Difficulties in understanding

In spite of the progress described above, certain fundamental problems in flow and heat transfer are still unclear. This leads to difficulties in understanding the essence of micro-thermohydrodynamic phenomena. 

In Spite of the existence of numerous experimental and theoretical investigations, a number of principal problems related to micro-fluid hydrodynamics are not well-studied. There are contradictory data on the drag in micro-channels, transition from laminar to turbulent flow, etc. That leads to difficulties in understanding the essence of this phenomenon and is a basis for questionable discoveries of special microeffects (Duncan and Peterson 1994 Ho and Tai 1998 Plam 2000 Herwig 2000 Herwig and Hausner 2003 Gad-el-Hak 2003). The latter were revealed by comparison of experimental data with predictions of a conventional theory based on the Navier-Stokes equations. The discrepancy between these data was interpreted as a display of new effects of flow in micro-channels. It should be noted that actual conditions of several experiments were often not identical to conditions that were used in the theoretical models. For this reason, the analysis of sources of disparity between the theory and experiment is of significance. 

To be able to explain chemical reactions, students will have to develop mental models of the submicroscopic particles of the substances that undergo rearrangement to produce the observed changes. However, students have difficulty in understanding submicroscopic and symbolic representations as these representations are abstract and carmot be directly experienced (Ben-Zvi, Eylon, Silber-stein, 1986, 1988 Griffiths Preston, 1992). As a result, how well students understand chemistry depends on how proficient they are in making sense of the invisible and the untouchable (Kozma Russell, 1997 p. 949). 

Despite several decades of studies devoted to the characterization of Fe-ZSM-5 zeolite materials, the nature of the active sites in N20 direct decomposition (Fe species nuclearity, coordination, etc.) is still a matter of debate [1], The difficulty in understanding the Fe-ZSM-5 reactivity justifies a quantum chemical approach. Apart from mononuclear models which have been extensively investigated [2-5], there are very few results on binuclear iron sites in Fe-ZSM-5 [6-8], These DFT studies are essentially devoted to the investigation of oxygen-bridged binuclear iron structures [Fe-0-Fe]2+, while [FeII(p-0)(p-0H)FeII]+ di-iron core species have been proposed to be the active species from spectroscopic results [9]. We thus performed DFT based calculations to study the reactivity of these species exchanged in ZSM-5 zeolite and considered the whole nitrous oxide catalytic decomposition cycle [10],... 

You will have no difficulty in understanding our new methods. They look rather complicated when they are in print, but they are easily explained with words. 7... 

The heart of the difficulty in understanding the concept of increase in entropy is a verbal one. It is difficult to dissociate the unconscious verbal implications of a word that we have used aU of our fives in other contexts without critical analysis. In speaking of increase in entropy, we are using language appropriate for the description of material bodies. Automatically, therefore, we associate with entropy other characteristics of material bodies that are at variance with the namre of entropy and hence that are a source of confusion. 

In determining experimentally the rate of a reaction, it is imperative to define the reaction completely, with respect to the reaetants, the stoichiometry and even the products. This is preferably carried out before any detailed rate measurements are made, otherwise difficulties in understanding the rate data (particularly if these are eomplex) are likely to arise. 

There is no reason to believe that replacement of water by the donor groups of a chelating agent is fundamentally different from replacement when only unidentate ligands are involved. However, the multiplicity of steps may increase the difficulty in understanding the detailed mechanism, and mainly for this reason the simpler bidentate ligands have been most studied. 

The great difficulty in understanding the evolution of a protein can be illustrated by cytochromes c, which are known to have a methionine and a histidine as heme ligands. The position of the methionine, especially, in the sequence of the proteins is very variable. Again cytochromes c have very different molecular weights. If this is true what is the implication for the overall fold The immediate neighbors of the heme, not bound to iron, include a tryptophan, almost invariable in structure but not in sequence position. How has this been evolved There is but one cytochrome c that has no tryptophan. I do not believe that we could follow evolution with confidence from sequences of single enzymes. Cytochrome c3 sequences show this very clearly, for these proteins, all from sulfur bacteria, show an enormous sequence variation [R. P. Ambler, Syst. Zool. 22, 554 (1974)]. There is no evidence that this is related to their evolution. 

There is a certain difficulty in understanding a basic fact of combustion theory, that the reaction always occurs primarily at a temperature close to the combustion temperature. 

The Oxidative Pathway. For a long time, one of the difficulties in understanding the mechanism of the superacid-catalyzed transformations of alkanes was that no... 

The main difficulty in understanding the causes of climate change is connected with the impossibility of considering climatic feedbacks sufficiently reliably. Primarily, this refers to cloud-radiation feedback, direct and indirect (by the effect on radiation properties of clouds) impacts of atmospheric aerosols on climate, and the impact of the atmosphere-ocean interaction on climate formation. 

The basic Nordic School approach, that of assessing satisfaction with a cross-section survey or appraisal raises one important dilemma. A criticism of the approach is that, when analysts and stakeholders obtain a set of scores, they may have difficulty in understanding their meaning and lack a context of interpretation. This issue is at the core of the charge made by Parasuraman and colleagues that the expectations approach is superior because it offers valuable contextual information (Parasuraman et al, 1985, 1994b). There are, however, three kinds of extensions to the basic Nordic School system that can augment the value of the post hoc only appraisals. 

In general, despite the difficulties in understanding the intricate mechanisms involved, ES provides a wide range of tools for structuring a fiber surface or composing a mesh at different length scales. 

One of the difficulties in understanding typical aragonitic ooid formation is the fact that the aragonite needles, or battens, are arranged with their c crystallographic axes parallel to the solid surface. Bathurst (1974) observed that the... 

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