Introduction: Many of us would have seen the demonstration of a needle being supported on water surface without it being wetted. This is due to the surface tension of water.
All liquids exhibit a free surface known as meniscus when in contact with vapor or gas. Liquid molecules exhibit cohesive forces binding them with each other. The molecules below the surface are generally free to move within the liquid and they move at random. When they reach the surface they reach a dead end in the sense that no molecules are present in great numbers above the surface to attract or pull them out of the surface. So they stop and return back into the liquid. A thin layer of little atomic thickness at the surface formed by the cohesive bond between atoms slows down and sends back the molecules reaching the surface. This cohesive bond exhibits a tensile strength for the surface layer and this is known as surface tension. Force is found necessary to stretch the surface. Surface tension may also be defined as the work in Nm/m2 or N/m required creating unit surface of the liquid. The work is actually required for pulling up the molecules with lower energy from below, to form the surface. Another definition for surface tension is the force required to keep unit length of the surface film in equilibrium (N/m). The formation of bubbles, droplets and free jets are due to the surface tension of the liquid.
Surface Tension Effect on Solid-Liquid Interface: In liquids cohesive forces between molecules lead to surface tension. The formation of dropletsis a direct effect of this phenomenon. So also the formation of a free jet, when liquid flows outof an orifice or opening like a tap. The pressure inside the droplets or jet is higher due to thesurface tension.
Liquids also exhibit adhesive forces when they come in contact with other solid or liquid surfaces. At the interface this leads to the liquid surface being moved up or down forming a curved surface. When the adhesive forces are higher the contact surface is lifted up forming a concave surface. Oils, water etc. exhibit such behavior. These are said to be surface wetting. When the adhesive forces are lower, the contact surface is lowered at the interface and a convex surface results as in the case of mercury. Such liquids are called no wetting. These are shown in Fig.
The angle of contact “β” defines the concavity or convexity of the liquid surface. It can be shown that if the surface tension at the solid liquid interface (due to adhesive forces) is σ s1and if the surface tension in the liquid (due to cohesive forces) is σ 11 then Cosβ = [(2 σ s1 / σ 11) – 1]
At the surface this contact angle will be maintained due to molecular equilibrium. The result of this phenomenon is capillary action at the solid liquid interface. The curved surface creates a pressure differential across the free surface and causes the liquid level to be raised or lowered until static equilibrium is reached.