Aims and techniques:

Experimental and numerical studies were performed on diffusion limited growth processes. The main goal is to understand the instabilities of a moving solidification front.

The experiments involved the observation and analysis of the moving front in a quasi-two-dimensional sample, that is either isothermally undercooled (free growth) or is continously translated in a temperature gradient (directional solidification). Various organic crystals and liquid crystals (the nematic - smectic B first order phase transition) was investigated.

The numerical calculations were carried out using the phase-field model, that is based on the Ginzbug-Landau formalism. It has the advantage of not tracking the front explicitly (as it is done in a sharp interface model).  At the same time it can take into account the anisotopies of the surface tension and that of the phase transformation kinetics.



Results:

Crystallization of impure biphenyl was studied in thin-sample directional solidification (T-DS) experiments. The platelike growth shape of the monoclinic biphenyl crystals includes two low-mobility (001) facets and four high-mobility {110} facets. Upon T-DS, biphenyl plates oriented with (001) facets parallel to the sample plane can exhibit either a strong growth-induced plastic deformation (GID), or deformation-free weakly faceted (WF) growth patterns. We determine the respective conditions of appearance of these phenomena. GID is shown to be a long-range thermal-stress effect, which disappears when the growth front has a cellular structure. An early triggering of the cellular instability allowed us to avoid GID and study the dynamics of WF patterns as a function of the orientation of the crystal. [Phys. Rev. E 80, 051601 (2009)]   (download pdf)
The growth of polycrystals and grain boundary dynamics was studied in directional solidification for CCH4. At faceted grain boundaries the undercooling might become sufficiently large for the nucleation of new germs.  The angular distribution of the in plane oriantation of the nucleated germs is remarkably different for samples with different surface treatment.   For polyimid-coated samples, the orientation effect of SmB crystals is mediated by the nematic, whereas, in poly(tetrafluoroethylene)-coated samples, it results from a homoepitaxy phenomenon occurring for two degenerate orientations. [Phys. Rev. E 66, 051709 (2002)]   (download pdf)

In directional solidification the nematic-smectic B  interface of CCH4 exhibits a faceted growth in the planar configuration. Near the Mullins-Sekerka threshold a new type of solitary wave, called "faceton", has been observed which consists essentially of an isolated macroscopic facet traveling laterally at such a velocity that its growth rate with respect to the liquid is small.  [Phys. Rev. E 65, 011702 (2001)]   (download pdf)

A review article summarizes experimental and numerical (phase-field simulation) results about solidification in systems with large anisotropy (faceted growth), dendritic growth, solidification from anisotropic melt and crystal growth in the presence of time-periodic external modulations.
[Physics Reports 337, 37 (2000)] 
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The effect of oscillatory pressure and periodic heating on the dendritic morphology in solidification was analized in details. The sensitivity of this phenomenon to the relevant parameters, the frequency and amplitude of the modulation, the initial undercooling and the anisotropies of the interfacial free energy and molecule attachment kinetics, has been explored. It has been shown, that in addition the side-branching mode synchronous with the external perturbation, modes that oscillate with higher harmonic frequencies are also present with perceptible amplitudes. [Phys. Rev. E 62, 7817 (2000)]   (download pdf)
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The dendritic morphology is often observed in solidification. The noisy character of the sidearm formation (Fig.a.) was intensively studied by various groups. We have shown that spatially homogeneous time-periodic external modulations  (pressure oscillation, heat pulses) regulate the dendritic growth (Fig.b.).
[Phys. Rev. Lett. 83, 2853 (1999)]  (download pdf)
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The shape of quickly growing dendrites is also influenced by the diffusion anisotropy of the surrounding nematic phase. The analysis of four armed dendrites reveal, that the arms which propagate in the direction of slower diffusion are faster.
Journal of Crystal Growth  193, 712 (1998)
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The growth of a homeotropic smectic domain (smectic layers parallel to the plane of the sample) is associated with small surface tension anisotropy and kinetic anisotropy. This allows one to concentrate on the effects coming from the anisotropy of the liquid (nematic) phase.  One interesting result is that anisotropic diffusion in the liquid phase causes an elongation of the growing crystals in the direction of the slow diffusion. The experimental observations have been reproduced by phase-field simulations. Phys. Rev. E 58, 6236 (1998)  (download pdf)
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Dendritic and faceted growth in strongly anisotropic systems has been characterized experimentally and the observed growth forms have been reproduced numerically using phase-field simulations.
[Physica D 99, 359 (1996)]
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The growth of a planar smectic domain (smectic layers perpendicular to the plane of the sample) is associated with large surface tension anisotropy and kinetic anisotropy, often resulting in faceted growth. [Pys. Rev. E 54, 1574 (1996)]  (download pdf)
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