Fluids at Rest and in Motion

 Surface tension

Hey! You are finally here. So wonderful. Though the scientists are great, but God is always greater than the greatest. This post made me to remember a day at high school. I used to wonder why the water drops on leaves seem spherical. Later, I realized that it was due to the influence of surface tension. Surface tension arises due to the strong intermolecular forces that holds the liquid surface together.


This phenomenon called surface tension affects our day-to-day activities. Soaps and detergents lowers the surface tension of water. This is desirable for cleaning purposes because the surface tension prevent the soap or detergent from easily entering  between the fibres of materials. Dear, it is advisable to use hot water for cleaning purposes, because high temperatures decreases the surface tension of water.

Surface tension is also applied in making umbrellas, raincoats and other water-repellent object. Due to surface tension, the trapped water forms elastic skin on the surface and prevents further seepage of water.

Capillarity

Capillarity is the tendency of a liquid to rise or fall in a narrow tube (known as capillary tube). Cohesion, adhesion and surface tension are forces responsible  for the capillarity of liquid.

 Dear, the capillarity is also occur in our everyday activities. The ability of lamp-wick to absorb paraffin from the base of the lamp is due to the capillary action of the liquid paraffin within the fibres of the wick. The ability of a kitchen towel to absorb spilled liquid is also due the existence of capillarity.

Viscosity

Viscosity is the internal frictional force existing between two layers of fluid (Liquid or gas).  It is obvious that some liquids flow smoother than the others. For example, honey is more viscous than water and is harder to pour. The viscosity of a liquid is affected by temperature. The viscosity of a fluid tends to decreases as its temperature rises.

When a body drops into a viscous liquid, its motion is divided into two stages.

1.  Acceleration Stage:  As the body is dropped into the viscous liquid, it accelerates downward with its weight greater than the sum of the upthrust and viscous force. Thus, W > U + V
2. Equilibrium Stage: After some period of time, the body will stop and start accelerating, but will move at constant velocity known as the terminal velocity. During this stage, the weight is equal to the sum of upthrust and the viscous force. Thus, W = U + V

 Liquids with high viscosity such as engine oil and grease are used as lubricants. A parachute uses the drag force exerted by air on a free falling person to slow down its decent and cushion his fall, protecting him from injury.