Experimental Work on Particle Nucleation

Cavitation bubbles work as nucleation sites of particles. For example, in a supercooled sucrose solution, nucleation of ice crystals induced by cavitation bubbles has been experimentally observed [72], This phenomenon has been called sonocrys-tallization [73]. Although there are some papers on the mechanism of sonocrystal-lization, it has not yet been fully understood [74, 75]. It has been reported that the distribution of crystal size in sonocrystallization is narrower than that without ultrasound [73]. It may be related to the narrower size distribution of sonochemi-cally synthesized particles compared to that without ultrasound [76, 77]. Further studies are required for the mechanism of particle nucleation by ultrasound. 

Regarding nucleation of solid particles from liquid droplets (antisolvent, nebulization, PGSS, microencapsulation processes), it is obvious that the droplets size is a basic parameter in the process. Some investigators reported results of experimental works supported by theoretical considerations on droplet formation by injection of a liquid into a pressurized fluid using a simple capillary nozzle (78) or a coaxial nozzle (79). Both articles, following previous works on liquid droplets formation into an insoluble liquid or into a gas, suggest the correlation of the experimental results with two dimensionless numbers... 

Moreover, the effect of the volume of particle nuclei on the Rayleigh light scattering intensity overrides that of the concentration of particle nuclei, and this is reflected in the Rgo versus time profiles shown in Figure 3.9. Again, these experimental data demonstrate the important role of polarity of monomers in the particle nucleation period, and they are consistent with the theoretical work of Song and Poehlein [47,48]. 

The result obtained by the transmission electron microscope observations of the freeze-fracture replica on trehalose solutionssuggested that the ice-crystal nucleation is enhanced in the solution with higher trehalose concentration. Based on these experimental results and assuming a similar nucleation process between ice and clathrate hydrates, we can speculate that trehalose, as a kinetic inhibitor, may not inhibit the nucleation but may work as an anti-agglomerant, keeping small hydrate particles dispersed as they form. 

The overview provided above covers the classical perspective on crystallization and particularly nucleation. A rapidly growing body of work shows that this view is not complete. The alternative model for nucleation is often termed the two-step nucleation theory. In brief, this model sng-gests that nucleation is a multistep process where the first step involves phase separation via the formation of liquid or amorphous nanoparticles. This is then followed by crystallization within this particle. The activation energy for each of these steps is relatively small, and it is expected that the overall process would be faster when compared with a single-step (classical) process with the same overall activation energy. Experimental and theoretical evidence supports this mechanism, and it has been suggested that the nucleation process is likely to proceed in this way in most, if not all, cases. ... 

Unzueta and Forcada [31] studied the emulsion copolymerization of methyl methacrylate and n-butyl acrylate. It was assumed that both micellar nucle-ation and homogeneous nucleation are operative in this emulsion polymerization system. Based on the experimental data and computer simulation results, the values of the free radical capture efficiency factors for monomer-swollen micelles (f ) and polymer particles (Fj) that serve as adjustable parameters in the kinetic modeling work are approximately 1(T and 10, respectively. The reason for such a difference in the free radical capture efficiency factors is not available yet. Table 4.2 summarizes some representative data regarding the absorption of free radicals by the monomer-swollen micelles and polymer particles obtained from the literature. 

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