Nucleating point

The term, metal dusting, was first used about this time to describe the phenomenon associated with hydrocarbon processing. Butane dehydrogenation plant personnel noted how iron oxide and coke radiated outward through catalyst particles from a metal contaminant which acted as a nucleating point. The metal had deteriorated and appeared to have turned to dust. The phenomenon has been called catastrophic carburization and metal deterioration in a high temperature carbonaceous environment, but the term most commonly used today is metal dusting. 

Centrosomes, also called the microtubule organizing centre, are protein complexes that contain two cen-trioles (ringlike structures) and y- tubulin. They serve as nucleation points for microtubular polymerization and constrain the lattice structure of a microtubule to 13 protofilaments. They are responsible for organizing the mitotic spindle during mitosis. 

Dynamic crystal growth is initiated at several distant nucleation points, the individual monocrystalline areas grow in all directions until the front edges of neighboring crystals meet [138] Electrostatic interactions are the dominant binding forces [118,122]... 

Supercooling The hot water sample may have a greater tendency to supercool, because it has less dissolved gas which can act as nucleation points for ice to form. Water that does not supercool may form a thin layer of ice at the surface which can insulate the rest of the water from the freezer and delay the freezing process. 

Don t let this happen to you. Use a brand new boiling stone every time you have to boil a liquid. A close up comparison between a boiling stone and the inner walls of a typical glass vessel reveals thousands of tiny nucleating points on the stone where vaporization can take place, in contrast to the smooth glass surface that can hide unsightly hot spots and lead to BUMPING, a massive instantaneous vaporization that will throw your product all over. 

In order to understand better what happens when a nucleation point, say x = Xo, is selected, let us focus on the small time behavior of the nontrivial self-similar solution. Consider a solution (2.5) at time t = At. It is convenient to parametrize the functions w(x,Ar) and v x,At) by x and present them as a curve in the (w,v) plane. It is not hard to see that one then obtains a loop, beginning and ending in a point (Wo,0) (see Fig. 8b) the details of the loop depend, of course, on the fine internal structures of shocks and kinks (see Fig. 8a). 

Phosphatidylinositol bisphosphate is hydrolyzed to yield two intracellular messengers, diacylglycerol and inositol 1,4,5-trisphosphate. Phosphatidylinositol 3,4,5-trisphosphate is a nucleation point for supramolecular protein complexes involved in biological signaling. 

A full analysis of the assembly process resulted in a growth model, in which a dimer acts as a nucleation point for further growth in two dimensions as a result of the strong, hydrophobic interactions between the large naphthalene surfaces forming a so-called dimeric... 

In the same samples, a second absorption feature was detected that is associated with the dopant ions themselves. These ligand-field transitions allow distinction among various octahedral and tetrahedral Co2+ species and are discussed in more detail in Section III.C. The three distinct spectra observed in Fig. 4(b) correspond to octahedral precursor (initial spectrum), tetrahedral surface-bound Co2+ (broad intermediate spectrum), and tetrahedral substitutional Co2+ in ZnO (intense structured spectrum). Plotting the tetrahedral substitutional Co2+ absorption intensity as a function of added base yields the data shown as triangles in Fig. 4(b). Again, no change in Co2+ absorption is observed until sufficient base is added to reach critical supersaturation of the precursors, after which base addition causes the conversion of solvated octahedral Co2+ into tetrahedral Co2+ substitutionally doped into ZnO. Importantly, a plot of the substitutional Co2+ absorption intensity versus added base shows the same nucleation point but does not show any jump in intensity that would correspond with the jump in ZnO intensity. Instead, extrapolation of the tetrahedral Co2+ intensities to zero shows intersection at the base concentration where ZnO first nucleates, demonstrating the need for crystalline ZnO to be... 

The notion that folding proceeds from a condensed state rather than from a random coil state is somewhat heretical in an age dominated by the secondary structure nucleation point of view. However, the question has not been addressed experimentally, and the Dill model is in accord with the thermodynamics of unfolding for many globular proteins. 

Two short pathways that link the a-helical and /3-hairpin macrostates without making use of microstates with an instantaneous temperature above 488K are shown in Fig.5.1. The path shown in Fig.5.1 (upper) involves the unwinding of both ends of the helix, leaving approximately one turn of helix in the middle of the molecule. This turn then serves as a nucleation point for the formation of the /3-turn, which is stabilized by hydrophobic interactions between the side chains of Y45 and F52. The native hydrogen bonds nearest to the turn then form, after which the remainder of the native hairpin structure forms. This pathway is similar to previously proposed mechanisms for the folding of the G-peptide /3-hairpin from a coil state, which emphasize the formation of hydrophobic contacts before hydrogen bond formation [17,18, 140-143] and the persistence of the /3-turn even in the unfolded state [143]. 

The foregoing discussion serves to show that disordered carbon structures are oxidized more readily than well-ordered graphite planes and that dislocations and active sites provide nucleation points for attack of the carbon crystallite. Another factor that must be considered is that dispersed electrocatalysts, such as platinum, on the carbon surface are not benign. The electrocatalysts interact with the carbon causing local oxidation or corrosion, i.e., the platinum catalyzes the corrosion of the carbon itself. In the presence of oxygen, which is the condition under which the electrocatalyst will operate, reduction intermediates from the oxygen (e.g., HOj) can have an accelerated corrosion effect. 

One mechanism for surface area loss is crystallite migration, for which Kinoshita et al.66 concluded that the mechanism of surface area loss was two-dimensional Ostwald ripening by means of ad-atom migration on the carbon surface. Nevertheless, trap sites for the migrating ad-atoms on the surface of the carbon can produce nucleation points for generation of... 

The Si02 walls are built up by condensation processes, for which the anchored silicate species act as nucleation points. The enrichment of the oxide at the templating crystals occurs by an Ostwald ripening mechanism the particles grow in size at the tube walls and highly soluble very small silica particles re-dissolve to the reaction solution. The polycondensation reaction is finished within about I2h. 

In general, the initiation of the precipitation process may result from the presence of particulate matter in the bulk water that seeds the crystallization. The process is usually termed heterogeneous nucleation. It is possible for homogeneous nucleation to occur when the nucleation is spontaneous. Once nucleation has occurred, crystals can grow, provided that the solution is supersaturated. Suitable nucleation points on the heat transfer surface facilitate deposit formation on the surface. In turbulent flow, it is possible that crystallites that are formed in the bulk fluid may be carried into regions, where they can redissolve. 

Circular volcano -shaped deposits can occur as a result of bubble formation in connection with boiling heat transfer and these deposits can act as nucleation points for further evaporation and deposition. As the process of deposition continues, the surface conditions will be modified and the scale may contain fissures, so that steam formation may occur within the deposit. The process is generally referred to as wick boiling that may influence the mechanism of heat transfer. The process of scale formation under boiling conditions is complex. 

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