Macroscale Friction

To summarize, macroscale adhesion results have shown a higher adherence for PDMS 17 and a large effect of extraction and separation speed. Macroscale friction results have indicated a higher friction resistance for PDMS 6 than for PDMS 17, and a slight effect of extraction and friction speed. 

Miyata and Yamaoka [152] used scanning probe microscopy to determine the microscale friction force of silicone-treated polymer film surfaces. Polyurethane acrylates cured by an electron beam were used as polymer films. The microscale friction obtained by scanning probe microscopy was compared with macroscale data, such as surface free energy as determined by the Owens-Wendt method and the macroscale friction coefficient determined by the ASTM method. These comparisons showed a good linear relationship between the surface free energy and friction force, which was insensitive to the nature of polymer specimens or to silicone treatment methods. Good linearity was also observed between the macroscale and microscale friction force. It was concluded that scanning probe microscopy could be a powerful tool in this field of polymer science. Evrard et al. [153] reported coefficient of friction measurements for nitrile rubber. Frictional properties of polyacetals, polyesters, polyacrylics [63], reinforced and unreinforced polyamides, and polyethylene terephthalate [52] have also been studied. 

In this chapter, we are concerned with the difference in the macroscale frictional behavior between the MoDTC/ZDDP and ZDDP tribofiims formed on sliding steel snrfaces under bonndary Inbrication. We describe several novel and practical SPM techniqnes, snch as the AFM phase image method and the nanoindentation and nanos-oatch methods. The primary emphasis is on how to apply these methods to determine nanometer-scale stmctnial and mechanical properties. By combining a nanoprobing techniqne with a beam-based analytical method, we explain how to determine the nanometer-scale controlling factors involved in reducing friction. The relationships between nanometer-scale propaHes, micrometer-scale phenomena, and macroscale frictional effect are explained by elnddating the friction reduction due to the MoDTC additive. 

These presumptions are based on the results of chanical composition analyses and structural determinations by beam-based analytical methods. To verify the mechanisms involved, it is essential to clarify the frictional and mechanical properties of each layer in the multilayered structure of the tribofilm. Most importantly, with respect to the uppermost surface and the underlying area, which are thought to control the macroscale friction behavior, it is indispensable to determine differences not only in surface roughness, but also in shear strength and frictional behavior relative to depth on a nanometer scale. It is also important to make clear the distribution of the chemical composition relative to the tribofilm depth and to make this chemical distribution consistent with the nanometer-scale frictional and mechanical properties. However, estimating these properties experimentally on a nanometer scale is a difficult task, although several attempts have been made [24-28],... 

The aforementioned nanoprobe and beam-based analyses results made clear the differences in surface roughness, mechanical/frictional properties, surface nanostructure, chemical composition, and surface chemical state on a nanometer scale between the MoDTC/ZDDP and ZDDP tribofilms. These results demonstrated that the nanostructural, nanometer-scale mechanical and chemical properties acted as the controlling factors in the macroscale friction behavior of the tribofilms. From these results, a microstructure model of the tribofilms was developed to explain the mechanism of friction reduction, as shown in fig. 9.20 [3, 34]. 

The flow of the continuous phase is considered to be initiated by a balance between the interfacial particle-fluid coupling - and wall friction forces, whereas the fluid phase turbulence damps the macroscale dynamics of the bubble swarms smoothing the velocity - and volume fraction fields. Temporal instabilities induced by the fluid inertia terms create non-homogeneities in the force balances. Unfortunately, proper modeling of turbulence is still one of the main open questions in gas-liquid bubbly flows, and this flow property may significantly affect both the stresses and the bubble dispersion [141]. 

