Macroscopic analogies

Contemporary research trends related to the development of mesoscopic materials, which contain nanosized inclusions of guest compounds and phases in a host matrix, are among the most prospective fields for technological applications. The physical characteristics of nanoobjects in such materials can substantially differ from those of their macroscopic analogs. As a result there is a strong need to investigate both the properties of individual nano-objects and their parameter modifications when they interact with their environment. 

All aspects of molecular shape and size are fully reflected by the molecular electron density distribution. A molecule is an arrangement of atomic nuclei surrounded by a fuzzy electron density cloud. Within the Born-Oppenheimer approximation, the location of the maxima of the density function, the actual local maximum values, and the shape of the electronic density distribution near these maxima are fully sufficient to deduce the type and relative arrangement of the nuclei within the molecule. Consequently, the electronic density itself contains all information about the molecule. As follows from the fundamental relationships of quantum mechanics, the electronic density and, in a less spectacular way, the nuclear distribution are both subject to the Heisenberg uncertainty relationship. The profound influence of quantum-mechanical uncertainty at the molecular level raises important questions concerning the legitimacy of using macroscopic analogies and concepts for the description of molecular properties. ... 

In view of general experience of the ultimate failure of all macroscopic analogies to explain microscopic phenomena, and indeed of the essentially illogical character of the demand that they should, one can hardly regard the analogies which suggested Maxwell s equations as more than suggestive. The possibility of other formal schemes which express the properties of radiation and its interaction with matter in quantum phenomena lies open. Such field theories lie, however, beyond the present bounds of physical chemistry. 

Fora macroscopic analog of the importance of size, think of a pile of oranges at a grocery store. If you were to replace an orange near the bottom and still maintain the pile, which would be easier to use—a tangerine (about the same size) or a large grapefruit ... 

Molecular surface is one such concept, derived from macroscopic analogies, where some of the quantum mechanical aspects of molecules are often disregarded. The approximate nature of the model, however, does not lessen its value in many practical applications, as long as its limitations are well recognized. The molecular surface concept is very useful for the interpretation of molecular size and shape properties within approximate models. 

All didactic approaches have to face these problems, especially the dilemma of visualization. Some researchers try to avoid any visualization of the micro-world (Buck, 1990) some do not use macroscopic analogies of the micro-world. Other researchers discuss hybrid models between a macro- and micro-world (Justi Gilbert, 1998). However, students are confronted every day with a lot of colourful pictures of micro-objects in the media and also in common schoolbooks. In any case, we have to deal with the meaning of all fliese visualizations and discuss them with students. 

Various techniques have been used to create a macroscopic analogy to the crack tip bare surfaces on which the oxidation and reduction rates can be measured. These include mechanical methods to rupture the surface oxide that involve slowly [87-91] or rapidly [92,93] straining the alloy, complete fracturing of the specimen to create a bare fracture surface [94,95] cyclic straining [88,96], scratching the alloy surface [97-102], and grinding [103]. Electrochemical methods have also been used to cathodically reduce the oxide [1,104-106] and then pulse to the potential of interest. Copyright 2002 Marcel Dekker, Inc. 

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