Diffusion
limited crystal growth
processes have been studied both experimentally and numerically for better
understanding of the instabilities on a moving solidification front.
In the
experiments quasi-two-dimensional samples of various organic crystals and
liquid crystals (the nematic - smectic B first order phase transition) have
been investigated. The peculiarity of liquid crystals for this field of
research is that both the low-temperature phase (‘crystal’) and the
high-temperature phase (‘melt’) may be anisotropic.
The numerical calculations were carried out using the phase-field model based on the Ginzburg-Landau formalism which takes into account the anisotropies of the surface tension and that of the phase transformation kinetics in the same time.
Main
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 (
[Phys. Rev.
E 80, 051601/1-11 (2009)] (pdf)
The growth of polycrystals and grain boundary dynamics
was studied in directional solidification for the
liquid crystal
[Phys. Rev. E 66, 051709/1-14 (2002)] (pdf)
In directional solidification
the nematic-smectic B interface of
[Phys. Rev.
E 65, 011702/1-11 (2001)] (pdf)
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)] (pdf)
Two review articles summarize 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.
[Phys. Rep.
337, 37-65 (2000)] (pdf)
[Mol. Cryst.
Liq. Cryst. 339, 175-208 (2000)]
(pdf)
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)] (pdf)
An aging behaviour was
observed in alkyl-bicyclohexyl-carbonitrile compounds which strongly influences
the growth of smectic B germs in the undercooled nematic phase. The aging also
results in a broadening of the phase coexistence temperature ranges and in a
saturating shift of phase transition temperatures with a temperature dependent
dynamics. An experimental evidence has been given that the air atmosphere is
responsible for the thermally induced ageing.
[Mol. Cryst.
Liq. Cryst. 328, 467-477 (1999)]
(pdf)
The growth of a homeotropic smectic domain 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, 6236-6245 (1998)] (pdf)
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.
[J. Cryst. Growth 193,
712-719 (1998)] (pdf)
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,
359-368 (1996)] (pdf)
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.
[Phys. Rev. E 54, 1574-1583 (1996)] (pdf)
Three types of
smectic-B germs with respect to the director orientation inside and outside the
germ have been found. The planar
germs undergo a dramatic change of morphology as a function of undercooling,
starting from an elongated, rectangle-like shape to dendritic growth of
fourfold symmetry. Near equilibrium the homeotropic
germs have a circular form with a small hexagonal modulation, and show a dense
branching morphology for larger undercoolings.
[Phys. Rev. E 51, 571-578 (1995)] (pdf)
An interesting “inverse
process” has been detected in melting of the rapidly grown smectic B phase. The
stable nematic phase nucleated substancially below the phase transition temperature. Most of these nematic germs
had uniform oval shape, totally different from that of the equilibrium smectic
seed.
[Mol. Cryst.
Liq. Cryst. 261, 349-369 (1995)] (pdf)
Equilibrium
shapes of the nematic–smectic-B liquid-crystal interface have been
investigated. From the shape of the germs the angle dependence of the surface
tension was determined.
[Phys. Rev. E 49,
5271-5275 (1994)] (pdf)
The interface of
a growing smectic phase in a supercooled nematic liquid crystal was studied. A
change in the morphology and the side-branching activity was observed with
the undercooling. Nonparabolic dendrites with faceted dissymmetric tips were
found for small undercooling.
[Europhys. Lett. 21, 477-482 (1993)] (pdf)