Nucleation on the surface

The primary nucleation on the surface of the NA is a kind of heterogeneous nucleation [25]. NA is a so called heterogeneity . In this study, all nucleation is limited to the heterogeneous nucleation. The shape of a heterogeneous nucleus is assumed parallelepiped with length of a stem l and the number of stems m and n. Here /, m, and n are counted by the number of atom... 

If has been observed fhaf by adsorbing surfactants onto the crystal surfaces of poorly water-soluble drugs, dissolution rate can be enhanced. Chen et al. " showed that the dissolution rate of Cl-1041, a poorly water-soluble compound, in 0.1 N HCl may be affected by the surfactant Tween 80. The effects of surfactant are complicated, and many factors are involved. Above the CMC of Tween 80, the adsorption of the surfactant onto the crystal surface may inhibit crystal nucleation on the surface, and causes the dissolution rate to increase. By adsorbing a very small amount of poloxamer onto the hydrophobic drug particle surface. 

The calcite cement within calcite concretions is polycrystalline, although calcite crystal size may be up to several centimetres (Hudson Andrews, 1987). This implies that new calcite crystals nucleated on the surface of older crystals during concretion growth, even though calcite supersaturations would be at a minimum at this location. A possible solution to this problem may be that dislocations on the surfaces of the growing crystals acted as nucleation points for new crystals. 

During nucleation ordered nuclei are formed as a result of matter fluctuations. If these are formed in the volume of ideally pure solutions, for example, containing only the protein and solvent molecules, the process is called homogeneous nucleation. If monomers nucleate on the surface of seeds added to the solution or on substrates that provide active centers for nucleation, it is referred as heterogeneous nucleation (Figure 3.2). 

The crystallisation from strained melt as for instance in a blown film or in the jet during fibre spinning produces a row nucleated structure. " Linear nuclei are formed parallel to the strain direction. They contain more or less extended polymer chains. Secondary epitaxial nucleation on the surface of such linear row nuclei produces folded chain lamellae which are oriented perpendicular to the strain (Fig. 6). In such a case the sample exhibits a high uniaxial orientation of chain axes in the strain direction with random orientation of the a- and b-axes perpendicular to it. If the growing lamellae exhibit a helical twist the chain orientation in the strain direction is very soon replaced by the orientation of the axis of maximum growth rate (b-axis in the case of polyethylene) perpendicular to the strain direction and a more random orientation of the remaining two axes (a- and c-axes in the case of polyethylene) with a maximum in the strain direction. Such a row nucleated structure has parallel cylindrical spherulites (cylindrites) as its basic supercrystalline element. 

The occurrence of crystallographic mismatch branching is controlled by the following two steps (1) the growth of the surface of parent crystals (2) the crystallographic mismatch nucleation on the surface. 

A composite material is composed of a blend of PVDF homopolymer or copolymer crystallized in the P-form and a conductive filler, such that the crystals in the P-form are nucleated on the surface of the particles of the conductive filler. " Graphite is the conductive filler. " ... 

It will be shown first that the growth is governed by the growth of flat F faces and by the generation steps on these faces. Two sources for these steps can be distinguished First, the steps can be generated by 2D nucleation on the surface or by screw dislocations. Second, the rate of incorporation of the building blocks into the steps at the so-called kink positions will be described. 

When spiral steps from two dislocations are separated by a distance larger than 9pJ2, two-dimensional re-entrant comers may appear and preferential growth will start after that, i.e. the steps will advance faster at the re-entry comers in comparison with other directions. When the step separation is much wider than a critical value, it becomes possible that the supersaturation between neighboring steps can achieve a level sufficient to overcome the barrier of 2D nucleation and to permit nucleation on the surface of spiral layers. 

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