Surface Tension: Surface tension can be easily observed in our daily life activities. Situations like needles floating over the water surface, small insects, like water striders, walking over the surface of the water, mercury not wetting the glass, but water sticking to it, oil rising in a cotton wick despite gravity, oil and water not mixing, sap and water rise to the top of the leaves of the tree, hair of a paintbrush not clinging together when dry, and even when dipped in water but form a fine tip when taken out of it.

What is the reason behind these bizarre yet common observations? The phenomenon behind all these is SURFACE TENSION.

All these and many more such experiences are related to the surfaces of liquids. As liquids have no definite shape but have a substantial volume, they acquire a surface when poured into a container.

This fundamental phenomenon due to which a liquid surface at rest tends to acquire the minimum possible area is known as surface tension. It is concerned with only liquid as gases do not have surfaces.

What is Surface Tension?

The attraction forces between particles in the surface layer and the particles in the bulk of the liquid, which tends to minimize its surface area, causes surface tension. Due to the surface tension, the surface of a liquid allows it to resist an external force due to the cohesive nature of its molecules.

Cohesive force is the force of attraction acting between the molecules of the same substance. These forces between molecules in a liquid are shared with all neighbouring molecules. Those on the surface have no adjacent molecules above.

Hence, these molecules have an unbalanced force acting on them compared to the molecules present in bulk.  The unbalanced forces acting on the surface molecules tend to pull them inwards, which is why a liquid’s surface behaves like a stretched membrane.

Thus, the surface tension of a liquid mainly acts to reduce the surface area of a liquid. Raindrops and soap bubbles are perfectly spherical because, for the given volume, the surface area of the sphere is the least. Hence, the surface of a liquid tends to contract to a minimum possible area. Thus, we can say that the directed contracting force which attracts the molecules at the surface of a liquid towards the interior of the liquid is surface tension.

What Causes Surface Tension?

Surface tension has been well- explained by the molecular theory of matter, and Laplace explained this phenomenon based on intermolecular forces. For example, we know that if the distance between two molecules is less than the molecular range “\(d\)”\((≈ 10^{-9}\,\rm{meter})\), they attract each other. Still, if the distance is more than this, they attract each other, but the attraction force decreases considerably with distance.

Therefore, if we draw a sphere of radius \(d\) with a molecule at its centre, only those enclosed within this sphere can attract, or be attracted by, the molecule placed at the sphere’s centre. This is called the ‘sphere of molecular activity.

To understand the tension acting in a liquid’s surface, let us consider four liquid molecules like \(A, B, C\) and \(D\) along with their spheres of molecular activity. The molecule \(D\) is well inside the liquid, and so it is attracted equally in all directions. Hence the resultant force acting on it is zero. The sphere of molecule \(C\) is just below the liquid surface, and the resultant force on it is also zero.

Molecule \(B\), which is a little below the liquid surface, has its sphere of molecular activity partly outside the liquid. The number of liquid molecules in the upper half (attracting downward) is less than upward attraction. Hence the molecule \(B\) experience a resultant downward force. Molecule \(A\) is on the surface of the liquid so that its sphere of molecular activity is half outside the liquid and half inside. As such, it experiences a maximum downward force. Thus all the molecules situated between the surface and a plane \(XY\), distance \(d\) below the surface, experience a downward cohesive force.

When the surface area of liquid is increased, molecules from the interior of the liquid rise to the surface; as these molecules reach near the surface, work is done against the (downward) cohesive force. This work is stored in the molecule in the form of potential energy.

Thus the potential energy of the molecules lying on the surface is greater than that of the molecules in the interior of the liquid. But a system is in stable equilibrium when its potential energy is minimum. Hence, to have minimum potential energy, the liquid surface tends to have a minimum number of molecules. In other words, the surface tends to be in contact with the minimum possible area. This tendency is exhibited as surface tension.

How to Calculate Surface Tension? (Surface Tension Formula)

Surface tension is a force per unit length acting in the plane of the interface between the plane of the liquid and any other substance. Thus, mathematically:

\(T = \frac{F}{L}\)

Where, \(T\) is the surface tension, \(F\) is the force, and \(L\) is the length of the liquid surface. The value of the surface tension of a liquid depends on the liquid’s temperature and the medium on the other side of the surface. It decreases with temperature rise and becomes zero at the critical temperature.

