Chemical-mechanical alteration

Atmospheric particulates (sea salt, carbonaceous soot, and sulfuric acid aerosols) are known to provide a condensed phase for conq>lex heterogeneous chemistry to occur. Although the presence of atmospheric particulates are known to alter trace gas concentrations, details of the specific chemical mechanisms for condensed phase chemistry have not been identified. 

Moisture can significantly affect loose materials, particularly their flowability. Low temperatures, particle bridging, and caking can alter interparticle void fractions and cause dramatic changes in bulk density. Moisture becomes bound to solids because of mechanical, physicochemical, and chemical mechanisms. Moisture retained... 

Natural mobilization includes chemical, mechanical, and biological weathering and volcanic activity. In chemical weathering, the elements are altered to forms that are more easily transported. For example, when basic rocks are neutralized by acidic fluids (such as rainwater acidified by absorption of CO2), the minerals contained in the rocks can dissolve, releasing metals to aqueous solution. Several examples are listed below of chemical reactions that involve atmospheric gases and that lead to the mobilization of metals ... 

Plisson, H. and M. Mauger (2001), Chemical and mechanical alteration of microwear polishes An experimental approach, Chimia 55, 931-937. 

To examine the effect of turbulence on flames, and hence the mass consumption rate of the fuel mixture, it is best to first recall the tacit assumption that in laminar flames the flow conditions alter neither the chemical mechanism nor the associated chemical energy release rate. Now one must acknowledge that, in many flow configurations, there can be an interaction between the character of the flow and the reaction chemistry. When a flow becomes turbulent, there are fluctuating components of velocity, temperature, density, pressure, and concentration. The degree to which such components affect the chemical reactions, heat release rate, and flame structure in a combustion system depends upon the relative characteristic times associated with each of these individual parameters. In a general sense, if the characteristic time (r0) of the chemical reaction is much shorter than a characteristic time (rm) associated with the fluid-mechanical fluctuations, the chemistry is essentially unaffected by the flow field. But if the contra condition (rc > rm) is true, the fluid mechanics could influence the chemical reaction rate, energy release rates, and flame structure. 

Other sorbent tubes are designed to trap vapors by a chemical mechanism. Chemical sorption is almost always accomplished by coating a solid sorbent or support with the desired reactant. In this way, the substance of interest is not only removed from the gas stream being sampled, but is altered chemically. This type of sorption has the advantages of being more selective and rendering reactive substances stable. Many variables may affect the ability of a sorbent to collect an analyte. Some of the relevant factors are discussed below. 

The adhesion to substrates may be increased by (i) mechanical alterations of the substrate, (ii) polar interactions with the bonding agent, or (iii) chemical bonding facilitated by an adhesion promoter [1], Although both mechanisms (i) and (ii) are effective in improving adhesion, this paper will focus on the use of chemical bonding (iii). 

Tumor promoters alter pathways that cooperate with the genetic changes present in the initiated cell to produce the selective growth of that cell. Generic mechanisms of tumor promotion are shown in Table 24.6. Although the precise molecular/bio-chemical mechanisms of tumor promotion are unknown, we do know that certain receptors, growth factors, transcription factors, cytokines, and kinases are involved in tumor promotion induced by a particular tumor promoter. 

Experimental evidence strongly suggests that material removal in chemical-mechanical polishing (CMP) processes is a result of one or more chemical steps that alter the wafer surface combined with a mechanical step that removes the altered material. Chemical action by itself also removes material by static etching, but generally at a much lower rate than is observed when mechanical action is also present. Similarly, polishing rates observed when a minimally reactive fluid such as water is used instead of slurry are also low. Both chemical and mechanical processes are therefore involved in material removal at commercially practical rates, and the model we describe reflects this dual nature of the process. 

It is well known that good flame velocity results do not mean that the chemical mechanism is correct. Similarly, lists of reactions with high flame velocity sensitivity do not show all the important parameters of a flame mechanism. Tuning of rate parameters having high-flame velocity sensitivities would alter significantly the flame propagation speed, but there are other parameters in the mechanism which have to be known precisely for the calculation of realistic concentration vs. distance profiles. 

If the sponge is left to dry in the sun, this adsorbed water will evaporate, leaving only a small proportion of water bound chemically to the salts and to the cellulose of the sponge fibers. Like water in sponge, water is held in food by various physical and chemical mechanisms (Table 3.1). It is a convenient oversimplification to distinguish between free and bound water. The definition of bound water in such a classification poses problems. Fennema (1985) reports seven different definitions of bound water. Some of these definitions are based on the freezability of the bound component, and others rely on its availability as a solvent. He prefers a definition in which bound water is that which exists in the vicinity of solutes and other non-aqueous constituents, exhibits reduced molecular activity and other significantly altered properties as compared with bulk water in the same system, and does not freeze at -40"C."... 

Mechanisms by which chemicals may alter carcinogenic response... 

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