Nucleus, phase particle

The growth stage of phase particle formation begins once a nucleus has exceeded the critical radius (r ). 

Crystal Formation There are obviously two steps involved in the preparation of ciystal matter from a solution. The ciystals must first Form and then grow. The formation of a new sohd phase either on an inert particle in the solution or in the solution itself is called nucle-ation. The increase in size of this nucleus with a layer-by-layer addition of solute is called growth. Both nucleation and ciystal growth have supersaturation as a common driving force. Unless a solution is supersaturated, ciystals can neither form nor grow. Supersaturation refers to the quantity of solute present in solution compared with the quantity which would be present if the solution were kept for a veiy long period of time with solid phase in contac t with the solution. The latter value is the equilibrium solubility at the temperature and pressure under consideration. The supersaturation coefficient can be expressed... 

During the red giant phase of stellar evolution, free neutrons are generated by reactions such as C(a,n) and Ne(a,n) Mg. (The (ot,n) notation signifies a nuclear reaction where an alpha particle combines with the first nucleus and a neutron is ejected to form the second nucleus.) The neutrons, having no charge, can interact with nuclei of any mass at the existing temperatures and can in principle build up the elements to Bi, the heaviest stable element. The steady source of neutrons in the interiors of stable, evolved stars produces what is known as the "s process," the buildup of heavy elements by the slow interaction with a low flux of neutrons. The more rapid "r process" occurs in... 

A subcritical aggregate having fewer subunit components than a nucleus. When this term is applied in the kinetics of precipitation, n refers to the number of subunits in a particle and n defines the number of subunits in a particle of critical size. This definition avoids confusion by distinguishing between subcritical (n < n subunits), critical (n = n subunits), and supercritical (n > n subunits) particle sizes. If a nucleus is defined as containing n n subunits, then an embryo contains n n subunits. Note that in this treatment, we are not using a phase-transition description to describe nucleation, and we are focusing on the smallest step in the process that leads to further aggregation. 

There is a distinct region of small aggregates or clusters which falls between the atomic (or molecular) domain and that of condensed matter. These small particles and clusters possess unique properties and have several technological applications. The formation of these particles involves a vapour-solid, a liquid-solid, a solid-solid or a vapour-liquid-solid type of phase change governed by nucleation and it is important that the size of the growing nucleus is controlled (Multani, 1981 Hadjipanyas Siegel, 1994). 

A recent study has been performed on various spray dried orange flavors(20% oil fixed in the powder) encapsulated on various Acacia gums and on mixtures of Acacia gums and malto-dextrins. The aromas in powder were tested by chromatography and then examined by electronic microscopy. This clearly shows the regular and uniform film covering each particle 10 to 40 microns in diameter. Prints of broken particles reveal numerous oil droplets (1 micron in diameter) spread in a gum nucleus. Protection of the oil phase then becomes fully effective. 

Crystallization occurs in two phases nucleation and particle growth. During nucleation, molecules in solution come together randomly and form small aggregates. Particle growth requires the addition of more molecules to a nucleus to form a crystal. When a solution contains more solute than should be present at equilibrium, the solution is said to be supersaturated. 

Crystallization in general is a two-step process involving (1) nucleation and (2) growth of the nucleus to a macro size. Nucleation involves the activation of small, unstable particles with sufficient excess surface energy to form a new stable phase. This may occur in supersaturated solutions as a result of mechanical shock, the introduction of small crystals of the desired type, or the presence of certain impurities that can act as centers for growth. 

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