Phase transitions in liquid crystals under negative pressures

Organic Glasses with Tunable Liquid-Crystalline Order

Liquid crystals (LCs) are known to undergo rapid ordering transitions with virtually no hysteresis. We report a remarkable counterexample, itraconazole, where the nematic to smectic transition is avoided at a cooling rate exceeding 20  K/s. The smectic order trapped in a glass is the order reached by the equilibrium liquid before the kinetic arrest of the end-over-end molecular rotation. This is attributed to the fact that smectic ordering requires orientational ordering and suggests a general condition for preparing organic glasses with tunable LC order for electronic applications.

Hydrodynamic cavitation in Stokes flow of anisotropic fluids

Cavitation, the nucleation of vapour in liquids, is ubiquitous in fluid dynamics, and is often implicated in a myriad of industrial and biomedical applications. Although extensively studied in isotropic liquids, corresponding investigations in anisotropic liquids are largely lacking. Here, by combining liquid crystal microfluidic experiments, nonequilibrium molecular dynamics simulations and theoretical arguments, we report flow-induced cavitation in an anisotropic fluid. The cavitation domain nucleates due to sudden pressure drop upon flow past a cylindrical obstacle within a microchannel. For an anisotropic fluid, the inception and growth of the cavitation domain ensued in the Stokes regime, while no cavitation was observed in isotropic liquids flowing under similar hydrodynamic parameters. Using simulations we identify a critical value of the Reynolds number for cavitation inception that scales inversely with the order parameter of the fluid. Strikingly, the critical Reynolds number for anisotropic fluids can be 50% lower than that of isotropic fluids.

Cavitation is the formation of vapour bubbles within a liquid and is undesirable in many industrial applications. Here Stieger et al. show how the anisotropic fluids influence this process in a nematic liquid crystal and find that orientational ordering of molecules can tune the onset of cavitation.

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

We report filling-fraction dependent dielectric spectroscopy measurements on the relaxation dynamics of the rod-like nematogen 7CB condensed in 13 nm silica nanochannels. In the film-condensed regime, a slow interface relaxation dominates the dielectric spectra, whereas from the capillary-condensed state up to complete filling an additional, fast relaxation in the core of the channels is found. The temperature-dependence of the static capacitance, representative of the averaged, collective molecular orientational ordering, indicates a continuous, paranematic-to-nematic (P-N) transition, in contrast to the discontinuous bulk behaviour. It is well described by a Landau-de-Gennes free energy model for a phase transition in cylindrical confinement. The large tensile pressure of 10 MPa in the capillary-condensed state, resulting from the Young-Laplace pressure at highly curved liquid menisci, quantitatively accounts for a downward-shift of the P-N transition and an increased molecular mobility in comparison to the unstretched liquid state of the complete filling. The strengths of the slow and fast relaxations provide local information on the orientational order: the thermotropic behaviour in the core region is bulk-like, i.e. it is characterized by an abrupt onset of the nematic order at the P-N transition. By contrast, the interface ordering exhibits a continuous evolution at the P-N transition. Thus, the phase behaviour of the entirely filled liquid crystal-silica nanocomposite can be quantitatively described by a linear superposition of these distinct nematic order contributions.

Thermotropic nematic order upon nanocapillary filling

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)=305 K. The capillary-condensed state exhibits a more pronounced, albeit still continuous P-N reordering, located 4 K below T(IN). This shift vanishes abruptly upon complete filling of the capillaries. It could originate in competing anchoring conditions at the free inner surfaces and at the pore walls or result from the 10-MPa tensile pressure release associated with the disappearance of concave menisci in the confined liquid upon complete filling. The study documents that the thermo-optical properties of nanoporous systems (or single nanocapillaries) can be tailored over a surprisingly wide range simply by variation of the filling fraction with liquid crystals.

Influence of pressure on the electric-field-induced phase transitions in liquid crystals

Within the framework of Landau-de-Gennes formulation, we analyse the effect of pressure on electric-field-induced phase transitions in a liquid crystal which shows spontaneously an isotropic-smectic A transition. Inferring from the experimental pressure dependences on the layer spacing in smectic A phase, as well as the nematic-smectic A metastable temperature T*(AN), we incorporated the pressure dependence in the free energy through (the surface energy term) and the coupling between the quadrupolar nematic ordering Q(ij) and the smectic order parameter psi. From the S-T phase diagram, we found that the stability of field-induced nematic phase increases with pressure, whereas the discontinuity of the transition decreases. Also, the region where paranematic phase transits directly to smectic A phase increases with pressure.