Lec.5 Surface Tension

An object less dense than water, such as an air-filled beach ball, floats with part of its volume below the surface. Conversely, a paper clip can rest atop a water surface even though its density is several times that of water. This is an example of surface tension:
The surface of the liquid behaves like a membrane under tension. Surface tension arises because the molecules of the liquid exert attractive forces on each other.
There is zero net force on a molecule inside the volume of the liquid, but a surface molecule is drawn into the volume . Thus the liquid tends to minimize its surface area. A sphere has a smaller surface area for its volume than any other shape.
To wash clothing thoroughly, water must be forced through the tiny spaces between the fibers . To do so requires increasing the surface area of the water, which is difficult to achieve because of surface tension. The job is made easier by increasing the temperature of the water and adding soap, both of which decrease the surface tension.