Rheologic response, friction mechanics

Unlike traditional textbooks of tribology, in this book we regard boundary lubrication as a limit state of hydrodynamic lubrication when film thickness is down to molecular dimension and independent of the velocity of relative motion. The discussions are based on the existing results, some from literatures but mostly from the authors own work. The topics are mainly focused on the mechanical properties of boundary films, including rheology transitions, molecular ordering, and shear responses. Ordered molecule films, such as L-B films and SAM, are discussed, with emphasis on the frictional performance, energy dissipation and the effects from structural features. Boundary films can be modeled either as a confined substance, or an adsorbed/reacted layer on the... 

For two-phase cocurrent gas-liquid flow, there is the wide variety of possible flow patterns which are governed principally by the physical properties (density, surface tension, viscosity of gas, rheology of liquid), input fluxes of the two phases and the size and the orientation of the pipe. Since the mechanisms responsible for holdup and momentum transfer (or frictional pressure drop) vary from one flow pattern to another, it is essential to have a method of predicting the conditions under which each flow pattern may occm. Before developing suitable methods for the prediction of flow pattern, it is important briefly to define the flow patterns generally encountered in gas-liquid flows. Horizontal and vertical flows will be discussed separately as there are inherent differences in the two cases. 

Although the choice of the rheological model varies quite significantly with tp and Pe, some broad albeit approximate trends are also evident in Fig. 27.10. Most importantly, it appears that the poroelastic response is bracketed between Pe = 0.4 and 2. At higher values of Pe one of the viscoplastic models (including purely viscous) applies depending on the value of p. At lower values of Pe bed friction and percolation are the two important dissipation mechanisms. 

Rheological measurements under oscillating conditions yield the dynamic mechanical properties of polymers, i.e. the storage modulus, G, the loss modulus, G , and a mechanical damping or internal friction, tan 5. In studies of the response of a material to vibrational forces, stress, strain, frequency and temperature are the key variables. When a material is subjected to cyclical stress under conditions analogous to those encountered in the intended applications, the data reflect both... 

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