Progress Article

I wear eyeglasses (you may too) and blow on them when they are dusty.The large dust particles blow off easily but the small particles do not.But why not? This familiar occurrence illustrates the no-slip boundary condition of fluid flow past a solid surface, which states that flowing fluid (air, in this example) comes to rest just at the point where it meets the solid surface.There is no relative motion of the fluid to the impenetrable boundary that it flows past; molecules at the boundary move on average with the same velocity as that boundary,and is therefore zero at a solid (the normal velocity component is also zero but physically trivial).We therefore realize that small dust particles do not extend far enough beyond an adsorbing surface to experience a large enough airflow to be blown off. Schematically, this is illustrated in Fig. 1. The same situation arises elsewhere in everyday life. When we take a shower,wash our hands,wash dishes... it’s difficult to wash soap off simply by rinsing. To remove soap from the surfaces on which it sits, it’s much more effective to scrub with a washcloth.A less mundane example is the energy lost to pump fluids. One hardly questions that a jet of fluid flows through air almost without frictional loss but that this is impossible for flow through a pipe.Why? Why,as we age,does fatty detritus tend to accumulate on our arteries? This is a core concept in fluid mechanics — ‘no slip at the wall’. It forms the basis of our understanding of the flow of simple low-viscosity fluids and comprises a springboard for much sophisticated calculation. Although it is true that at some level of detail this continuum description must fail and demand description at the molecular level, it is tremendously successful as the basis for continuum-based calculations.As expressed in a prominent fluid dynamics textbook: