Nucleation active sites

Cracking tendency, crystallization temperature, in situ atmosphere, active nucleation sites. 

Figure 14.15 Mechanism of PET nucleation by sodium ionomers. A particular advantage of such compounds is that they provide active nucleation sites without molecular chain scission...

Activation of latent sites — The theory accounts for non-stationary effects due to the appearance and disappearance of active nucleation sites on the electrode surface as a result of chemical and/or electrochemical reactions parallel to the process of nucleus formation. Under such circumstances the time dependence of the number N(t) of nuclei is given by a second-order differential equation ... 

The function JAS (f)A.r under the integral gives the differential increase of the surface occupied by new island patches nucleated in the time interval between t and r + dr. There are cases, however, particularly in the presence of a limited number of very active nucleation sites, where nuclei are formed immediately after pulse application only. This case is known as instantaneous nucleation in contrast to the previous case which is known as progressive nucleation (cf. Section 8.7). If Zq is the... 

Surface nucleation processes can be described with two quantities surface nucleation density, Nj (cm ), and surface nucleation rate, (cm h" ). The nucleation density is the number of nuclei grown on unit substrate surface, and the nucleation rate is the number of nuclei formed per unit substrate surface in unit time. The nucleation density depends on the number of activated nucleation sites available on the substrate surface. Nucleation will stop when crystals have nueleated on all available nucleation sites or when the diffusion zones of nuclei overlap eaeh other (as discussed above), whichever occurs first. Both the nucleation density and rate are determined by substrate surface conditions and deposition parameters. 

FIGURE 15.16 Number density of active nucleation sites as a function of site radius calculated from the wall superheat using Eq. 15.14 (from Klausner et al. [34], with permission from ASME). 

Effect of the Heat Transfer Surface. As we saw from the discussion on pages 15.9-15.18, the number of active nucleation sites on the surface at a given wall superheat and heat flux depends on a variety of factors. First, there is the population of potentially active sites, which is a function of the nature and preparation of the surface. Second, there are the wetting characteristics associated with the fluid/surface combination. These characteristics are often expressed in terms of the contact angles (<( , <]> , and < >,). In general, the heat flux (or heat transfer coefficient) in nucleate boiling heat transfer is strongly dependent on the number of active sites. 

For multicomponent mixtures, the mechanisms are even more complex, involving mass transfer in both liquid and vapor phases. Detailed measurements of bubble frequency (/), bubble departure diameter (dd), and number of active nucleation sites ((V ) are reported by Bier and Schmidt [127] for the propane/n-butane mixtures studied, the bubble departure diameter initially increases (relative to its value for n-butane) with increasing mole fraction of propane and then decreases to a value less than that for pure propane before increasing rapidly with concentration and propane mole fractions greater than about 0.9. Ilie bubble frequency shows the opposite trends. The number of active sites (Na) passes through a minimum as the mole fraction of propane is increased, the maximum reduction being around a factor of 3. It is clear, therefore, that the effects of having a multicom-... 

L The number of active nucleation sites is estimated as a function of heat flux using relationships for heterogeneous nucleation similar to those given earlier. 

C. H. Wang and V. K. Dhir, On the Prediction of Active Nucleation Sites Including the Effect of Surface Wettability, in Pool and External Flow Boiling, V. K. Dhir and A. E. Bergles eds., ASME, New York, 1992. 

Little has yet been achieved in the way of making an explicit identification of these active nucleation sites, though materials like silver iodide are known to show heterogeneous surface effects in water vapour adsorption which may be related to their ice nucleation behaviour (Zettlemoyer et al. 1963). Other recent... 

In addition to the boiling curve measurements, the bubble dynamics have been photographed along the entire heated surface of the platinum wire at a saturated boiling heat flux of 0.358 + 0.006 W.mm for the Natrosol 250 HHR and Separan AP-30 solutions. Both polymer solutions have been tested only at a relative viscosity of 1.08. Slow motion films of the bubble dynamics have been analyzed to determine the average number density of active nucleation sites, and the frequency distribution of bubble departure diameters. 

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