Repeated nucleation

M. J. Uttormark, J. W. Zanter, J. H. Perepezko. Repeated nucleation in an undercooled aluminum droplet. J Cryst Growth 777 258, 1997. 

Cavitation is a form of corrosion that is caused by the repeated nucleation, growth, and violent collapse of vapor bubbles in a liquid against a metal surface. Cavitation erosion arises when a solid and fluid are in relative motion, and bubbles formed in the fluid become unstable and implode against the surface of the solid. Cavitation erosion is similar to surface wear fatigue.75 A metal having undergone a cavitation erosion has an appearance similar to a pitted metal.31... 

It is observed that the normal craze fibril structure can be observed just behind the craze tip where the craze is as thin as 5—lOnm . This observation was difficult to reconcile with early models of craze tip advance which postulated that this occurred by repeated nucleation and expansion of isolated voids in advance of the tip. One problem was to explain how the void phase became interconnected while the craze was still so thin. Another was that the predicted kinetics of craze growth appeared to be incorrectly predicted indeed since this mechanism almost involves the same steps as the original craze nucleation, it is hard to understand how craze growth could be so much faster than craze nucleation as observed experimentally. 

The motion of domain walls in ferroelectrics is not simple. In an electric field a 180° wall in BaTiOs appears to move by the repeated nucleation of steps by thermal fluctuations along the parent wall. Domains misoriented by 180° tend to switch more easily than 90° domain walls since no net physical deformation is required domains misoriented by 90° are inhibited from changing orienta-... 

The above mechanism was consistent with several experimental observations. MgO was known to be preferentially formed on the surface of Al-Mg alloys under many circumstances, as described earlier, either because of cation demixing in spinel, to form periclase, or due to rapid diffusion of Mg in the solid state and subsequent surface segregation. The single crystal oxidation product clearly precluded any mechanism that involved repeated nucleation of corundum (alumina) grains, which in any event is known to he difficult at temperatures below 1200°C. Finally, the presence of a film of molten alloy at a distance of about 1 pm from the surface accounted for the relative absence of a growth rate dependence on composite thickness (a rather slow decrease is actually observed). 

In the sections that follow, we consider the kinematics and kinetics of plastic flow by repeated nucleation of STs, primarily in metallic glasses and amorphous silicon as key examples. Similar corresponding applications to glassy polymers are covered in Chapter 8. 

FIG. 1 Schematic drawing of the temperature-dependent microslructure of a mixture of water, oil, and a nonionic surfactant. The temperature-induced phase sequence from lamellar to droplet phase microemulsion to two-phase microemulsion is shown. In the 20 region constant heating leads to a decrease in the average size of droplets in a stepwise manner. This is due to the process of repeated nucleation and growth, as indicated by the arrow below the schematically drawn test tubes. The mechanism is discussed in detail in Section III. 

Polypropylene molecules repeatedly fold upon themselves to form lamellae, the sizes of which ate a function of the crystallisa tion conditions. Higher degrees of order are obtained upon formation of crystalline aggregates, or spheruHtes. The presence of a central crystallisation nucleus from which the lamellae radiate is clearly evident in these stmctures. Observations using cross-polarized light illustrates the characteristic Maltese cross model (Fig. 2b). The optical and mechanical properties ate a function of the size and number of spheruHtes and can be modified by nucleating agents. Crystallinity can also be inferred from thermal analysis (28) and density measurements (29). 

In the secondary nucleation stage, the remaining amorphous portions of the molecule begin to grow in the chain direction. This is schematically shown in Fig. 16. At first, nucleation with the nucleus thickness /i takes place in the chain direction and after completion of the lateral deposition, the next nucleation with the thickness k takes place, and this process is repeated over and over. The same surface nucleation rate equation as the primary stage can be used to describe these nucleation processes. 

It should be noted that the critical nucleation process does not depend on M. This can be explained by our model of surface diffusion (Fig. 27). In the model a nucleus will be formed from the absorbed chains. We can estimate the number of repeating units within a critical nucleus (N ) using parameters a, ae, and Ah given in [14]. N is the order of 102-103 for the range of AT in our experiment, which is much smaller than the number of repeating units within a molecule (103-104). This indicates that a critical nucleus should be formed by a part of a molecular chain. Therefore, the nucleation process of the critical nucleus will not depend on M. Thus, it is a natural result that B does not depend on M in this study. This is consistent with the discussion by Hoffman et al. [28] on FCC. They showed that the nucleation process of an FCC does not depend on Mn in the case of Mn > 104. On the contrary they showed that it depends on Mn for Mn < 104, because ae depends on Mn due to the effect of chain ends on the end surface of the critical nucleus. 

They have many of the morphological and ultrastructural characteristics of disease filaments [11, 12] (Fig. 45-5). Assembly is a nucleation-dependent process that occurs through its amino-terminal repeats. The carboxy-terminal region, in contrast, is inhibitory. Assembly is accompanied by the transition from random coil to a [3-pleated sheet. By electron diffraction, a-synuclein filaments show a conformation characteristic of amyloid fibers. Under the conditions of these experiments, P- and y-synucleins failed to assemble, consistent with their absence from the filamentous lesions of the human diseases. When incubated with a-synuclein, P- and y-synucleins inhibit the fibrillation of a-synuclein, suggesting that they may indirectly influence the pathogenesis of Lewy body diseases and multiple system atrophy. 

Jiang, Y., Li, H., Zhu, L., Zhou, J. M., and Perrett, S. (2004). Amyloid nucleation and hierarchical assembly of Ure2p fibrils. Role of asparagine/glutamine repeat and nonrepeat regions of the prion domains./. Biol. Chem. 279, 3361-3369. 

Fig. 38 General mechanism of the nucleation and elongation stages of polymerization generating a helical structure (the arrows represent the interactions among repeating units). Secondary interactions light arrows), absent in the first turn of the helix, are the molecular origins of a less favorable nucleation event (i.e.,the critical chain length) beyond which propagation becomes more favorable...

Initiation of reverse transcription in HIV-infected cells relies on a critical RNA-RNA interaction between tRNA y s, which is preferentially packaged into the viral particle, and a specific viral RNA seqnence. The 3 -terminaI 18 nucleotides of tRNA y are complementary to the primer binding site (PBS) sequence located in the 5 -Iong terminal repeat (LTR) of the viral RNA genome (Figure 10.3). The UUU anticodon of the tRNA is complementary to and binds to an adenosine rich loop located 8 nucleotides upstream (5 ) of the PBS. This RNA-RNA duplex which is formed when tRNA y s binds to the PBS fits within the active site of HIV-1 reverse transcriptase, bnt mnitiple interactions between the viral RNA and tRNA y are necessary for efficient initiation of reverse transcription. This interaction nucleates the reverse transcription complex which contains viral RNA, reverse transcriptase, tRNA y pl , nncleocapsid p7, and Vpr (Viral protein R), as well as multiple host factors." ... 

A plot of the In n versus L yields a straight line where the slope is - 1/G t and the intercept is n (the population density of nuclei). Since the MSMPR is at steady state, the supersaturation is known. This experiment can then be repeated at a number of different supersaturations and fit to growth and nucleation expressions of the form below ... 

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