Shear nucleation

Shear nucleation caused by flow across a ciystal surface. 

STUDY OF SHEAR NUCLEATION THEORY IN CONTINUOUS MICROCELLULAR FOAM EXTRUSION... 

A theoretical analysis based on shear energy was carried out with the aim of illustrating the mechanism of shear nucleation through a slit channel die in continuous microcellular foam extrusion. The shear energies for different positions and for different experimental conditions were calculated to demonstrate the various contributions to the surface free energy required for bubble nucleation. 14 refs. 

Figure 3 Polarizing optical micrographs of poly(ethylene terephthalate) (PET) crystallized at 240°C in the absence (A) and in the presence (B) of a shearing. As a consequence of the shearing, nucleation becomes increasingly profuse, and the shape of the spherulites becomes elliptical. (From ref. 11)...

FIG. 21-165 Two-stage continuous granulation process, consisting of high-shear nucleation (pin mixer) and moderate shear growth and consolidation (plough shear). [Mart et al.. Powder Technoh, 117,173 (2003).]... 

Fluid-shear nucleation. This type is known to take place under certain conditions and is suspected in others. When supersatured solution moves past the surface of a growing crystal at a substantial velocity, the shear stresses in the boundary layer may sweep away embryos or nuclei that would otherwise be incorporated into the growing crystal and so appear as new crystals. This has been reported in work on sucrose crystallization. It also has been demonstrated in the nucleation of MgS04 7H20, if the solution is subjected to shear at the crystal face at one supersaturation and then quickly cooled to a higher supersaturation and allowed to stand while nuclei grow to macroscopic size. 

Abuzaina, F., Fitz, B. D., Andjelic, S. and Jamiolkowski, D. D., Time resolved study of shear-induced crystallization of poly(p-dioxanone) polymers imder low-shear, nucleation-enhancing shear conditions by small angle Ught scattering and optical microscopy. Polymer, 43, 4699, 2002. 

Guo, M. C. and Y. C. Peng. 2003. Study of shear nucleation theory in continuous microcellular foam extrusion. Polym Test 22 705-9. 

Secondary nucleation is the formation of nuclei attributable to the influence of the existing microscopic crystals in the magma. The first type of known secondary crystallization is attributable to fluid shear, the other due to collisions between already existing crystals with either a solid surface of the crystallizer or with other crystals themselves. Fluid shear nucleation occurs when liquid travels across a Crystal at a high speed, sweeping away nuclei that would otherwise be incorporated into a Crystal, causing the swept-away nuclei to become new crystals. Contact nucleation has been found to be the most effective and common method for nucleation. The benefits include the following... 

However, in the case of mini- and microemulsions, processing methods reduce the size of the monomer droplets close to the size of the micelle, leading to significant particle nucleation in the monomer droplets (17). Intense agitation, cosurfactant, and dilution are used to reduce monomer droplet size. Additives like cetyl alcohol are used to retard the diffusion of monomer from the droplets to the micelles, in order to further promote monomer droplet nucleation (18). The benefits of miniemulsions include faster reaction rates (19), improved shear stabiHty, and the control of particle size distributions to produce high soHds latices (20). 

Dick et al. [29] present additional data on the <100) shock compression of LiF which further establishes a threshold shear stress of between 0.24 GPa and 0.30 GPa for nucleation of dislocations in the shock front. 

Martensitic phase transformations are discussed for the last hundred years without loss of actuality. A concise definition of these structural phase transformations has been given by G.B. Olson stating that martensite is a diffusionless, lattice distortive, shear dominant transformation by nucleation and growth . In this work we present ab initio zero temperature calculations for two model systems, FeaNi and CuZn close in concentration to the martensitic region. Iron-nickel is a typical representative of the ferrous alloys with fee bet transition whereas the copper-zink alloy undergoes a transformation from the open to close packed structure. ... 

Onset of Nucleate Boiling in Conventional Size Channels where / = 2Tw/(pt/ ), tw is the wall shear stress. 

The kinetics and mechanisms of the C —> G transition in a concentrated solution of PS-fr-PI in the PS-selective solvent di-n-butyl phthalate was studied [137,149]. An epitaxially transformation of the shear-oriented C phase to G, as previously established in melts [13,50,150], was observed. For shallow quenches into G, the transition proceeds directly by a nucleation and growth process. For deeper quenches, a metastable intermediate structure appears, with scattering and rheological features consistent with the hexag-onally perforated layer (PL) state. The C -> G transition follows the same pathways, and at approximately the same rates, even when the initial C phase is not shear-oriented. 

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