Macroscale observations

The subject of this chapter is the relationship between macroscale observations and the underlying microscale processes in shock compression. Since the greater part of our current experimental knowledge of the shock compression process involves macroscale observations, we try to infer microscale phenomena from these data. A much more satisfactory approach is the direct real-time observation of microscale processes themselves. This is difficult to do in most cases, so we must still rely on a combination of macroscale measurement, microscale theory, and whatever direct observations of microscale processes that can be made. 

Unfortunately, in many cases, one or more of the above three components are unavailable, and we resort to substantial subjective guesswork in establishing underlying micromechanical causes of a particular macroscale observation. 

When (7.10)-(7.12) are combined with the expressions for mass and momentum conservation, we are then able to compare assumptions regarding and v with macroscale observations such as wave profiles, for example. The conservation laws are (in Lagrangian form Pq dX = p dx )... 

Unfortunately modern methods of mineral analysis on a microscopic scale, electron microbeam and others, do not allow the determination of the different oxidation states of iron especially for nonstoichiometric minerals. One can use Mossbauer spectral analysis, but the scales of observations are not the same (Mossbauer needing more material) one method used for observations on a microscale, the other on a macroscale. Given the problems of micro- and macroscale observations, oxidation state information is almost excluded from data gathered since the 1980s or so, and hence information... 

Understanding how chemistry and the environment can influence macroscale observations. 

We will attempt to address a number of these phenomena in terms of their micromechanical origins, and to give the essential quantitative ideas that connect the macroscale (continuum description) with the microscale. We also will discuss the importance of direct observations, wherever possible, in establishing uniqueness of scientific interpretation. 

Much of what we currently understand about the micromechanics of shock-induced plastic flow comes from macroscale measurement of wave profiles (sometimes) combined with pre- and post-shock microscopic investigation. This combination obviously results in nonuniqueness of interpretation. By this we mean that more than one micromechanical model can be consistent with all observations. In spite of these shortcomings, wave profile measurements can tell us much about the underlying micromechanics, and we describe here the relationship between the mesoscale and macroscale. 

Underlying all continuum and mesoscale descriptions of shock-wave compression of solids is the microscale. Physical processes on the microscale control observed dynamic material behavior in subtle ways sometimes in ways that do not fit nicely with simple preconceived macroscale ideas. The repeated cycle of experiment and theory slowly reveals the micromechanical nature of the shock-compression process. 

In recent years there has been a renewed appreciation of potential beneficial effects of roughness on a macroscale. For example Morris and Shanahan worked with sintered steel substrates bonded with a polyurethane adhesive [61]. They observed much higher fracture energy for joints with sintered steel compared with those with fully dense steel, and ascribed this to the mechanical interlocking of polymer within the pores. Extra energy was required to extend and break these polymer fibrils. 

The group in the Swiss Federal Institute of Technology [55] has fabricated a macroscale device by depositing the conducting polymer (poly(/j-phenylenevinylene)) on the MWCNT film (Fig. 16). They have observed the characteristic rectifying effect from the l-V curve, which suggests the CNTs inject holes efficiently into the polymer layer. However, due to the difficulty in... 

A method has recently been described for wrapping polymers around metal atoms and very small metal clusters using both matrix and macroscale metal vapor-fluid polymer synthetic techniques. Significant early observations are that (i) the experiments can be entirely conducted at, or close to room temperature, (ii) the resulting "pol5aner stabilized metal cluster combinations are homogeneous liquids which are stable at or near room temperature, and (,iii) the methodology is easily extended to bimetallic and trimetallic polymer combinations. ... 

Macroscale reduction of the monoacids 56 could lead to a variety of products, depending on the extent of electrolysis. For instance, reduction to the carbinols (29-31) would seem likely further reduction might also give the picolines 15, 16, and 19. Reduction of the picolines could also give the piperidines or piperdeines (see Schemes 5 and 10). Experimentally, all of these processes have been observed at one time or another in addition to formation of the isomeric aldehydes 41,42, and 58. 

