MECHANICAL PROPERTIES OF FLUIDS: VISCOSITY:Just as surface tension is the property of liquid at rest, viscosity is the property of fluid in motion. Most of the fluids are not ideal ones. They offer some resistance to the fluid motion. When the fluid moves in the form of layers having different velocity, tends to destroy the relative motion of the different layers of the fluid. This resistance is an internal friction of fluid, similar to the friction when a solid moves on the surface of another solid. This internal friction of moving fluid is called viscosity of fluid.Thus, viscosity is the property of the fluid liquid or gas by virtue of which an internal fictional force comes into play when the fluid is in motion in the form of layers having relative motion. It opposes the relative motion of the different layers. Viscosity is also called us fluid friction.To understand viscosity of fluid, take a fluid i,e. liquid oil enclosed between two parallel glass plates as shown in Figure. Keep the bottom plate fixed and move the top plate by applying some force with a constant velocity u, relative to the fixed plate. Replace the oil with honey, we will note that a greater force is required to move the top plate with the same velocity. It shows that the honey has more internal frictional force than oil, i.e., honey is more viscous than oil. In the above experiment, the layer of liquid in contact with the surface has the same velocity as that of the surface. It means, the layer of the liquid in contact with top glass plate moves with the velocity ‘u’ and the layer of liquid in contact with the fixed glass plate is stationary. The velocity of the different layers of liquid increases uniformly from bottom zero velocity to the top layer velocity ‘u’.Every fast moving layer tends to pull forward its lower slow moving layer and every slow moving layer tends to pull backwards the upper fast moving layer. This produces a viscous force called internal frictional force or viscosity between the two layers of the liquid, which tends to destroy the relative motion of the two layers of liquid. Therefore, to maintain the relative velocity between the two layers of liquid flow, a force F equal to the viscous force acting between these two layers is to be applied tangentially on the liquid layer. Let initially BCDE be the portion of the liquid in between two parallel glass plates and be the thickness of liquid. Let A be the area of the liquid in contact with glass plate.

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