Surface Tension

Objective

To determine the surface tension of a liquid by capillary rise method.

Theory

How do you define surface tension?

Surface tension is the property of a liquid, by virtue of which its free surface at rest behaves like an elastic skin or a stretched rubber membrane, with a tendency to contract so as to occupy minimum surface area. This property is caused by cohesion of molecules and is responsible for much of the behaviors of liquids.

The property of surface tension is revealed, for example, by the ability of some objects to float on the surface of water, even though they are denser than water.  Surface tension is also seen in the ability of some insects, such as water striders, and even reptiles like basilisk, to run on the water’s surface.

The Theory Behind Surface Tension

Surface tension has been well- explained by the molecular theory of matter. According to this theory, cohesive forces among liquid molecules are responsible for the phenomenon of surface tension. The molecules well inside the liquid are attracted equally in all directions by the other molecules. The molecules on the surface experience an inward pull.

So, a network is formed against the inward pull, in order to move a molecule to the liquid surface.  It results in a greater potential energy on surface molecules. In order to attain minimum potential energy and hence stable equilibrium, the free surface of the liquid tends to have the minimum surface area and thereby it behaves like a stretched membrane.

Surface tension is measured as the force acting normally per unit length on an imaginary line drawn on the free liquid surface at rest. It is represented by the symbol T (or S). It's S.I. The unit is Nm-1 and dimensional formula is M1L0T-2.

Capillarity in Liquids

When a capillary tube is dipped in a liquid, the liquid level either rises or falls in the capillary tube.  The phenomena of rise or fall of a liquid level in a capillary tube is called capillarity or capillary action.

How do we define the surface tension of a liquid through the capillary rise method?

When a liquid rises in a capillary tube, the weight of the column of the liquid of density ρ inside the tube is supported by the upward force of surface tension acting around the circumference of the points of contact.

Thus, surface tension;

$T=\frac{r(h+\frac{r}{3})\rho g}{2 cos\theta }$

Where,  h - height of the liquid column above the liquid meniscus
ρ - Density of the liquid
r - Inner radius of the capillary tube
θ - Angle of contact

Learning Outcomes

• Students understand the theory of the surface tension of liquids.
• Students correlate the property of surface tension with different natural phenomena.
• Students understand the concept of capillarity in liquids.
• They are able relate surface tension and capillarity.

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