Mechanical Properties of Fluids Test - 4

The surface tension of a liquid results from an imbalance of intermolecular attractive forces, the cohesive forces between molecules:A molecule in the bulk liquid experiences cohesive forces with other molecules in all directions.A molecule at the surface of a liquid experiences only net inward cohesive forces.The unbalanced attraction of molecules at the surface of a liquid tends to pull the molecules back into the bulk liquid leaving the minimum number of molecules on the surface. It required energy to increase the surface area of a liquid because a larger surface area contains more molecules in the unbalanced situation and  this may results as the surface being stretched.

In fluids there are primarily 2 forces cohesive force and adhesive force. Cohesive force is the one which act between same molecules and adhesive force is the one which acts between different molecules. 

The cohesive force gives rise to a phenomenon called surface tension. In this phenomenon a fluid surface tends to pull itself inwards, causing reduction of exposed surface.

For a water droplet which is left alone, cohesive forces undoubtedly dominate, leading to surface tension . Surface tension makes all the molecules of water to stick with each other closely, this results in its spherical shape.

Let L be the width of the film. Since the film has two surfaces, the total length along which the surface force acts on the slider is 2L. The surface tension S in the film is defined as the ratio of the surface force F to the length d (perpendicular to the force) along which the force acts

  S = F/d

Hence, in the case, d = 2L

S=F/2L

Capillary action(sometimes capillarity or wicking) is the ability of a liquid to flow in narrow spaces without the assistance of,or even in opposition to, external forces like gravity. The effect can be seen in the drawing up of liquids between the hairs of a paint-brush, in porous materials such as paper and plaster, in some non-porous materials such as sand and liquefied carbon fiber, or in a cell. It occurs because of intermolecular forces between the liquid and surrounding solid surfaces. If the diameter of the tube is sufficiently small, then the combination of surface tension (which is caused by cohesion within the liquid) and adhesive forces between the liquid and container wall act to propel the liquid.

when any liquid is poured onto another liquid there are different phenomena that can take place depending on the properties of liquids:In case of crude oil and water -

The two liquids are non-miscible and the first is lighter than the second. The first liquid then spreads on the surface of the second liquid forming a thin layer. The thickness of this layer depends on the properties of the two liquids. This is the case for most hydrocarbons, including both refined products and crude oil of API gravity greater than 10.