Light scattering near the nematic-smectic-A liquid-crystal phase transition

Elastic response of colloidal smectic liquid crystals: Insights from microscopic theory

Elongated colloidal rods at sufficient packing conditions are known to form stable lamellar or smectic phases. Using a simplified volume-exclusion model, we propose a generic equation of state for hard-rod smectics that is robust against simulation results and is independent of the rod aspect ratio. We then extend our theory by exploring the elastic properties of a hard-rod smectic, including the layer compressibility (B) and bending modulus (K_{1}). By introducing weak backbone flexibility we are able to compare our predictions with experimental results on smectics of filamentous virus rods (fd) and find quantitative agreement between the smectic layer spacing, the out-of-plane fluctuation strength, as well as the smectic penetration length λ=sqrt[K_{1}/B]. We demonstrate that the layer bending modulus is dominated by director splay and depends sensitively on lamellar out-of-plane fluctuations that we account for on the single-rod level. We find that the ratio between the smectic penetration length and the lamellar spacing is about two orders of magnitude smaller than typical values reported for thermotropic smectics. We attribute this to the fact that colloidal smectics are considerably softer in terms of layer compression than their thermotropic counterparts while the cost of layer bending is of comparable magnitude.

Influence of phase sequence on focal conic domains in smectic films

We present a study of smectic focal conic domains in films of various liquid-crystal compounds on a solid substrate. Focal conic domains are generated as a result of the antagonistic anchoring conditions of the film surfaces, random planar at the substrate interface and homeotropic at the air interface. The curved arrangement of the smectic layers in focal conic domains leads to a depression in the film/air interface above each domain. Using atomic force microscopy, we determine the temperature dependence of the depth of the depressions in different smectic phases and at different phase transitions. In most cases, our results are not in accordance with the assumption of strictly incompressible smectic layers, and we discuss the observed behavior with respect to the known results on the layer compressibility modulus B .

AFM study of defect-induced depressions of the smectic-A/air interface

The smectic-A/air interface of liquid-crystal droplets with antagonistic boundary conditions is studied by atomic force microscopy (AFM). The droplets are prepared on coated silicon wafers on which a planar alignment is preferred in contrast to the homeotropic alignment at the air interface. As a result, focal conic defects appear in the smectic-A phase causing a characteristic pattern of depressions in the droplet surface. The dimensions of the defect-induced depressions are measured by AFM as a function of temperature for two different compounds possessing a smectic-A-isotropic and a smectic-A-nematic transition. Whereas the results are independent of temperature in the smectic-A-isotropic case, reflecting the first-order nature of the transition, a pronounced temperature dependence is observed for the second compound, where the depth of the defect-induced depressions decreases continuously with increasing temperature and vanishes at the second-order transition to the nematic phase. These observations can be qualitatively explained through the behavior of the layer compressional elastic constant at the smectic-A-nematic transition.

Behavior of the layer compression modulus as a function of frequency near the nematic-smectic-A and re-entrant nematic-smectic-A phase transitions

A progressive cross-over of the layer compression modulus B from a simple power law behavior to a saturation behavior when the frequency is increased from 1 Hz to 2 x 10(3) Hz is observed for the first time near the nematic-smectic-A and the re-entrant nematic-smectic-A phase transitions of a mixture of 4-cyano-4'-( n-hexyloxy)biphenyl and 4-cyano-4'-( n-octyloxy)biphenyl (6OCB-8OCB). This result shows that the saturation of B determined by second sound near the nematic-smectic-A phase transition of a similar mixture is dynamic in nature, and cannot be associated with the static saturation effect predicted by the dislocation loop model, as proposed previously.