip, it will float without even being partially submerged. These examples occur due to the physical phenomenon that can be termed surface tension. It is an inherent property of water due to the behaviour of its surface molecules.
What is the unit of surface tension?
The surface tension of a liquid is generally denoted by γ. Surface tension is measured using the formula that defines it as the force applied per unit length. Hence, the SI unit of surface tension is Newton per metre (N/m).
γ = F/2l = W/ΔA
F = Force acting on the water molecules
l = Length
W = Work stored as potential energy
ΔA = Change in surface area
As a result, surface tension can also have an SI unit of Joules per square metre (J/m²).
What causes surface tension?
When water or a liquid is at rest, the surface molecules experience two different forces. These are:
Van Der Waals forces act on 80% of the molecules on the water surface due to the nearby water molecules surrounding them.
The remaining 20% of the surface water molecules are exposed to the atmosphere and experience a downward force due to atmospheric pressure.
All the forces acting in the opposite direction nullify each other. However, two force components – the downward force experienced due to atmospheric pressure and the upward force due to the Van der Waals bond balance each other. Hence, the water molecules do not move from their position and behave like a stretched membrane.
This is what causes surface tension in water at rest. As a result, the water molecules tend to stay close, behaving like a stretched membrane. When a material with a higher density is kept on this membrane, it floats rather than becoming partially submerged.
Forces defining surface tension
Even though the intermolecular forces are the main causes of surface tension, they can be divided into two parts. These forces are based on the interaction between liquid molecules and other molecules coming into contact.
Cohesive forces
Water molecules are made of two hydrogen atoms and one oxygen atom. Since it is a covalent compound, the hydrogen and oxygen atoms are held together by weak covalent bonds. The hydrogen atoms exhibit a partial positive charge, while the oxygen atom has a divalent partial negative charge. Let’s consider three water molecules placed linearly along a horizontal line.
The partially positive hydrogen atom on one water molecule is attracted by the partially negative oxygen atom of the adjacent water molecule. This creates a strong intermolecular force of attraction, also known as the cohesive forces. They exist between molecules of the same type like water, mercury, benzene, etc.
Adhesive forces
The water molecules also experience another type of force with other molecules. This is called the adhesive force. For example, when the water molecules are exposed to the atmosphere, the surface molecules directly contact both the positive and negative ions in the air. This gives rise to the adhesive forces between two different types of molecules.
Examples to understand surface tension
Formation of liquid drops
One of the ways to understand surface tension is by understanding how liquid drops behave. Let us consider mercury and water to explain how this physical phenomenon occurs. When you see water drops on a surface, you can see that they don’t have a perfectly spherical shape, and if you add more water, it will burst open and form a rivulet. Even though the cohesive forces between the water molecules are high, the adhesive force between water and surface is more, so the pressure inside the water drop is high, and it tends to burst open.
On the other hand, if we consider the mercury drops, they will have a perfect spherical shape. This is because the mercury molecules are strongly held together with the cohesive forces compared to the force of attraction between the mercury molecules and the surface.
Liquid kept in a glass
When you keep water in a glass, you will see a downward bulge or concave meniscus forming at the surface. This is because the water molecules close to the glass exhibit stronger adhesive forces than the cohesive forces shared with the surrounding water molecules. Therefore, the sides tend to rise upwards, creating a depression at the centre.
Similarly, if you keep mercury in the glass, you can see the meniscus taking a convex shape. This is because the cohesion forces between the mercury molecules are more in magnitude than the adhesive forces between the mercury and glass molecules.
Conclusion
Surface tension can also be defined as the work done to increase the surface area of the liquid surface. At rest, the surface molecules experience two forces: cohesive forces between the liquid molecules and adhesive forces between the air and water molecules. The surface tension is an interesting property of liquids which are important in understanding many phenomena in real life.