Surface Tension Units

The surface tension is defined as the force per unit length in the plane of the liquid surface, acting at right angles on either side of an imaginary line drawn in that surface. The SI unit of force is newton \((N)\), and the SI unit of length is meter \((m)\). Therefore, the SI unit of surface tension becomes:

\({\text{Surface Tension}} = \frac{{{\text{Force}}}}{{{\text{Length}}}} = \frac{{{\text{newton}}}}{{{\text{meter}}}} = \frac{{\left[{{\text{ML}}{{\text{T}}^{ – 2}}} \right]}}{{[{\text{L}}]}}\)

Thus, surface tension is measured in terms of newton per meter, and the dimension of surface tension is \({\text{ML}}{\text{T}}^{ – 2}\)

Relation Between Surface Energy and Surface Tension

When the surface area of a liquid is increased, the molecules from the interior rise to the surface. This requires work against the force of attraction of the molecules just below the surface. This work is stored in the form of potential energy in the newly formed surface. Thus the molecules on the surface have some additional energy due to their position. This extra energy per unit area of the surface is called surface energy.
Consider a liquid film trapped in a metal frame, with a moveable bar as shown in the figure:

Now, as we move the bar by a small distance \(d\), the area of the surface increases; this will lead to a rise in energy stored within the system; thus, work has been done against an internal force. If \(F\) is the internal force, then work done by the applied force is

CLEAR YOUR CONCEPTUAL DOUBTS ON SURFACE TENSION

\(W = F.d = Fd\).

According to the conservation of energy, this work will be equal to the additional energy in the film. Let \(S\) be the surface energy per unit area, and the extra area is, \(A = 2dl\) since a film has two sides and the liquid in between, so there are two surfaces. Thus, if \(S\) is the magnitude of surface tension in the film, then the extra energy is

\(S.(2dl) = W = U\)

\(U = S × A\)

\(S = \frac{U}{A}\)

Hence, surface tension is also equal to the surface energy per unit area.

Surface Tension Example

1. Insect walking on water:  Small and light insects can be seen walking on water because their weight is not enough to break through the surface. For example, a water strider.
2. Floating paper clip: A carefully placed small clip can be made to float on the water’s surface even though it is several times as dense as water. If the surface is agitated to break up the surface tension, then the clip will quickly sink.
   
3. “Touch me not”, said the tent: The tents commonly used by hikers are waterproof. But if there is any physical contact between the hand and the tent material, the surface tension between the tent fibres and water breaks and the rain drip through.
4. Using disinfectants: Disinfectants spread out on the cell walls of bacteria and disrupt them. For this, solutions of low surface tension are necessary.
5. Soaps and detergents: commonly used for washing dishes or clothes, are also based on surface tension. When these are added to water, they lower the surface tension of the water so that it more readily soaks into pores and soiled areas.
6. Washing with hot water:  The surface tension of hot water is lower than that of cold water, and it is a better wetting agent. Thus, we often use hot water for especially during winters.
7. Why it is required to blow to form soap bubbles: When you blow air into soapy water, the air stretches the surface. The surface tension in this layer captures the air and pulls the soapy water into the shape of a sphere.
8. The shape of droplets: Liquid droplets acquire their shape because of the surface tension. Water drops come together due to their forces of attraction, and because of the surface tension, they take up the spherical shape to have the least possible area.

FAQs on Surface Tension

Following are the frequently asked questions on surface tension:

Q.1. What is definition of surface tension in chemistry?
Ans: Surface tension definition chemistry: surface tension in chemistry is measured as the energy required to increase the surface area by a unit of area. The surface tension of liquid results from an imbalance of attractive intermolecular forces

Q.2. What is surface tension definition?
Ans: Surface tension is the property of any liquid by which it tries to minimize its surface area.

Q.3. What is the surface tension of water?
Ans: The surface tension of water is about \(72\,\rm{mN/m}\) at room temperature, the second-highest surface tension for liquid.

Q.4. What is surface tension? Explain with example.
Ans: It is the tension of the surface film caused by the cohesion forces between the liquid molecules, due to which the liquid surface tries to attain the minimum possible area. It is due to the surface tension that liquids droplets take up a spherical shape.

Q.5. Does salt water increase surface tension?
Ans: The presence of soluble impurities increases the surface tension of a liquid. Thus, the surface tension of water increases when salt is added to it.

Q.6. What increases the surface tension of water?
Ans: As the temperature decreases, surface tension increases. Thus, the surface tension of water can be increased by cooling it; that is why we use hot water for washing. Also, the presence of soluble impurities like \(\rm{NaCl}\) can increase the surface tension of water.

Q.7. Which liquid has the highest value of surface tension?
Ans: Mercury is a liquid metal with a surface tension of almost \(500\,\rm{mN/m}\).

Q.8. How can we measure surface tension?
Ans: We can measure the surface tension by using these methods:
1. Spinning drop method
2. Stalagmometric method
3. Pendant drop method
4. Capillary rise method
5. Bubble pressure method
6. Sessile drop method.

Q.9. What is the SI unit of Surface tension?
Ans: The surface tension of a liquid is measured as the force acting per length on an imaginary line drawn tangentially on the surface of the liquid. Its SI unit is newton per meter \((\rm{Nm}^{-1})\).