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, thus viscosity 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
- 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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