The fluids like water or oil, flow on the surface in the form of layers with the top layer moving at the fastest speed and the bottom layer moving at slow speed. This shows that as we move from the top layer to the bottom layer the speed of the flow of fluid reduces. There is some sort of resistance to the flow of various layers of the fluid, which is offered by the adjoining layers, this property of the fluids is called as viscosity of the fluid.
The viscosity of the fluid is defined as the property of the fluids that offers resistance to the movement of one layer of the fluid over another adjacent layer of the fluid.
In the figure various layers of the liquid are shown. There will be viscous forces between various layers that will resist the movement of the adjoining layers. Let us consider the two layers separated by small distance dy. If the velocity of the lower layer is u, the velocity of the layer just above it will be u+du, where du is the small incremental velocity. This is because the velocity of the upper layer of the fluid is more than the velocity of the lower layers of the fluid.
The SI unit of measurement of viscosity of the fluid is Ns/m2.
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