The no-slip boundary condition in fluid mechanics

Ideas leading to the resolution of the problem of no-slip condition for fluid velocity at a solid surface are traced in this concluding part of the article. In the continuum limit velocity slip being zero is established beyond any doubt now. Even turbulent flows which have a large velocity gradient near a wall have to satisfy the no-slip condition at every instant. From molecular considerations, on the other hand, we know that the velocity slip is proportional to the mean free path which may not be negligible in rarefied gas flows. Experimental verification of no-slip has been only indirect and it is only recently that slip velocity in non-wettable liquids has been measured directly.