Homogeneous nucleation, See

Homogeneous Nucleation a process by which gases interact and combine with droplets made up of their own kind. For instance, the collision and subsequent adherence of water vapor to a water droplet is a homogeneous nucleation. See nucleation. 

Fig. 5 Schematic plot showing reported crystallization temperatures for PEO in the bulk and as a component of block copolymers of varying compositions. The morphology of the PEO block is indicated on the x-axis. The filled bars are for data where isothermal crystallization measurements were performed and Avrami indexes of 1 or less were reported. The horizontal lines indicate the maximum temperature range that can be associated with PEO homogeneous nucleation, see text...

Nucleation a process by which a gas interacts and combines with droplets. See homogeneous nucleation. 

Fig. 2 Optical microscopy image of a small section of a poly(ethylene oxide) (PEO) droplet dispersion sample, see text (1000-mm wide) obtained at Tc = - 2.6 °C. Amorphous droplets appear dark and semicrystalline droplets appear white under nearly crossed polarizers. The plot shows the fraction of crystallized droplets as a function of temperature upon cooling (0.4 °C min-1) for homogeneous nucleation. (Reprinted with permission from [84], Copyright 2004 by the American Physical Society)...

In this article, we suggest that a modified superheated-liquid model could explain many facts, but the basic premise of the model has never been established in clearly delineated experiments. The simple superheated-liquid model, developed for LNG and water explosions (see Section III), assumes the cold liquid is prevented from boiling on the hot liquid surface and may heat to its limit-of-superheat temperature. At this temperature, homogeneous nucleation results with significant local vaporization in a few microseconds. Such a mechanism has been rejected for molten metal-water interactions since the temperatures of most molten metals studied are above the critical point of water. In such cases, it would be expected that a steam film would encapsulate the water to... 

A modified superheat theory was proposed by Shick to explain molten salt (smelt)-water thermal explosions in the paper industry (see Section IV). (Smelt temperatures are also above the critical point of water.) In Shick s concept, at the interface, salt difiuses into water and water into the salt to form a continuous concentration gradient between the salt and water phases. In addition, it was hypothesized that the salt solution on the water side had a significantly higher superheat-limit temperature and pressure than pure water. Thicker, hotter saltwater films could then be formed before the layer underwent homogeneous nucleation to form vapor. 

Several investigators studied R-12. Holt and Muenker (1972) and Rausch and Levine (1973) made simple spills of this cryogen into water. The highest water temperature used by both teams was —342 K and weak explosions were noted. From Table XVI, it can be seen that this water temperature was barely within the range of the superheat-limit temperature, so no or only minor explosions might have been expected. Henry et al. (1974) spilled R-12 on top of a hot mineral oil. For oil temperatures less than about 409 K, there was little interaction except rapid boiling. Above 409 K, explosions resulted. Henry et al. state that this oil temperature would lead to an interface temperature [see Eq. (1)] close to the expected homogeneous nucleation temperature (—345 K) so that the explosions were to be expected. 

From nucleation theory (see Section IX), one can estimate the expected rate of formation of critical-sized vapor embryos in a liquid as a function of temperature. This rate is a very strong function of temperature emd changes from a vanishingly low value a few degrees below the homogeneous nucleation temperature to a very large value at this temperature. 

The condensation of a low-vapor-pressure species to form a new particle is known as homogeneous nucleation. Recall that the vapor pressure of a substance over the curved surface of a droplet is greater than over a flat surface of the same substance (e.g., see... 

Note that this predicts critical concentrations that are more than an order of magnitude below the experimentally observed concentrations of Wyslouzil et al. (1991) shown in Fig. 9.30 (see Problem 3). For a recent treatment of the binary homogeneous nucleation of H2S04 and H20, see Kulmala et al. (1998). 

The atmospheric situation is complicated by varying conditions of temperature, relative humidity, and concentrations of other gases such as NH3 which can enhance nucleation rates over those expected for a well-mixed air mass at a fixed temperature and RH (e.g., see Nilsson and Kulmala, 1998). However, there is a general consensus that the observed rates of nucleation of H2S04 often, indeed usually, exceed those expected from classical binary homogeneous nucleation theory. (Note that this is not always the case. For example, Pirjola et al. (1998) reported that the measured formation of nuclei in the Arctic boundary layer... 

As we have seen in our earlier discussion of the size distribution of tropospheric particles, the chemical components are not generally distributed equally among all sizes but, rather, tend to be found in specific size ranges characteristic of their source. Generally, the smallest ultrafine particles are produced by homogeneous nucleation and hence tend to contain secondary species such as sulfate and likely organics (see Section A.2). Particles in the Aitken nuclei range are produced... 

CD can occur either by initial homogeneous nucleation in solution or by het-eronucleation on a substrate, depending on the deposition mechanism (see Chapter 3). For this reason, we consider both types of nucleation. 

The homogeneous nucleation of martensite in typical solids is too slow by many orders of magnitude to account for observed results. Calculations of typical values of AQc using the classical nucleation model of Section 19.1.4 (see Exercise 19.3) yield values greatly exceeding 76 kT. Furthermore, nearly all martensitic transformations commence at very sparsely distributed sites. Small-particle experiments [14] have yielded typical nucleation densities on the order of one nucleation event per 50 pm diameter Fe-Ni alloy powder particle.3 Thus, nucleation of martensite is believed to occur at a small number of especially potent heterogeneous nucleation sites. 

Thus in the emulsifier-free emulsion copolymerization the emulsifier (graft copolymer, etc.) is formed by copolymerization of hydrophobic with hydrophilic monomers in the aqueous phase. The ffee-emulsifier emulsion polymerization and copolymerization of hydrophilic (amphiphilic) macromonomer and hydro-phobic comonomer (such as styrene) proceeds by the homogeneous nucleation mechanism (see Scheme 1). Here the primary particles are formed by precipitation of oligomer radicals above a certain critical chain length. Such primary particles are colloidally unstable, undergoing coagulation with other primary polymer particles or, later, with premature polymer particles and polymerize very slowly. 

The class of methods used for preparing colloidal dispersions in which precipitation from either solution or chemical reaction is used to create colloidal species. The colloidal species are built up by deposition on nuclei that may be of the same or different chemical species. If the nuclei are of the same chemical species, the process is referred to as homogeneous nucleation if the nuclei are of different chemical species, the process is referred to as heterogeneous nucleation. See also Dispersion Methods. An empirical or qualitative term referring to the relative ease with which a material can be deformed or made to flow. It is a reflection of the cohesive and adhesive forces in a mixture or dispersion. See also Atterberg Limits. 

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