Causes of Viscosity - contd from previous slide...
As the random molecular motion increases with a rise in temperature, the viscosity also increases accordingly. Except for very special cases (e.g., at very high pressure) the viscosity of both liquids and gases ceases to be a function of pressure.
For Newtonian fluids, the coefficient of viscosity depends strongly on temperature but varies very little with pressure.
For liquids, molecular motion is less significant than the forces of cohesion, thusviscosity of liquids decrease with increase in temperature.
For gases,molecular motion is more significant than the cohesive forces, thus viscosity of gases increase with increase in temperature.
Fig 1.8: Change of Viscosity of Water and Air under 1 atm
No-slip Condition of Viscous Fluids
It has been established through experimental observations that the relative velocity between the solid surface and the adjacent fluid particles is zero whenever a viscous fluid flows over a solid surface. This is known as no-slip condition.
This behavior of no-slip at the solid surface is not same as the wetting of surfaces by the fluids. For example, mercury flowing in a stationary glass tube will not wet the surface, but will have zero velocity at the wall of the tube.
The wetting property results from surface tension, whereas the no-slip condition is a consequence of fluid viscosity.
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