Surface Tension

                                                        Surface tension                      
Surface tension can be defined as the force acting per unit length perpendicular on an imaginary line drawn on the liquid surface, tending to pull the surface apart along the line.

     Surface tension = F / L

It is a property of the surface of a liquid that allows it to resist an external force.It is an attractive property of the surface of a liquid.
For example floating of some object on the surface of water, even though they are denser than water, is due to surface tension of water.
The cohesive forces among the liquid molecules are responsible for this phenomenon of surface tension. In the bulk of the liquid, each molecule is pulled equally in every direction by neighboring liquid molecules, resulting in a net force of zero. The molecules at the surface do not have other molecules on all sides of them and therefore are pulled inwards. This creates some internal pressure and forces liquid surfaces to contract to the minimal area.

In figure molecules present in the lower part experience balanced forces as compared to the surface molecules. There is a inward force on the surface water molecules, this force causes surface tension.
Surface tension is also responsible for the shape of liquid droplets. Droplets of water tend to be pulled into a spherical shape by the cohesive forces of the surface layer. In the absence of other forces, including gravity, drops of virtually all liquids would be perfectly spherical because the sphere has the smallest surface area for its volume and this is due to surface tension.
This can also be understand in term of energy as, a molecule in contact with a neighbor is in a lower state of energy than if it were alone. The interior molecules have as many neighbors as they can possibly have, but the boundary molecules are missing neighbors (compared to interior molecules) and therefore have a higher energy. For the liquid to minimize its energy state, the number of higher energy boundary molecules must be minimized. The minimized quantity of boundary molecules results in a minimized surface area.
Surface tension, is not a property of the liquid alone, but a property of the liquid's interface with another medium.For example The surface of any liquid is an interface between that liquid and some other medium. If a liquid is in a container, then besides the liquid/air interface at its top surface, there is also an interface between the liquid and the walls of the container. The surface tension between the liquid and air is usually different (greater than) its surface tension with the walls of a container
Different liquids have different values of surface tension due to different nature of intermolecular forces present in them.
For example Pure water has high surface tension due to strong Hydrogen Unit & dimension:
Its unit is N m-1 but since a Force times a length is Energy, the surface tension is also an energy per unit area, J/m2 . and dimensional formula is MT-2

Applications of Surface Tension:

v  Surface tension of soap solution is less, it can spread over large areas and wash clothes more effectively, since the dirt particles stick to the soap molecules.

v  In soldering, addition of flux reduces the surface tension of molten tin. Hence, it spreads.

v  Antiseptics like dettol have low surface tension, so that they spread faster.

v  Surface tension prevents water from passing through the pores of an umbrella.

v  A duck is able to float on water as its feathers secrete oil that lowers the surface tension of water.

 

Factors Affecting Applications of Surface Tension:

1)Impurities:

            Impurities present in a liquid appreciably affect surface tension. A highly soluble substance like salt increases the surface tension and the sparingly soluble substances like soap decreases the surface tension.

2)Temperature:

            The Surface tension decreases with the increase of temperature  The temperature at which the surface tension becomes zero is known as Critical temperature of the liquid.

By increasing temperature intermolecular forces of liquid decreases due to which force of attraction among surface molecules decreases, that causes a decrease in surface tension of liquid.

                       

Temperature dependence of the surface tension of pure water

3) Effects of Solute:

 If the solute is less soluble in liquid, the surface tension of liquid decreases. For example by  mixing soap, detergent powder or phenol in water, the surface tension of water decreases but if the solute is very soluble in liquid, the surface tension of liquid increases.

Capillary Action

Capillary action, or capillarity, is the ability of liquid to flow against gravity where liquid spontaneously rises in a narrow space such as a thin tube.

In the capillary action of mercury, the mercury level at the center of the tube is higher than at the edges, making the upper surface of the mercury dome-shaped. The center of mass of the entire column of mercury would be slightly lower if the top surface of the mercury were flat over the entire crossection of the tube. But the dome-shaped top gives slightly less surface area to the entire mass of mercury. Again the two effects combine to minimize the total potential energy. Such a surface shape is known as a convex meniscus.If we replace mercury with water then it was observed that water level is upper at the edges sides, it is called convex meniscus.

Some important questions related to the topic,,,

What is the relationship between cohesion and capillary action?
Cohesion is the attraction between molecules of the same substance. Capillary action is the tendency of water to rise in a thin tube. As one molecule of water is pulled up the plant's capillary, it...
 Which feature of water explains why water has high surface tension?
Water molecules can make hydrogen bonds with other water molecules. Hydrogen bonds are the strongest type of intermolecular forces. This explains the high surface tension of water.

What is the difference between surface tension and interfacial tension?
interfacial tensions are similar to surface tensions in which cohesive forces are also involved but the major forces are adhesive forces (tension) between liquid phase of one substance and either...