Pressure-driven reentrant phenomena in liquid crystals: the role of inverse layer spacing

Abstract

The occurrence of pressure-driven reentrant phenomena observed in high-pressure experimental studies in some achiral mesogenic materials has been explained using a thermodynamic model based on Landau-de Gennes theory. In this approach, the free-energy is expanded in terms of nematic, smectic A order parameters and the couplings (cubic and biquadratic) between them. The basic theme here is that the 'inverse layer spacing', which mimics an order parameter, becomes coupled to the nematic and smectic A order parameters. Secondly, in addition to the order parameter couplings the N-S A metastable temperature (which appears due to elimination of inverse layer spacing from the free-energy density expression) becomes pressure dependent. The occurrence of a pressure-driven reentrant nematic phase is explained in terms of these three pressure-dependent parameters. They all show smooth but rapid variation at the critical pressure beyond which nematic reentrance appears.

The influence of high pressure on isotropic–smectic A transition and nonlinear dielectric effect in homologous series of nDBT

“…As that assumed elsewhere [9][10][11][12], we consider that the orientational and positional orders are spatially invarient, i.e. S = const.…”

“…A close agreement between theory and experiment shows that the characteristic features of the effect of hydrostatic pressure on the S A I transition and NDE in homologous series of isothiocyanato series nDBT can be explained using a phenomenological free-energy density expansion. [9][10][11][12] The pressure dependence of the properties of the mesogens has been incorporated in the theory via pressure-dependent Landau coupling coefficients.…”

“…[13][14][15][16] Adopting the procedure as outlined in previous works [9][10][11][12], the total free-energy density of an S A phase in the presence of pressure is written as…”

“…Considering the application of same Landau-de-Gennes formulation, Srivastava et al [11] analyzed the effect of pressure on the electric-field-induced phase transition in a mesogen exhibiting spontaneously IS A transition. In terms of the pressure-dependent Landau coupling coefficients, Singh et al [12] explained the occurrence of pressure-driven reentrance observed in high-pressure experimental studies in some achiral mesogenic materials.…”