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What is the reason for different volecity of different layers of liquid?I have read...

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What is the reason for different volecity of different layers of liquid?

I have read that viscosity in a liquid is due to the relative motion of different layers of liquid but what is the reason the for the relative motion of the layers.

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When a liquid comes into contact with a surface (such as the walls of a pipe, tube or a container), both cohesive and adhesive forces act on it. These forces govern the shape which the liquid takes on. These forces are also responsible for the difference in velocity of different layers inside a flowing tube, causing viscous dragging forces. Layers in contact with the inner walls of the tube experience adhesive forces (forces of attraction between like molecules) at the junction of the tube material and liquid molecules, and cohesive forces (forces of attraction between unlike molecules) between the two layers of liquid molecules. Whereas, those in the middle layers experience adhesive forces both in upper and lower direction. In addition to that thermal diffusion forces act on all the layers, albeit to varying extents at different points. The result is that, different layers of liquids experience different resultant forces, thus giving different velocities to them.  

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