Nucleation experimental test

The extensive use of the Young equation (Eq. X-18) reflects its general acceptance. Curiously, however, the equation has never been verified experimentally since surface tensions of solids are rather difficult to measure. While Fowkes and Sawyer [140] claimed verification for liquids on a fluorocarbon polymer, it is not clear that their assumptions are valid. Nucleation studies indicate that the interfacial tension between a solid and its liquid is appreciable (see Section K-3) and may not be ignored. Indirect experimental tests involve comparing the variation of the contact angle with solute concentration with separate adsorption studies [173]. 

An extensive formulation of classical and nonclassical models for homogeneous nucleation, as well as experimental tests of their validity, have been carried out for the Co-Cu precipitation system in which coherent Co-rich nuclei form [15]. 

Kinetics of Homogeneous Nucleation 277 Experimental Test of Nucleation Theory 280 Heterogeneous Condensation 283 Growth Laws 284... 

It is today well accepted that a control of the heterogeneous primary nucleation can favor the formation of different polymorphs. In this context, the ability of membrane-based CTystalUzers to promote the selective formation of stable or metastable polymorphs on polymeric hydrophobic manbranes was confirmed by experimental tests on glycine with selective CTystalUzation of forms a and y (Di Profio et al. 2007a), on paracetamol with the possibility to disamtinate between the monocline stable form of... 

Until the early 80s, experimental tests of classical nucleation theory were restricted to measuring the supersaturation required to trigger an easily measurable nucleation rate, typically 1 cm sec This is known as the critical supersaturation. Using the density and surface tension of water. Equation (23) predicts an increase of seventeen orders of magnitude in the nucleation rate as a result of a 10% change in supersaturation, from 2 to 2.2. It follows that the critical supersaturation is not a sensitive probe of the accuracy of nucleation theories. It also follows from equation (23) that this constitutes an incomplete test of the theory, insofar as it does not test the actual J(S) functionality. A review by McGraw [36] is a particularly useful and comprehensive survey of experimental measurements of critical supersaturations it also proposes a useful corresponding states correlation for this quantity. 

Figure 3. Experimental test of classical nucleation theory. Comparison between experimental (exp) and theoretical (th) nucleation rates for droplet condensation from n-nonane vapor. Subscript int denotes the integrated theoretical nucleation rate along the height of the thermal diffusion cloud chamber [37].

Zanotto E.D. and Galhardi A., "Experimental Test of the General Theory of Transformation Kinetics Homogeneous Nucleation in a Na20-2Ca0 3Si02 Glass," J. Non-Cryst. Solids, 104, 73-80 (1988). 

Experimental tests, performed by changing the configuration of the capillary (length and diameter), showed that the pressure drop and the pressure gradient was varied and this resulted in different nucleation rate. Therefore, foams prepared with the longer capillary (1 = 30 mm) had a very fine cellular structure with 80 pm average diameter of cells as compared to those (300 pm diameter) obtained by using a shorter capillary (1 = 10 mm). 

Several criticisms of these parameters have recently been pointed out. First, they have no specific association with a material plane (i.e., they are scalar parameters), despite the fact that cracks are known to nucleate on specific material planes. With traditional parameters it is difficult to account for the effects of crack closure under compressive loading. Traditional parameters have not been successful at unifying experimental results for simple tension and equibiaxial tension fatigue tests. Finally, a nonproportional loading history can always be constmcted for a given scalar equivalence parameter that holds constant the value of the scalar parameter, but which results in cyclic loading of material planes. For such histories, scalar parameters incorrectly predict infinite fatigue life. 

The experimental conditions and results of the analysis of the purity of separated benzene crystals are shown in Table 1. In tests of No. 1-1 to 1-4, and 3-1 and 3-2, the melt was compressed to the pressure shown in Table 1 and kept on the same value, without seed crystals. Nucleation occured on the wall and crystals grew there. In tests of No.2-1, 2-2 and 3-3, seed crystals were made as described above they grew both inside the optical cell and on the wall. In these tests, since the melt around benzene crystals was replaced by the water, the crystals were taken out without serious destruction. The shapes of benzene crystals were dendritic, and purity of it was over 99.9 mole percent, independent from the operational conditions and the feed compositions as shown in Table 1. Therefore, crystals obtained by high pressxire crystallization is considered to be very pure due to the complete removement of mother liquid from crystal surface. 

Crystal stmcture prediction by computer has made great steps forward in the last 10 years, with progress toward consistent success in blindfold tests. Fundamental uncertainties still remain, due to the unknown role of nucleation kinetics and to the neglect of temperature effects in the calculations. Success or failure still depends to some extent on hardly predictable factors and on the extent to which the experimental polymorph screening has been carried out. Presently, some of the best computational tools are not yet available to the general community of solid state scientists, being implemented in commercial, strictly copyrighted software. 

The selectivity of the nickel(l 1 1) surface may thus be controlled by modification of the number of free step sites, and this notion was tested experimentally by blocking the steps with small amounts of silver (84). In other STM investigations it was found that when silver was deposited on nickelfl 1 1) at room temperature, the silver preferentially nucleated and grew as islands at the step edges. When this system was post-annealed to 800 K, the silver atoms were observed to become highly mobile and decorate all the step edges of nickelfl 1 1), as shown in Fig. 6b. 

The Cu-Co system is a particularly simple precipitation system in which a Corich /3 phase precipitates in a Cu-rich terminal a phase. The f.c.c. lattices of both phases are well matched in three dimensions, so that the precipitate interfaces are coherent with respect to either lattice as a reference structure and the interfacial energy is sufficiently isotropic so that they are almost spherical, as in Fig. 19.2. Both the interfacial energy and strain energy are therefore relatively low and the nucleation of the f3 phase is therefore relatively easy and occurs homogeneously. This system has been used to test the applicability of the classical nucleation theory (Section 19.1.1) [11, 12]. In this work, the experimental conditions under which... 

Table 3. The toughness improvement induced by the -modification, rationalized in terms of the ratio Gt0t(/ -nuclcalcd)/Gtot (non-nucleated), is markedly stronger for the series tested under biaxial conditions. As both sets of investigated samples were injection molded, the skin-core structure (discussed in Sect. 3.2.3) could be inferred to be at least partly responsible for this feature. This was, however, confirmed in a recent work, where impacted /3-PP samples (tested at 5 ms-1, T = 23 °C and - 20 °C) with or without skin layers showed the same fracture performance within the limits of the experimental error [116].

It is well known that the mechanical behavior of glassy amorphous polymers is strongly influenced by hydrostatic pressure. A pronounced change is that polymers, which fracture in a brittle manner, can be made to yield by the application of hydrostatic pressure Additional experimental evidence for the role of a dilatational stress component in crazing in semicrystalline thermoplastics is obtainai by the tests in which hydrostatic pressure suppresses craze nucleation as a result, above a certain critical hydrostatic pressure the material can be plastically deformed. 

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