Solidification Front Technique

Table 14.3. Summary of contact angle, 6, and surface tension, ypv, data obtained by the Wilhelmy balance technique and the solidification front technique for carbon and Kevlar fibres, respectively (from ref. (75))...

Table 14.5. Comparison of the surface tensions of fixed erythrocytes obtained by the droplet sedimentation and solidification front techniques, using water/ice (from ref. (70))...

In the third section, two additional matrix materials are Introduced, benzophenone and blbenzyl. However, this is not done through measurements of the temperature dependence of contact angles as in the first section above, but through the solidification front technique Itself, using particles of known surface tension as probes. 

Melt-spinning techniques that use a solid quenching medium can only be used to produce filaments of flat cross-sections. Various techniques that differ in small details (single roller, twin roller) are in use. They all have in common that the ribbon forms from a liquid puddle that remains in a fixed position on the moving quench medium which is usually a rotating Cu wheel. The width of the ribbon varies with the width of this puddle and the thickness with the penetration depth of the solidification front that moves into the puddle. Only the solidified metal moves with the wheel and this... 

Overall, wettability measurement of small particles is a difficult problem that is further aggravated in the case of heterogeneous surfaces. Some of these problems can result from the presence of patches of different composition in the same particle. It is considered that if these patches are below a critical size of 0.1 mm, the surface is homogeneous regarding its wettability. Several indirect techniques have been developed to measure the surface tension, and thus the wettability of small particles. In these techniques, the surface tensions of the particles are derived from thermodynamic models and include the advancing solidification front or freezing front, sedimentation volume, and particle adhesion techniques [44, 45]. 

In addition to the aforementioned techniques, most commonly used for the characterization of fibers, fiber assemblies, and fabrics in terms of their wetting behavior or surface tension, there are also other methods available the determination of the contact angle from the equilibrium meniscus near a floating fiber [80], the reflection method initially developed by Jones and Porter [137] (reviewed in Refs. 1 and 32), the tilted fiber or rotating stage technique [138,139], and the solidification front method (recently reviewed in Ref. 140). 

The scheme of this technique is to first determine the so-called critical velocity, Vc, of a solidification front. A particle which is pushed by a solidification front through the liquid matrix may be engulfed by the solid phase when the repulsion (i.e. a positive value of is just balanced by the viscous drag. 

As shown by Fig. 22.16, the concentrations of solute atoms are significantly reduced in the material that is solidified early in the solidification process when k < 1. One-dimensional plane-front solidification can therefore be used as a method of purification. However, purification is carried more effectively out by modifying the process and adopting a zone-melting technique. 

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