smectic-A smectic-Ctransition Research Articles

Smectic-A–smectic-C–smectic-C⁎ Lifshitz point in mixtures of chiral and achiral smectic liquid crystals

A phenomenological theory to describe the smectic-A–smectic-C transition, smectic-A–smectic-C⁎ transition and smectic-C–smectic-C⁎ transition in liquid mixtures is proposed. The influence of the concentration of the chiral liquid crystal on these phase transitions is discussed by varying the coupling between various order parameters. The phase diagrams of the thermodynamic parameters, wave vector, helix pitch, tilt angle and polarization are studied and a possible smectic-A–smectic-C–smectic-C⁎ Lifshitz point is predicted. We present a detailed analysis of the different phases that can occur and analyze the question under which conditions a Lifshitz point is observed in the phase diagram. Calculations based on this model agree qualitatively with experiment.

Carbon nanotube-induced chirality in an achiral liquid crystal

A small quantity of carbon nanotubes was dispersed in an achiral liquid crystal (LC), and the mixture was found to exhibit a weak degree of chirality. The induced chirality in the LC was probed by means of the electroclinic effect in the LC’s smectic-A phase, which showed significant pretransitional behavior on approaching the smectic-A–smectic-C transition temperature from above. The results suggest that there is a net chirality associated with the carbon nanotubes, which is transmitted into the LC.

Static and dynamic critical behaviours in de Vries liquid crystals

Measurements of dielectric dispersion have been carried out on two liquid crystal compounds 8422[2F3] and 8O23[7F8-], showing non-layer-shrinkage (NLS) behaviour through the smectic-A (Sm-A)–smectic-C (Sm-C) transition. The critical exponent γ for the susceptibility determined from the data lies in the range 1.9–2.0 for 8422[2F3] and 1.1–1.2 for 8O23[7F8-]. While the former agrees with the general trend reported so far for this group of liquid crystals, the latter result is distinct from them, the γ value being definitely smaller. These observations indicate that the Sm-A–Sm-C transition with NLS behaviour can exhibit quasi two-dimensional Ising critical behaviour under certain conditions. The critical exponent Δ for the relaxation frequency has also been obtained as Δ = 2.0–2.1 for 8422[2F3] and 1.2–1.3 for 8O23[7F8-]. The results support the general expectation Δ > γ rather than the conventional behaviour Δ = γ.

The existence of a tricritical point in the smectic A-smectic C transition line

In the light of a recent x-ray study on the smectic layer spacing in the vicinity of the smectic A-smectic C (SmA-SmC) transition of the TBBA (terephthal-bis-(4n)-butylaniline) homologous series, a Landau theory with coupled order parameters is proposed to provide a unique explanation of the x-ray data. It is found that as the length of the alkyl-tails increases the SmA-SmC transition change from being second-order to being first-order with a tricritical point in between.