• 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

 

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.

 

End of Lecture 1! To start next lecture click next button or select from left-hand-side.