Surface Tension:

You might have observed some of the interesting characteristics of liquids. For example, drops of mercury on a plane surface always assume spherical shape. Similarly, raindrops and dewdrops on the leaves of plants appear spherical. Did you ever ask yourself: Why a small amount of liquid gathers together into a spherical drop? In addition, you also might have observed that small insects could move on the surface of water without sinking! Similarly, sewing needle floats on the surface of water. Instead, heavier objects could neither walk nor float on the water surface; they simply sink. On the primary of these observations, you might conclude which

• There exists a force along the surface of the liquid which tends to shrink the liquid surface so that it has minimum area, and
• The surface of a liquid behaves such as a stretched membrane.

A logical question you may ask now is : Why does a liquid surface behave in this manner? What causes such behaviour of liquids? The phenomenon responsible for above characteristics of liquids is called surface tension. Due to this phenomenon, the free surface of a liquid behaves like a stretched elastic membrane tending to contract so as to have minimum surface area.