Surface-induced nucleation

Table 1 Examples of surface-induced nucleation and growth of fibrils... 

There has been little work done to date on the nature of the three-phase substrate-solid-liquid intersection, and there is little guidance in predicting the magnitude of the equilibrium contact angle 9 that enters into classical nucleation theory in the factor f(0). Considerably more work has been done on the contact angle in substrate-liquid-vapor equilibrium, which determines the wetting properties of the liquid for the substrate. It is possible that some of the techniques developed for that problem may be applicable to the study of surface-induced nucleation of crystallization. 

In conclusion, it is evident from Figs. 22 and 23 that the rate of heat-transfer increases with increasing interfacial turbulence, irrespective of whether this turbulence is induced by high heat fluxes (as in Gordon s mercury surface), induced boiling by artificial nucleation sites (gas or solid contaminants), or mechanical agitation at the liquid-liquid interface. 

Finally, shear viscosity is strongly affected by the clay in the blends, especially at high PEN contents. A lubricating effect rather than a filler effect reveals the possibility that the clay is not well dispersed in the polymer blend, and migration of particles in the flow to the wall region can explain the observed reduction in shear viscosity. When MMT clay is mixed with crystallizable polymers such as polyesters, some processing problems arise because the crystallization process is modifled by nucleation effects induced by the nanoparticles. Moreover, these particles also influence the kinetics of transesteriflcation between PET and PEN, besides other factors such as the reaction time and extruder processing temperature. In Reference 72, a quaternary alkyl ammonium compound (Cl8) and MAH were used to modify the surface properties of the clay... 

PBT fibers nucleate fiber surface-induced crystallization of isotactic polypropylene. PBT fiber has the highest nucleating abihty of the three polyesters studied (PET, PTT, and PBT). Remarkable nucleating ability of PBT fibers is associated with epitaxial crystalU-zation. ... 

Boundary conditions of the type (2.7.12) are important in crystallization where secondary nucleation, as pointed out by Randolph and Larson (1988), may be governed by the growth rate of existing particles. For example, consider a well-mixed crystallizer where the number density is only a function of the sole internal coordinate selected as particle size x as represented by a characteristic length, which should satisfy a population balance equation of the type (2.7.6). Randolph and Larson discuss a variety of nucleation mechanisms and conclude that contact nucleation is the most significant form of nucleation. Thus, the mechanical aspects of the crystallizer equipment which provide contact surfaces contribute to increased nucleation rate. When growing crystals, containing adsorbed solute on their surfaces, come into contact with other solid surfaces, nucleation is induced. The boundary condition for the formation of new nuclei in a real crystallizer is therefore considerably more complicated than that implied by (2.7.7). Instead, the boundary condition must read as... 

IV. Crystaltization of the dispersed phase in the presence of a solidified matrix. In cases tike this, heterogeneous nucleation is induced by the solid surface and increased the nucleating activities. The dispersed phase may or may not show fractionated aystaUization depending on other factors as imposed by other heterogeneities. 

---