Viscous flow is familiar and useful, yet the underlyingphysics is surprisingly subtle and complex. Recent experiments and simulationsshow that the textbook assumption of ‘no slip at the boundary can fail greatlywhen walls are sufficiently smooth. The reasons for this seem to involvematerials chemistry interactions that can be controlled — especiallywettability and the presence of trace impurities, even of dissolved gases. Todiscover what boundary condition is appropriate for solving continuum equationsrequires investigation of microscopic particulars. Here, we draw attention tounresolved topics of investigation and to the potential to capitalize on ‘slipat the wall’ for purposes of materials engineering.
粘性流是我们熟悉的和有用的，但其底层的物理却令人惊讶的微妙和复杂。最近的实验和模拟表明，当壁面足够光滑时教科书的无滑移边界假设可以被认为是错误。这种情况的原因似乎涉及材料的可控化学相互作用，尤其是润湿和痕量杂质的存在时，或者含有溶解气。需要进行微观细节的研究才能找到什么边界条件适合求解连续方程。在这里，我们将注意力集中到研究这个未解决的问题和其在材料工程中潜在的壁滑行能力。
I wear eyeglasses (you may too) and blow on them when theyare dusty. The large dust particles blow off easily but the small particles donot. But why not? This familiar occurrence illustrates the no-slip boundarycondition 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 solidsurface. There is no relative motion of the fluid to the impenetrable boundarythat it flows past; molecules at the boundary move on average with the samevelocity as that boundary, and is therefore zero at a solid (the normalvelocity component is also zero but physically trivial).We therefore realizethat small dust particles do not extend far enough beyond an adsorbing surfaceto experience a large enough airflow to be blown off. Schematically, this isillustrated in Fig. 1.
我戴眼镜（你也可能也带着），我对着眼镜吹气来除去上面的尘土。大的灰尘颗粒吹掉很容易，但小颗粒则不易。但是，为什么不易呢？这个熟悉的现象发生说明了流体流过固体表面上的无滑移边界条件，其指出，流动的流体（空气，在这个例子中）只会在它碰到固体表面上停留下来。流体和不可穿透的边界没有相对运动；分子具有平均的与边界相同的移动速度（法向速度分量也为零，实际上也微不足道），因此，我们认识到，细小的灰尘颗粒没能延伸得足够远超出吸附面，从而不会被大的气流吹走。可见如图1所示。

图1 吹气来清洁表面尘土颗粒。大颗粒很容易被除去，但非常小的颗粒依然附着。它们不能被吹除，因为由箭头长度指示的流动空气的速度将在表面变为零。