The earliest studies related to thermophysieal property variation in tube flow conducted by Deissler [51] and Oskay and Kakac [52], who studied the variation of viscosity with temperature in a tube in macroscale flow. The concept seems to be well-understood for the macroscale heat transfer problem, but how it affects microscale heat transfer is an ongoing research area. Experimental and numerical studies point out to the non-negligible effects of the variation of especially viscosity with temperature. For example, Nusselt numbers may differ up to 30% as a result of thermophysieal property variation in microchannels [53]. Variable property effects have been analyzed with the traditional no-slip/no-temperature jump boundary conditions in microchannels for three-dimensional thermally-developing flow [22] and two-dimensional simultaneously developing flow [23, 26], where the effect of viscous dissipation was neglected. Another study includes the viscous dissipation effect and suggests a correlation for the Nusselt number and the variation of properties [24]. In contrast to the abovementioned studies, the slip velocity boundary condition was considered only recently, where variable viscosity and viscous dissipation effects on pressure drop and the friction factor were analyzed in microchannels [25]. 

The influence of speed is obvious (contrary to the macroscopic results), with an increase in friction with speed for both grades of PDMS. This effect is similar to the influence of speed observed in macroscale adhesion, for which a higher separation speed induces an increase in the tack energy. Chain motions during nano-friction and pull-out mechanisms, which are speed-dependent, could explain this behavior. 

For turbulent flows, the friction factor is a function of both the Reynolds number and the relative roughness, where s is the root-mean-square roughness of the pipe or channel walls. For turbulent flows, the friction factor is found experimentally. The experimentally measured values for friction factor as a function of Re and are compiled in the Moody chart [1]. Whether the macroscale correlations for friction factor compiled in the Moody chart apply to microchannel flows has also been a point of contention, as numerous researchers have suggested that the behavior of flows in microchannels may deviate from these well-established results. However, a close reexamination of previous experimental studies as well as the results of recent experimental investigations suggests that microchannel flows do, indeed, exhibit frictional behavior similar to that observed at the macroscale. This assertion will be addressed in greater detail later in this chapter. 

Judy et al. measured friction pressure drop of fluid flow in microtubes and microchaimels with hydraulic diameters between 15 and 150 pm for three different fluids (water, methanol, and isopropanol), two different tube materials (and hence two different surface roughnesses), and two different tube cross-sectional geometries (circular and square) and found no significant deviation from macroscale viscous flow theory. 

Hetsroni et al. [6] also reexamined previous studies of friction factor in microchannels and drew the same conclusions that they did for transition in microchannels. They found that the anomalous results reported in some studies could be explained by the same factors that contributed to the observation of anomalous transitional behavior. Indeed, in the only study performed to date combining both microPIV and extensive pressure drop measurements. Sharp and Adrian [8] found that transition as measured by microPIV agreed well transition as inferred from friction factor data and also found that their measured friction factors agreed well with macroscale results. As with transition to turbulence, the experimental evidence on friction factors in turbulent microchannel flow shows that microscale flow exhibits the same behavior as macroscale flows. 

The studies referenced up to this point indicate that the bulk characteristics of turbulent microchannel flow, specifically the onset of transition and the frictional pressure drop, tend to agree well with classical results for macroscale pipe and channel flow. However, the question remains as to whether the microscale turbulence is statistically and structurally similar to tiubu-lence at the macroscale. 

Although for many years there were some questions about whether turbulent microchannel flows exhibited behavior similar to macroscale flows, recent experiments using new experimental techniques and a reexamination of data collected in previous studies suggest that, at least for microchannels as small as 50 pm in diameter, these questions are unfounded. Turbulent microchannel flow exhibits the same transitional behavior, displays similar frictional loss characteristics, and is statistically and structurally similar to macroscale pipe and channel flow. 

As mentioned in the Introduction, the recent interest for reducing friction — promoting slip — at solid-liquid interfaces was initially motivated by the ever growing field of microfluidic devices where the role of channel surfaces is considerably enhanced compared with the macroscale. It is in this particular context that super-hydrophobic surfaces have been introduced, and we have presented in Section 2 a review of the different theoretical and experimental works showing their remarkable frictional properties in laminar (low Reynolds numbers) flows. 

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