Viscosity and surface tension

1.            Surface tension is a property of a liquid by which the free surface of a liquid behaves like a stretched elastic membrane, having contractive tendency.

2.            The frictional force offered by liquid is called viscous force.

3.            The liquid flows in different layers. The layers at the top will be flowing with maximum velocity.  The lower layers with be flowing with lesser and lesser velocities.

4.            The reason for this change in velocity from layer to layer is the frictional force between liquid layers (viscous force)

5.            The property of a liquid by which force are exerted between layers so as to reduce their relative motion is called viscosity.  The word "viscosity" is derived from the Latin "viscum", meaning mistletoe.

6.            Different liquids possess different viscosity.

7.            A fluid that has no resistance to shear stress is known as an ideal fluid or inviscid fluid. Zero viscosity is observed only at very low temperatures, in super fluids.

8.            Viscous liquid: Liquids of large viscosity are called viscous liquid.  Eg:  Glycerin, honey, castor oil etc

9.            Mobile liquids: Liquids of small viscosity are called mobile liquids: They can easily flow eg: Water, Kerosene etc.

10.          Viscosity is measured with various types of viscometers and rheometers.

11.          Viscosity of a liquid decrease with temperature

12.          Surface tension is caused by unbalanced molecular cohesive force. (cohesive force is the action or property of molecules sticking together, being mutually attractive. Adhesion is the tendency of dissimilar particles or surfaces to cling to one another.)

13.          The surface tension decrease with rise of temperature.

14.          The larger the surface the more energy there is.  To minimize energy most fluids assume the shape with the smallest surface area.  For a given volume the sphere has the minimum surface area.  Hence it assumes spherical shape.  Hence rain drops and mist drops are spherical in shape.

15.          A liquid of smaller surface tension wets a surface easily.

When soap water is added to water the surface tension of water decreases.  Therefore soap water can wet a surface more easily than pure water.