Free-standing Smectic Films Research Articles

Self-Tuning Behavior of Vibrating Smectic Films

The wave equation that rules the drumhead oscillations of freestanding smectic films is similar to the two-dimensional Schr\"odinger equation. This makes vibrating smectic films the classical analog of appropriate quantum billiards. In particular, a rectangular smectic film carrying a very small inclusion, like a small metallic ball, is equivalent to a rectangular quantum billiard with a quasi-point-like scatterer. We point out that the ball, free to move in the plane of the film, tends to adjust its position as a function of the excitation frequency.

Phase behavior of liquid-crystal films exhibiting the surface smectic-Lphase

Free-standing smectic films of a liquid crystal exhibiting the hexatic smectic-$L$ phase have been studied using electron diffraction and optical textures. The films show a rich phase diagram as a function of thickness and temperature. A possible phase transition within the surface smectic-$L$ phase is observed.

Experiments on Tracer Diffusion in Thin Free-Standing Liquid-Crystal Films

Using laser-induced photobleaching of a fluorescent tracer molecule, we study diffusion in free-standing smectic films as thin as two layers. For films four or more layers thick, the in-plane diffusion constant increases in agreement with a hydrodynamic theory outlined here. Two- and three-layer films show different behavior.

Preliminary communication Anomalous behaviour of photoactive free-standing smectic films under illumination

Illumination of free-standing smectic films doped with azo dyes produces bubbles in the SmA phase; these cannot be observed in bulk samples. The bubbles appear in the films with some time delay after the beginning of illumination. This time delay depends linearly on (T where T is the smectic A-isotropic transition temperature. This dependence shows that IA illumination induces in the interior of the films a smectic A-isotropic phase transition. The SmC* in-plane director field configuration depends on the orientation of the polarization plane of the incident light. In the SmC* phase no bubbles are observed even on continuous illumination. T), IA

Model of Layer-By-Layer Thinning Transition in Thin Films

Free-standing smectic liquid-crystal films exhibit an interesting phenomenon of layer-by-layer thinning transition where, instead of abrupt rupture of the film at or below Sm-A–isotropic transition temperature T c, discrete jumps in film thickness are observed when temperature is increased, and the film eventually breaks at a temperature above the bulk value T, [4]. Starting with a molecular pair interaction potential of the McMillan form and deriving therefiom a single effective potential, the behavior of thin films with changing temperature is studied. The film thickness experimentally found [4] to exhibit a clear functional relationship to temperatures of layer transitions, h(t) = l 0 t −v, can be explained qualitatively, its exact form depending strongly on the parameters of the potential. Fluctuation profile of the order parameters within smectic layers is considered showing a critical behavior in the vicinity of the transition.

Destruction of a first-order smectic-A-->smectic-C* phase transition by dimensional crossover in free-standing films.

Shifting and destruction of phase transitions due to the finite thickness of free-standing smectic films are examined. The behavior of smectic films is studied as a function of the temperature and the thickness in a material showing a first-order smectic-A (Sm-A)\ensuremath{\rightarrow}smectic-${\mathit{C}}^{\mathrm{*}}$ (Sm-${\mathit{C}}^{\mathrm{*}}$) bulk phase transition. Measurements of the optical reflectivity and direct observations in an inverted polarizing microscope reveal that the first-order phase transition between Sm-A- and Sm-${\mathit{C}}^{\mathrm{*}}$-like structures takes place in films that are thick enough (N>${\mathit{N}}_{\mathit{c}}$=90). In thin films (N${\mathit{N}}_{\mathit{c}}$) this transition disappears. For N\ensuremath{\approxeq}${\mathit{N}}_{\mathit{c}}$, a critical behavior is detected. A theoretical scheme for interpretation of phase transitions in smectic films is proposed and on its basis a conjecture on tension-induced phase transitions is formulated.

Temperature dependence of XY-like order parameters in thin free-standing smectic liquid-crystal films.

The in-plane projection of the director in smectic-C liquid crystals and the hexatic order parameter in hexatic smectic liquid crystals are both analogous to the magnetization in an XY ferromagnet. Recent experimental results and theoretical arguments suggest that two-dimensional XY ferromagnets have a universal finite-size-induced effective magnetization exponent \ensuremath{\beta}\ensuremath{\approxeq}0.23 [S. T. Bramwell and P. C. W. Holdsworth, J. Phys. Condens. Matter 5, L53 (1993)]. From these results, we predict that XY-like order parameters in thin free-standing smectic liquid-crystal films should behave as \ensuremath{\propto}(${\mathit{T}}_{\mathit{C}}$-T${)}^{\mathrm{\ensuremath{\beta}}}$ in an intermediate temperature range below the relevant transition temperature ${\mathit{T}}_{\mathit{C}}$ if they exhibit a Kosterlitz-Thouless transition of a single XY-like order parameter.

Dimensionality of hexatic melting in liquid crystals by in situ electron diffraction

The theory of melting in two dimensions indicates that it is very different from the melting in three dimensions. An intermediate hexatic phase, with long range orientational but short range translational order, is proposed to exist between two dimensional solid and fluid phases. Because the long range order is limited experimentally by the domain or “grain” size, which is expected to be of sub-millimeter size, selected area electron diffraction is the method of choice for observing orientational order. Free-standing smectic liquid crystal films prove to be a most attractive system in which to verify this theory. However, multilayered liquid crystal samples are not strictly two dimensional systems because of interlayer interaction, their melting characteristics are expected to deviate from that of two dimensional systems. Furthermore, the outer layers may behave differently from the inner ones due to interfacial tension. We have applied low dose electron diffraction to study the effect of dimensionality, or number of layers, on the melting of multilayer organic liquid crystals.