Surface Tension (you know???)

Surface Tension (you know???)

water drops on glass surface

when  a drop of water is placed on the surface of cleaned glass,then the water spread as a thin film on  glass surface. But when a drop of mercury is placed then it forms a small SPHERE. Similarly if there is greece on  glass surface then water drop will also form a tiny sphere.WHY?????

mercury drop on glass surface

Now the conclusion comes out that "the behavior of liquids is not affected only by the GRAVITATIONAL FORCE but there is another factor called surface tension.

If a drop is of negligible weight then it has perfect spherical shape.

For examples: rain drops, soap bubbles, etc.

We should know that---------

                                  "For a given volume a sphere has minimum surface area"

Hence, it is the tendency of free surface to shrink to it's minimum surface area.

How to define the surface tension???????

Let an imaginary line AB on water surface.

In both direction of line the pulling force  F in opposite direction is working.

This force lies on the surface and perpendicular to the line AB.

if AB has length l then surface tension T

                 

                                             T=F/L

it's unit is (newton per meter).

• On heating the liquid the surface tension decreases.(because inter molecular attraction decreases.
• At the critical temperature of liquid the surface tension is Zero.

Beautiful phenomenon based on surface tension

1. The end of glass tube becomes rounded on heating.
   

   

   round end of glass

   When the glass tube is heating then it is melt into liquid. This liquid tries to acquire minimum surface and it converts into spherical shape.

2.small drops of mercury are spherical while big are flat,why??? 

    

Big drops have definite weight and therefore the gravitational force comes in the existence,

hence due to gravity the big drop becomes flat.

But small drops have negligible weight therefore the gravity do not affect the shape of drop and here the surface tension becomes major force, hence the small drops are spherical.