Showing papers by "Samo Kralj published in 2002"

Effect of dispersed silica particles on the smectic-A-smectic-C* phase transition.

Abstract: We study the influence of the dispersed aerosil particle concentration on the soft and Goldstone mode dynamics across the smectic-A-smectic-C* phase transition using dielectric spectroscopy. We use CE8 liquid crystal filled with aerosils of concentrations x=m(s)/(m(s)+m(LC))=0.025, 0.05, 0.1, and 0.15, where m(s) and m(LC) stand for masses of aerosil and liquid crystal, respectively. In dispersions with x 0 the degeneracy of the modes at the SmA-SmC* transition temperature is lifted. With increasing x the modes' intensities gradually decrease and the critical effective coefficient gamma tentatively approaches the value characterizing the three-dimensional XY universality class. For x=0.15, a drastic change in behavior is observed. The Goldstone mode is completely suppressed and the f(s)(T) dependence begins to deviate from the Arrhenius behavior. A simple phenomenological approach is used in the explanation of results.

Molecular dynamics study of the isotropic-nematic quench.

Abstract: Effects of cylindrical and spherical confinement on the kinetics of the isotropic-nematic quench is studied numerically. The nematic liquid crystal structure was modeled by a modified induced-dipole\char21{}induced-dipole interaction. Molecules were allowed to wander around points of a hexagonal lattice. Brownian molecular dynamics was used in order to access macroscopic time scales. In the bulk we distinguish between the early, domain, and late stage regime. The early regime is characterized by the exponential growth of the nematic uniaxial order parameter. In the domain regime domains are clearly visible and the average nematic domain size ${\ensuremath{\xi}}_{d}$ obeys the dynamical scaling law ${\ensuremath{\xi}}_{d}\ensuremath{\sim}{t}^{\ensuremath{\gamma}}.$ The late stage evolution is dominated by dynamics of individual defects. In a confined system the qualitative change of the scaling behavior appears when ${\ensuremath{\xi}}_{d}$ becomes comparable to a typical linear dimension R of the confinement. In the confining regime $({\ensuremath{\xi}}_{d}g~R)$ the scaling coefficient \ensuremath{\gamma} depends on the details of the confinement and also the final equilibrium nematic structure. The domain growth is well described with the Kibble-Zurek mechanism.

Core hysteresis in nematic defects.

Abstract: We study field-induced transformations in the biaxial core of a nematic disclination with strength m=1, employing the Landau-de Gennes order tensor parameter Q. We first consider the transition from the defectless escaped radial structure into the structure hosting a line defect with a negative uniaxial order parameter along the axis of a cylinder of radius R. The critical field of the transition monotonically increases with R and asymptotically approaches a value corresponding to xi(b)/xi(f) approximately 0.3, where the correlation lengths xi(b) and xi(f) are related to the biaxial order and the external field, respectively. Then, in the same geometry, we focus on the line defect structure with a positive uniaxial ordering along the axis, surrounded by the uniaxial sheath, the uniaxial cylinder of radius xi(u) with negative order parameter and director in the transverse direction. We study the hysteresis in the position of the uniaxial sheath upon increasing and decreasing the field strength. In general, two qualitatively different solutions exist, corresponding to the uniaxial sheath located close to the defect symmetry axis or close to the cylinder wall. This latter solution exists only for strong enough anchorings. The uniaxial sheath is for a line defect what the uniaxial ring is for a point defect: by resorting to an approximate analytic estimate, we show that essentially the same hysteresis exhibited by the uniaxial sheath is expected to occur at the uniaxial ring in the core structure of a point defect.

Smectic-A edge dislocations in a very thin cell

Abstract: We study stable “bookshelf” smectic-A structures within a very thin plane-parallel cell of thickness L in which the mismatch between surface preferred (d s) and intrinsic (d0) smectic layer thicknesses occurs. The Landau-Ginzburg approach based on a complex smectic order parameter is used. For a weak enough smectic positional anchoring strength W smectic layers adopt the modified bookshelf profile. In a thick enough cell with increasing W a lattice of edge dislocations is continuously formed at the confining surfaces and then depinned from them. The structure with dislocations is formed when the condition d 0/(ζ s |d 0/d s − 1|) ∼ 2 is fulfilled, where ζ s is the positional surface anchoring extrapolation length. If the cell is thin enough the dislocations formed at opposite cell plates annihilate and consequently the smectic layers adopt a locked bookshelf structure. This transition is discontinuous and takes place when d 0/(L|d 0/d s − 1|) ∼ 5 is realized. To observe these transitions in a cell of thickness L∼ 1μm the conditions W∼ 10-6 J/m 2 and |d 0/d s − 1| ∼ 5 · 10−4 have to be fulfilled. All the three qualitatively different structures coexist at the triple point.

Influence of the Polymer Network on the Switching Behaviors in the Polymer Stabilized Ferroelectric Liquid Crystals

Abstract: The electric field induced stripe texture in the ferroelectric liquid crystals can be efficiently stabilized by means of the built-in low concentration polymer network, which tends to phase separate predominantly along the stripe texture domain walls, leaving the quasibookshelf alignment as well as the dynamics of the ferroelectric liquid crystals within each stripe to be very little affected by the polymer orienting surface, while efficiently stabilizing the stripe texture. Moreover the oriented polymer network within the domain walls reduces the elastic stress and helps increasing the ferroelectric liquid crystals molecular effective cone angle. This in turn results in the increase of the threshold voltage for the switching between the two bistable molecular orientations, further resulting in significant improvement of the contrast of the ferroelectric liquid crystals switching element in the case of the multiplex driving. The resulting polymer stabilized quasibookshelf alignment represents therefore a very promising solution for display applications.