Thermotropic nematic order upon nano-capillary filling

TL;DR

This study investigates the thermotropic nematic order of a liquid confined in nano-capillaries, revealing how the phase transition behavior varies with filling state and capillary effects, including a pressure-related shift in transition temperature.

Contribution

It provides detailed insights into the nematic ordering and phase transition shifts of a liquid in nano-capillaries during sequential filling, highlighting the influence of confinement and pressure effects.

Findings

Weak paranematic-to-nematic transition in thin films

Shift of transition temperature below bulk value during partial filling

Abrupt disappearance of shift upon complete filling due to pressure release

Abstract

Optical birefringence and light absorption measurements reveal four regimes for the thermotropic behavior of a nematogen liquid (7CB) upon sequential filling of parallel-aligned capillaries of 12 nm diameter in a monolithic, mesoporous silica membrane. No molecular reorientation is observed for the first adsorbed monolayer. In the film-condensed state (up to 1 nm thickness) a weak, continuous paranematic-to-nematic (P-N) transition is found, which is shifted by 10 K below the discontinuous bulk transition at T_IN=305K. The capillary-condensed state exhibits a more pronounced, albeit still continuous P-N reordering, located 4 K below T_IN. This shift vanishes abruptly on complete filling of the capillaries, which we tentatively trace to a 10 MPa tensile pressure release associated with the disappearance of concave menisci in the confined liquid.