Phase Transitions in Liquid Crystals Under Negative Pressures

Abstract

We report the first measurements of orientational order parameters and phase transition temperatures in nematic and smectic A liquid crystals under negative pressures generated by an isochoric cooling of small droplets embedded in a glass former. Comparison of isobaric and isochoric measurements allows us to estimate the coefficients coupling the order parameter and density of an extended Landau--de Gennes model of the nematic phase.

On the pressure evolution of the melting temperature and the glass transition temperature

“…Reaching a negative pressure in 5CB was already reported in Ref. [64]. It was obtained for small 5CB inclusions in a sucrose matrix.…”

“…In Ref. [64] the negative pressure value was estimated from the downward shift of T IN assuming a linear T IN (P) dependence extrapolated from the stable P > 0.1 MPa region.…”

Thermotropic nematic order upon nanocapillary filling

“…It is interesting to note that this T -shift is much larger than any tensile pressure effect on liquid crystals that has been reported to date. Presumably, this results from the extreme challenges [45] associated with experiments aimed at reaching tensile pressures comparable to the ones established here in the partially filled state. Since the true curvature of the menisci at high f -values is not accessible in our experiments and since studies with higher f-resolution are experimentally demanding both due to the sizable light scattering in that regime and because of the error bars in the filling fraction (resulting from Tdependent volume changes in the liquid), this conclusion remains somewhat speculative and more rigorous studies are planned for the future.…”

“…It is known that this negative hydrostatic pressure causes not only subtle deformations of the rigid, nanoporous matrix [38][39][40], it also significantly affects density, and thus pressuredependent first-order transitions, most prominently the liquid-solid transition [41][42][43]. Therefore, it is in principal also expected to affect the isotropic-to-nematic transition [44,45]. According to the Laplace formula applied to the menisci in the capillaries, while assuming good wetting conditions (mean curvature radius = -pore radius = -6 nm, surface tension of 7 CB=31.7 mN/m [46]) the tensile pressure in the liquid bridges amounts to ∼-10.5 MPa for f c < f < 1.…”

Inhomogeneous relaxation dynamics and phase behaviour of a liquid crystal confined in a nanoporous solid

“…There is, however, another, quantitatively verifiable and arguably more simple explanation, which is related to the hydrostatics of the confined liquid: In the partially filled, capillary-condensed state, the liquid experiences a tensile pressure, dictated by the concave curvature of the menisci terminating the liquid bridges. This negative pressure causes not only subtle deformations of the rigid, nanoporous matrix [88][89][90], it also significantly affects density, and thus pressure-dependent phase transformations, most prominently the liquid-solid transition [91][92][93][94], but also the isotropic-to-nematic transition studied here [95,96]. According to the Young-Laplace formula applied to the menisci in the capillaries (mean curvature radius = -pore radius = -6.6 nm, surface tension of 7 CB=31.7 mN/m [97]) the tensile pressure in the liquid bridges amounts to ∼-9.6 MPa for f c < f < 1.…”