Challenging areas

Foam rheology has been a challenging area of research of interest for the yield behavior and stick-slip flow behavior (see the review by Kraynik [229]). Recent studies by Durian and co-workers combine simulations [230] and a dynamic light scattering technique suited to turbid systems [231], diffusing wave spectroscopy (DWS), to characterize coarsening and shear-induced rearrangements in foams. The dynamics follow stick-slip behavior similar to that found in earthquake faults and friction (see Section XU-2D). 

The fonnation of surface aggregates of surfactants and adsorbed micelles is a challenging area of experimental research. A relatively recent summary has been edited by Shanna [51]. The details of how surfactants pack when aggregated on surfaces, with respect to the atomic level and with respect to mesoscale stmcture (geometry, shape etc.), are less well understood than for micelles free in solution. Various models have been considered for surface surfactant aggregates, but most of these models have been adopted without finn experimental support. 

Finally the concepts of fragment size, and fracture number or frequency statistics, need to be included within the framework of continuum and computational modeling of dynamic fracture and fragmentation. This challenging area of research has the potential for addressing many needs related to dynamic fragmentation. 

The approach to the evaluation of vibrational spectra described above is based on classical simulations for which quantum corrections are possible. The incorporation of quantum effects directly in simulations of large molecular systems is one of the most challenging areas in theoretical chemistry today. The development of quantum simulation methods is particularly important in the area of molecular spectroscopy for which quantum effects can be important and where the goal is to use simulations to help understand the structural and dynamical origins of changes in spectral lineshapes with environmental variables such as the temperature. The direct evaluation of quantum time- correlation functions for anharmonic systems is extremely difficult. Our initial approach to the evaluation of finite temperature anharmonic effects on vibrational lineshapes is derived from the fact that the moments of the vibrational lineshape spectrum can be expressed as functions of expectation values of positional and momentum operators. These expectation values can be evaluated using extremely efficient quantum Monte-Carlo techniques. The main points are summarized below. 

Catalytic enantioselective nucleophilic addition of nitroalkanes to electron-deficient alke-nes is a challenging area in organic synthesis. The use of cinchona alkaloids as chiral catalysts has been studied for many years. Asymmetric induction in the Michael addition of nitroalkanes to enones has been carried out with various chiral bases. Wynberg and coworkers have used various alkaloids and their derivatives, but the enantiomeric excess (ee) is generally low (up to 20%).199 The Michael addition of methyl vinyl ketone to 2-nitrocycloalkanes catalyzed by the cinchona alkaloid cinchonine affords adducts in high yields in up to 60% ee (Eq. 4.137).200... 

Organic fruit production is one of the most important and challenging areas of organic production for a variety of reasons. These include ... 

The purpose of this chapter is to provide a critical assessment of the literature regarding the partial oxidation of methane and the C02 reforming of methane, with emphasis on the following challenging areas hot spots, 02 separation cost, and the issues of reaction pathways and catalyst selection we also address the issue of carbon deposition in the C02 reforming of methane. The reason why we review these two reactions together is that they have many common characteristics, including the catalysts, the products, and CH4 as reactant. 

The biosynthesis of this imine and its congener, nitropolyzonamine (22), would be another challenging area for future exploration. 

There is thus a great potential for using nanocarbon-semiconductor hybrids to develop improved devices, particularly for the more challenging area of using renewable energy, while most of the current studies are centered on environmental applications, for which there is limited need to develop advanced photocatalysts. 

Finite-time thermodynamies is one of the newest and most challenging areas in thermodynamics. A book entitled Recent Advances in Finite Time Thermodynamics (editors Chih Wu, Lingen Chen, and Jincan Chen, Nova Science Publishers, Inc., New York, USA, 1999, ISBN 1-56072-644-4) provides results from research, which continues at an impressive rate. The book contains many academic and industrial papers that are relevant to current problems and practice. The numerous contributions from the international thermodynamic community are indicative of the continuing global interest in finite-time thermodynamics. 

The dioxirane epoxidation of a prochrral alkene will produce an epoxide with either one new chirality center for terminal alkenes, or two for internal aUcenes. When an optically active dioxirane is nsed as the oxidant, expectedly, prochiral alkenes should be epoxi-dized asymmetrically. This attractive idea for preparative purposes was initially explored by Curci and coworkers in the very beginning of dioxirane chemistry. The optically active chiral ketones 1 and 2 were employed as the dioxirane precursors, but quite disappointing enantioselectivities were obtained. Subsequently, the glucose-derived ketone 3 was used, but unfortunately, this oxidatively labile dioxirane precursor was quickly consumed without any conversion of the aUcene . After a long pause (11 years) of activity in this challenging area, the Curci group reported work on the much more reactive ketone... 

These few cases indisputably illustrate the synthetic value of such enantioselective CH insertions, a challenging area of dioxirane chemistry that demands more intensive research activity. For that purpose more reactive and more effective chiral dioxiranes must be designed. 

The introduction of more porous wall structures necessitates a closer look at the soot particle fate inside the wall and the deep-bed filtration mode starts to be important. Flow in porous media represents a challenging area of fluid mechanics and initial approaches (Bissett, 1984 Konstandopoulos et al., 2001,... 

The encapsulation of food flavors presents a very unique and challenging area of investigation. Commercial production of encapsulated flavors is accomplished by a variety of methods. 

This is not to say that research should be stopped because of the fear of these various risks in an extremely complex and challenging area that has great promise for the psychology of religion. But while research is progressing on the theoretical or primary level and before projects for testing useful social... 

This brief review attempts to summarize the salient features of chemically modified electrodes, and, of necessity, does not address many of the theoretical and practical concepts in any real detail. It is clear, however, that this field will continue to grow rapidly in the future to provide electrodes for a variety of purposes including electrocatalysis, electrochromic displays, surface corrosion protection, electrosynthesis, photosensitization, and selective chemical concentration and analysis. But before many of these applications are realized, numerous unanswered questions concerning surface orientation, bonding, electron-transfer processes, mass-transport phenomena and non-ideal redox behavior must be addressed. This is a very challenging area of research, and the potential for important contributions, both fundamental and applied, is extremely high. 

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