Simple Planar Models of Slip

The term slip has acquired different meanings. It can mean solid-like sliding in the sense of friction and tribology. While such phenomena can definitely be investigated with shear-wave resonators, they are outside the scope of this chapter. Second, slip may denote interfacial shear thinning in complex liquids. The well-known everyday example is toothpaste. When squeezed out of the tube, toothpaste experiences plug flow because the large stress at the wall induces structural transformations inside the paste (such as 

slip can imply interfacial sliding between a simple liquid and a solid (see chapter 2). Generally speaking, slip in this sense is the exception rather than the rule. The interactions between small molecules and a solid wall are expected to be at least as strong as the interactions between the molecules in the bulk. The effective viscosity at the interface therefore should be similar to (or larger than) the bulk viscosity. Still, there is experimental evidence in favor of slip even in simple liquids. Acoustic shear waves should be a well-suited tool of investigation.  

This chapter is limited to simple liquids. We do, however, adopt a bit of an open-minded view of the term interfacial layer. Whether or not this layer consists of nanobubbles depends on the dehnition. The following text uses the term nanobubble, but this usage should not imply that these are just small bubbles. Nanobubbles are expected to behave peculiarly. Line tension should play a role. The Laplace pressure can be many atmospheres. The vertical scale of surface roughness compares to bubble size. Understanding nanobubbles comes down to understanding a significant portion of slip (see also chapter 7 for a general description of nanobubbles). 

The slip length can be viewed as an apparent negative hydrod5mamic thickness. 

The term slip length does not imply a statement about the physical origin of slippage. It only rephrases the phenomenon. 

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The chapter is organized as follows. Section 8.2 provides a short reminder of what acoustic shear waves can and cannot do. Shear waves have distinct advantages (compared to other surface anal3Ttical techniques like optical reflectometry or atomic force microscopy [AFM]), but there are also some caveats to be kept in mind. Section 8.3 briefly summarizes some predictions from simple planar models of slip. An experimental result, which stands as an example for an experience in the authors laboratory, is presented in section 8.4. Section 8.5 provides the results from FEM calculations. Section 8.6 discusses nonlinear phenomena and acoustic streaming, in particular. 

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