SURFACE TENSION

          




ABSTRACT 


  • Surface tension is defined as a cohesive molecular force acting on the surface of all liquids, which tends to force the solution to its smallest volume. The surface of the liquid, due to this force, becomes an elastic like film that exerts its force perpendicular to the surface of the solution. Surfactants are used to reduce surface tension, and their primary function is as an anti pitting agent, while the reduction of surface tension on the cathode surface also tends to enhance the effect of other additives on the character of the electrodeposit. The surface tension of a liquid influences the size of the drop that will be formed when the liquid is suspended from a glass tip. The capillary method, used to measure the surface tension of pure liquids very accurately, and stalagmometer is used to measure surface tension directly.




KEYWORDS



Surface Tension, miscible liquid, immiscible liquid, cohesive force




INTRODUCTION


Surface tension is the tendency of liquid surfaces at rest to shrink into the minimum surface area possible. Surface tension is what allows objects with a higher density than water such as razor blades and insects (e.g. water striders) to float on a water surface without becoming even partly submerged.The phenomenon of surface tension can be explained by the molecular theory of matter. There exist cohesive forces of attraction between molecules of the liquid.


OBJECTIVES

  • To understand the concept of surface tension and the effect of a surfactant
  • To understand cohesion and adhesion of water
  • To see the hydrophobic, polar and non-polar properties of water

METHODOLOGY

1.Spherical shape of drops: Surface tension tries to decrease the surface area of the liquid to the minimum.The drops of a liquid (or the bubbles of a gas) are spherical because for given volume, a sphere has 
minimum surface area.Inward pull of surface tension makes a drop spherical.

2. Rise of a liquid in a capillary tube: Suppose one end of a capillary tube is put into a liquid that wets 
glass (fig.). It is found that the liquid rises into the capillary tube to a certain height. This rise due to 
inward pull of surface tension acting on the surface which pushes the liquid into the capillary tube. It is because of the same reason that oil rises into the wick of an oil pump or water below the surface of earth rises to the plants through the roots or ink rises in a blotting paper.It may be mentioned here that in case of liquid which do not wet glass, e.g. Hg, the level inside the capillary falls below the level outside. Further whereas the upper surface of a liquid that wet glass is concave, that of Hg is convex. Such a surface is therefore, called Meniscus. The angle which the curved surface makes with the wall of the tube is called contact angle and is found by drawing a tangent at that 
point.

3. Interfacial Tension: If two immiscible or partially miscible liquids are taken in a vessel, then the surface tension acting along their surface of separation i.e. along the interface is called the interfacial tension. Its value is generally intermediate between the surface tension of the two liquids but sometimes it is less than both. This is due to fact that at the interface, the molecules of one liquid are attracted by the molecules of the other.In fact, the surface tension of a liquid is also an interfacial tension, the two phases involved being the liquid and the vapour above it.
4. Surface Active Agents: There are certain substances like soaps, certain sulphonic acids and certain other organic compounds like CH3OH, C2H5OH, CH3COCH3 etc. which when added to water even in small amount decreases the surface tension of water to a considerable extent. Such substances which lowers the surface tension of water are called as surface active agents. It is for this reason that the soap is acts
as detergent.

5. In the Functioning of Lung: A biologically relevant issue that is the stability of lung tissue. Lung tissue is composed of small water-lined, air-filled chambers called alveoli. According to Equation (Pinner = Pouter+2γ/r), the pressure difference across the surface of a spherical air-filled cavity is proportional to the surface tension and inversely proportional to the radius of the cavity. Therefore, if two air-filled alveoli, 
approximated as spheres with radii r and R and for which are interconnected, the smaller alveolus will collapse and the larger alveolus will expand because the pressure within the smaller alveolus is greater than the pressure within the larger alveolus. In theory, due to the surface tension of water that lines the alveoli, these cavities will “coarsen” as smaller alveoli collapse, leaving an ever diminishing number of alveoli of increasing size. Coarsening of the alveoli according to Equations (Pinner = Pouter +2γ/r) will result eventually in the collapse of the lungs.

CAUSES

Surface Tension is caused by the intermolecular force of the liquid particles such as the Van der Waals force. The molecules at the surface are pulled towards the bottom of the liquid. 

A molecule is tugged in all directions equally by neighboring liquid molecules due to cohesive forces, resulting in a net force of zero.As the molecules near the surface are not surrounded by identical molecules on all sides, they are pushed inward.This causes internal pressure which further causes the liquid surfaces to compress to the smallest possible area.Due to the cohesive structure of water molecules, there is also a tension parallel to the surface that will resist an external force.

EFFECTS

Rainwater beads on a waxy surface like a leaf. Water clusters into drops because it sticks weakly to wax but firmly to itself. Because a sphere has the smallest possible surface area to volume ratio, surface tension gives them its near-spherical shape.

When a liquid mass is stretched, drop formation happens. The water clings to the faucet and developing group until the surface tension can no longer keep the drop attached to the faucet. The drop then separates and is shaped into a sphere by surface tension. If a stream of water were to come out of the tap, it would break up into droplets as it descended. The stream is stretched by gravity and then compressed by surface tension into spheres.

Flotation of non-wettable items denser than water happens when the object’s weight is minimal enough to be supported by surface tension forces. Water striders, for example, walk on a pond’s surface using surface tension in the following fashion: Because there is no connection between the molecules of the water strider’s leg and the water molecules. When the leg presses down on the water, the surface tension of the water seeks to recover its flatness from the leg’s deformation. The water strider is pulled upward by the water’s behaviour, allowing it to stand on its surface as long as its mass is small enough for the water to support it. The water’s surface behaves like an elastic film: the insect’s feet make indentations in the surface, increasing the water’s surface area. In contrast, the water’s inclination to minimise surface curvature (space) pushes the insect’s feet higher.

Tension at the surface between different liquids causes oil and water separation (in this case, water and liquid wax). The term “interface tension” is used to describe this form of surface tension, but the chemistry is the same.

Wine tears are droplets and rivulets that form on the side of a glass containing an alcoholic beverage. It is caused by a complex interplay between the various surface tensions of water and ethanol, created by a combination of ethanol changing the surface tension of water and ethanol evaporating quicker than water.



CONCLUSION

Surface tension is the amount of energy necessary to raise the surface area by a certain quantity. The requirement for this to happen is that strong intermolecular interactions exist between distinct liquid particles. The phenomenon known as surface tension is caused by the cohesive forces between liquid molecules. Because other water molecules do not entirely encircle the molecules at the surface of a glass of water on all sides, they are called surface molecules. They cohere more strongly with those directly related to them (in this case, next to and below them, but not above). It is not true that a “skin” forms on the water surface; the stronger cohesiveness between water molecules, as opposed to the attraction of water molecules to the air. It is more difficult to move an object through the surface than it is to move it while completely immersed.

REFERENCES
https://www.wikipedia.org
https:// www.liquidstudy.com
https:// www.edu.nic.in
https:// www.surfacetension.org
https:// www.educare.com


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