Pre-transitional surface ordering and disordering at liquid crystal isotropic liquid interface

The existence of pre-transitional surface ordering and disordering at the liquid crystal-isotropic liquid interface has been established from the surface tension measurements. A spinning-drop technique has been used for this study. Measurements were carried out in the smectic A(S-A), nematic (N) and isotropic (I) phases of the liquid crystals and reproducible results were obtained. The results suggest that the equilibrium value of gamma changes discontinuously in the N and S-A phases. The slope d gamma/dT is positive for both the isotropic phase just above T-NI as well as for the nematic phase just below it. Similar positive slope has been obtained in the vicinity of T-AN. A positive value of d gamma/dt implies excess surface order than that of the bulk interior. From the density measurements it has been observed that there is a correlation between the anomalies in the temperature dependence of surface tension and density in the isotropic phase.