• The causes of viscosity in a fluid are possibly attributed to two factors:

(i) intermolecular force of cohesion
(ii) molecular momentum exchange

• Due to strong cohesive forces between the molecules, any layer in a moving fluid tries to drag the adjacent layer to move with an equal speed and thus produces the effect of viscosity as discussed earlier. Since cohesion decreases with temperature, the liquid viscosity does likewise.



Fig 1.7 Movement of fluid molecules between two adjacent

moving layers



• Molecules from layer aa in course of continous thermal agitation migrate into layer bb
  
  
• Momentum from the migrant molecules from layer aa is stored by molecules of layer bb by way of collision
  
  
• Thus layer bb as a whole is speeded up
  
  
• Molecules from the lower layer bb arrive at aa and tend to retard the layer aa
  
  
• Every such migration of molecules causes forces of acceleration or deceleration to drag the layers so as to oppose the differences in velocity between the layers and produce the effect of viscosity.