Instability growth, mechanism

The first steps involve coordination and cycloaddition to the metal. Insertion of a third molecule of ethene leads to a more instable intermediate, a seven-membered ring, that eliminates the product, 1-hexene. This last reaction can be a (3-hydrogen elimination giving chromium hydride and alkene, followed by a reductive elimination. Alternatively, one alkyl anion can abstract a (3-hydrogen from the other alkyl-chromium bond, giving 1-hexene in one step. We prefer the latter pathway as this offers no possibilities to initiate a classic chain growth mechanism, as was also proposed for titanium [8]. The byproduct observed is a mixture of decenes ( ) and not octenes. The latter would be expected if one more molecule of ethene would insert into the metallocycloheptane intermediate. Decene is formed via insertion of the product hexene into the metallo-cyclopentane intermediate followed by elimination. 

Lowenhaupt and Hellmann (1991) have determined whether microphase separation or macrophase separation occurs in blends of a PS-PMMA diblock with PMMA homopolymer with a < 1 and a > 1 using TEM. They found that the transition between purely microphase separation and macrophase separation occurs for a lower diblock content for blends with a smaller a, as supported by calculations of the instability limit using the random phase approximation. Blends with a < 1.4 were always initially microphase separated, although in a blend with a - 1.4 this was followed by macrophase separation. However, the macrophase-separated structure took the form of aggregates of micelles (see Fig. 6.1), suggesting a nucleation and growth mechanism for the secondary... 

Stability is an essential condition for PFC emulsions to be of practical use. The principal mechanism for irreversible droplet growth in submicronic PFC emulsions during storage is molecular diffusion (also known as Ostwald ripening or isothermal distillationj.P Coalescence may contribute to instability when mechanical stress is applied and at higher temperatures, as during heat sterilization. Sedimentation and flocculation are fully reversible and pose no problem. 

R. L. Holmes, G. Dimonte, B. Fryxell, M. L. Gittings, J. W. Grove, M. Schneider, D. H. Sharp, A. L. Velikovich, R. P. Weaver and Q. Zhang, Richtmyer-Meshkov Instability Growth Experiment, Simulation and Theory , Journal of Fluid Mechanics 9, 55-79 (1999). 

As an example, consider the study by Banerjee et al. (1984) on the effect of electron irradiation on the order-disorder transformation in (DIJ Ni4Mo. Electron micrographs and diffraction patterns were obtained during in situ electron irradiations at 50-1050 K in a HVEM. At temperatures below 200 K, the alloy completely disorders. At 200-450 K, only SRO was observed, and the transition between LRO and SRO, which occurs via the completely disordered state, is consistent with the concentration-wave description of the SRO structure and supports the concept of spinodal ordering. It is believed that an interstitial mechanism is responsible for maintaining the SRO. Above 450 K, LRO persisted for samples initially in this state and SRO was only preserved up to 550 K for samples initially in that state. Between 550 and 720 K, a mixed SRO-LRO state occurred, and at temperatures above 720 K a complete transition to SRO was obtained. It is believed that maintenance of LRO requires a vacancy mechanism. At temperatures below 800 K the SRO-LRO transition occurred in a continuous fashion, while above 800 K a nucleation and growth mechanism was operative. This behavior is characteristic of an ordering transition of the first kind below and above the coherent instability temperature. 

Growth inhibition by TGF- 3, associated with inhibition of c-myc, cdks, reduction in cyclin D1 levels, and inhibition of cdk-4-associated Rb kinase activity, as well as induction of cdk inhibitors pi5 and p27, has been noted in intestinal epithelial cells. Loss of responsiveness to growth inhibition from TGF- 3 occurs in many cell types including breast, colorectal carcinoma, and pancreatic carcinoma cells. Mutational inactivation of T 3RH represents one mechanism of this process, which in many cases, leads to the development of gastrointestinal cancer. Thirteen percent of colorectal carcinomas are thought to be associated with a replication error (RER) or microsatellite instability phenotype. Subsequent inactivation of T 3RII and... 

The pressure waves when reflected from the top wall interact with the flame again and cause a destabilizing effect on the flame front. The flame is accelerated toward a denser medium and the growth of the perturbations thus turbulizes the flame front via Rayleigh-Markstein instability mechanism. 

A second application of current interest in which widely separated length scales come into play is fabrication of modulated foils or wires with layer thickness of a few nanometers or less [156]. In this application, the aspect ratio of layer thickness, which may be of nearly atomic dimensions, to workpiece size, is enormous, and the current distribution must be uniform on the entire range of scales between the two. Optimal conditions for these structures require control by local mechanisms to suppress instability and produce layer by layer growth. Epitaxially deposited single crystals with modulated composition on these scales can be described as superlattices. Moffat, in a report on Cu-Ni superlattices, briefly reviews the constraints operating on their fabrication by electrodeposition [157]. 

Flat Sheets. Generally, the interface between a liquid sheet and air can be perturbed by aerodynamic, turbulent, inertial, surface tension, viscous, acoustic, or electrical forces. The stability of the sheet and the growth rate of unstable disturbances are determined by the relative magnitude of these forces. Theoretical and experimental studies 255112561 on disintegration mechanisms of flat sheets showed that the instability and wave formation at the interface between the continuous and discontinuous phases are the maj or factors leading to... 

As described previously, in the atomization sub-model, 232 droplet parcels are injected with a size equal to the nozzle exit diameter. The subsequent breakups of the parcels and the resultant droplets are calculated with a breakup model that assumes that droplet breakup times and sizes are proportional to wave growth rates and wavelengths obtained from the liquid jet stability analysis. Other breakup mechanisms considered in the sub-model include the Kelvin-Helmholtz instability, Rayleigh-Taylor instability, 206 and boundary layer stripping mechanisms. The TAB model 310 is also included for modeling liquid breakup. 

---