On the basis of the observations in the macroscale, the flow of a fast fluidized bed can be represented by the core-annulus flow structure in the radial direction, and coexistence of a bottom dense region and a top dilute region in the axial direction. Particle clusters are an indication of the heterogeneity in the mesoscale. A complete characterization of the hydrodynamics of a CFB requires the determination of the voidage and velocity profiles. There are a number of mathematical models accounting for the macro- or mesoaspects of the flow pattern in a CFB that are available. In the following, basic features of several types of models are discussed. 

Microreactors provide a safe means by which reactions, including multistage schemes, can be undertaken where, otherwise, products involving unstable intermediates may be formed. This is exemplified by Fortt who showed that for a serial diazonium salt formation and chlorination reaction performed in a microreactor under hydrodynamic pumping, significant yield enhancements (15-20%) could be observed and attributed them to enhanced heat and mass transfer [77]. This demonstrates the advantage of microreactor-based synthesis where diazonium salts are sensitive to electromagnetic radiation and static electricity, which in turn can lead to rapid decomposition. Microreactors facilitate the ability to achieve continuous-flow synthesis, which is often not possible with conventional macroscale reactors and batch production. 

In their pioneering work, Jensen et al. demonstrated that photochemical transformation can be carried out in a microfabricated reactor [37]. The photomicroreactor had a single serpentine-shaped microchannel (having a width of 500 pm and a depth of 250 or 500 pm, and etched on a silicon chip) covered by a transparent window (Pyrex or quartz) (Scheme 4.25). A miniature UV light source and an online UV analysis probe were integrated to the device. Jensen et al. studied the radical photopinacolization of benzophenone in isopropanol. Substantial conversion of benzophenone was observed for a 0.5 M benzophenone solution in this microflow system. Such a high concentration of benzophenone would present a challenge in macroscale reactors. This microreaction device provided an opportunity for fast process optimization by online analysis of the reaction mixture. 

In acidic medium (pH 0 2), 4-aminopyrimidine undergoes a two-step reduction. The pH-dependent wave I corresponds to a 3e process and wave II, corresponding to a le transfer, is accompanied by catalytic hydrogen discharge. Only wave I is observed between pH 2 and 7.5. Macroscale electrolysis at the potentials of waves I and II was followed electrochemically and by UV spectroscopy, with isolation and identification of wave I products, 02,. 

The most powerful property of the detailed microbalance models, especially in combination with visualization techniques, is the a priori prediction of (observable) macroscale phenomena. This can be particularly helpful in reducing the required experimental effort. Important problems are the amount of detailed information required for the microscale transport equations and the large progranuning and computational efforts required to solve specific problems. Nevertheless, these types of models, by generating insight in the micro- and... 

An analysis and a physical interpretation of these observations will now be proposed. In order to analyse and classify the different heat transfer coefficient behaviours observed on figures 4 and 5 it is useful to represent, for a given vapour quality, the heat transfer coefficient as a function of the heat flux and the heat flux as a function of the wall-fluid temperature difference. For Dj, = 2 mm, since Co < 0.5, the results were analysed in terms of macroscale boiling. This was done on figure 8, which exhibits two trends ... 

In macroscale reduction of unsubstituted phenylarsonic acid at a mercury pool electrode, the final product is an oxygen-containing polymer, which contains As—As as well as As—O—As bonds . Precipitation of PhAsH2 was also observed during the electrolysis , and it was suggested that products were formed by the following reactions ... 

Liquid flow behavior is far more complex and not nearly as well understood as gas flow, especially in microscale. Generally, when the dimensions of a flow channel are 10 pm or more, the fluid behavior is simple and laminar. However, as the channel dimensions shrink below 1 pm new processes start to occur. ° The presence of the wall can affect the phase behavior of the fluid causing colloidal behavior. Numerous and thus far unexplained deviations from classical laminar flow behavior have been observed. At very high strain rates, deviations from Newtonian behavior are observed in fluids that are Newtonian in macroscale systems. ... 

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