Level by diffusion

Polymerization proceeds in the polymer particles as the monomer concentration in the particles is maintained at the equilibrium (saturation) level by diffusion of monomer from solution, which in turn is maintained at the saturation level by dissolution of monomer... 

If electron transfer is dominated in fluid solution by reactants in close contact, those in close contact are quickly depleted and their statistical population is brought back to the equilibrium level by diffusion together of the reactants. As long as the timescale for the diffusional process is short compared with that for electron transfer, the equilibrium statistical distribution is maintained, equation (32) is valid, and the electron transfer rate constant is the product of and Kf or, as defined by Marcus, is given by equation (18). However, if the electron transfer process is sufficiently rapid, statistical equilibrium is not reached and the experimentally observed rate constant will include a contribution from the diffusion together of the reactants. When that situation exists, the experimentally observed rate constant kobs is given by equation (34) where ko is the diffusion limited rate constant,and is either or given by equation (18). ... 

Under these conditions the mixing process can be promoted only by the reduc-tion of the diffusion distances. Thus, the characteristic dimension (the diameter of the channel) directly influences the micromixing time, that is, the time necessary to achieve mixing on a molecular level by diffusion (Equation 4.17). 

Oxidation occurs at the surface. Deeper in the polymer mass, the oxygen supply is limited at a certain level by diffusion, therefore the oxygen uptake is dependent on the sample thickness. In articles with higher surface to volume ratios such as films and fibers, the physical properties deteriorate more rapidly upon oxidation. 

There is an intimate connection at the molecular level between diffusion and random flight statistics. The diffusing particle, after all, is displaced by random collisions with the surrounding solvent molecules, travels a short distance, experiences another collision which changes its direction, and so on. Such a zigzagged path is called Brownian motion when observed microscopically, describes diffusion when considered in terms of net displacement, and defines a three-dimensional random walk in statistical language. Accordingly, we propose to describe the net displacement of the solute in, say, the x direction as the result of a r -step random walk, in which the number of steps is directly proportional to time ... 

Histamine in the Blood. After its release, histamine diffuses rapidly into the blood stream and surrounding tissues (12). Histamine appears in blood within 2.5 min after its release, peaks at 5 min, and returns to baseline levels by 15 to 30 min. In humans, the diurnal mean of plasma histamine levels is 0.13 ng/g. In urine, elevations of histamine or metaboUtes are more prolonged than plasma elevations. Consequendy, abnormahties are more easily detected by urinary histamine assay. About one-half of the histamine in normal blood is in basophils, one-third in eosinophils, and one-seventh in neutrophils the remainder is distributed among all the other blood components. Increases in blood histamine levels occur in several pathological... 

The comparison of the results of very different methods has to be judged very precisely, as, e.g., the given thickness of a layer is a function of the limit of detection (EOD) of a method. Additionally, the detected areas vary from about 0.01 up to about 400 mm-. Therefore, the methods with a low level of detection and with a high sensitivity (high slope of the calibration function) give a higher value for the layer thickness. Furthermore, the layers are broadened with time by diffusion. 

Diffusion Flame. Wlien the fuel and oxidizer are initially unmixed and then mix in a thin region where the flame is located, the flame is called a diffusion flame (Figure 2). The word diffusion is used to describe the flame because the fuel and oxidizer are mixed on the molecular level by the random thermal motion of the molecules. An example of a diffusion flame is a candle flame or flares at an oil refinei y. 

If the major constituents of a solid alloy in contact with a liquid alloy are highly soluble in the latter without formation of compounds, progressive attack by solution is to be expected. If, on the other hand, a stable inter-metallic compound is formed, having a melting point above the temperature of reaction, a layer of this compound will form at the interface and reduce the rate of attack to a level controlled by diffusion processes in the solid state. By far the most serious attack, however, occurs in the presence of stresses, since in this case the liquid alloy, or a product of its reaction with the solid alloy, may penetrate along the grain boundaries, with resultant embrittlement and serious loss of strength. 

Regulation of neuropeptide expression is exerted at several levels. Control of neuropeptide function is mediated by factors controlling rates of prepropeptide gene transcription, translation, peptide degradation and secretion (Fig. 18-11). On the scale of seconds to minutes, peptide secretion is not always coupled lock-step with classical transmitter release (example above). Peptides are inactivated by diffusion and by proteolysis, so it would be expected that inhibition of specific extracellular proteases... 

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