Colloids of electrically charged nanorods can spontaneously develop a fluid yet ordered liquid crystal phase, but this ordering competes with a tendency to form a gel of percolating rods. The threshold for ordering is reduced by increasing the rod aspect ratio, but the percolation threshold is also reduced with this change; hence, prediction of the outcome is nontrivial. Here, we show that by establishing the phase behavior of suspensions of cellulose nanocrystals (CNCs) fractionated according to length, an increased aspect ratio can strongly favor liquid crystallinity without necessarily influencing gelation. Gelation is instead triggered by increasing the counterion concentration until the CNCs lose colloidal stability, triggering linear aggregation, which promotes percolation regardless of the original rod aspect ratio. Our results shine new light on the competition between liquid crystal formation and gelation in nanoparticle suspensions and provide a path for enhanced control of CNC self-organization for applications in photonic crystal paper or advanced composites.

The interaction between a phase separated polymer network and a liquid crystal was studied across the smectic-A* (Sm-A*) to smectic-C* (Sm-C*) phase transition of a polymer-stabilized ferroelectric liquid crystal polymerized in the Sm-A* phase. Using precise measurements of the tilt angle and the spontaneous polarization as functions of the external electric field and polymer concentration, the effective coefficients of the Landau expansion of the free energy of the Sm-C* phase have been determined experimentally. The observed polymer concentration dependence of the Landau expansion coefficients is explained using a more general theoretical model which incorporates the effect of polymer networks on the local liquid crystal director configuration. In particular, using experimental estimates of the penetration depth of the polymer network into the liquid crystal, it is shown that the b coefficient calculated from the Landau model increases with polymer concentration, evidencing the relationship determined experimentally.

A general phenomenological description and a simple molecular model is proposed for the "discrete" flexoelectric effect in tilted smectic liquid crystal phases. This effect defines a polarization in a smectic layer induced by a difference of director orientations in the two smectic layers adjacent to it. It is shown that the "discrete" flexoelectric effect is determined by electrostatic dipole-quadrupole interaction between positionally correlated molecules located in adjacent smectic layers, while the corresponding dipole-dipole interaction is responsible for a coupling between polarization vectors in neighboring layers. It is shown that a simple phenomenological model of a ferrielectric smectic liquid crystal, which has recently been proposed in the literature, can be used to describe the whole sequence of intermediate chiral smectic C* phases with increasing periods, and to determine the nonplanar structure of each phase without additional assumptions. In this sequence the phases with three- and four-layer periodicities have the same structure, as observed in the experiment. The theory predicts also the structure of intermediate phases with longer periods that have not been studied experimentally so far. The structures of intermediate phases with periodicities of up to nine layers are presented together with the phase diagrams, and a relationship between molecular chirality and the three-dimensional structure of intermediate phases is discussed. It is considered also how the coupling between the spontaneous polarization determined by molecular chirality and the induced polarization determined by the discrete flexoelectric effect stabilizes the nonplanar structure of intermediate phases.

We develop a molecular-statistical theory of the smectic-A-smectic-C transition which is described as a transition of the order-disorder type. The theory is based on a general expansion of the effective interaction potential and employs a complete set of orientational order parameters. All the order parameters of the smectic-C phase including the tilt angle are calculated numerically as functions of temperature for a number of systems which correspond to different transition scenario. The effective interaction potential and the parameters of the transition are also calculated for specific molecular models based on electrostatic and induction interaction between molecular dipoles. The theory successfully reproduces the main properties of both conventional and so-called "de Vries-type" smectic liquid crystals, clarifies the origin of the anomalously weak layer contraction and describes the tricritical behavior at the smectic-A-smectic-C transition. The "de Vries behavior," i.e., anomalously weak layer contraction is also obtained for a particular molecular model based on interaction between longitudinal molecular dipoles. A simple phenomenological model is presented enabling one to obtain explicit expressions for the layer spacing and the tilt angle which are used to fit the experimental data for a number of materials.

In the framework of molecular mean-field theory we study the effect of nanoparticles embedded in nematic liquid crystals on the orientational ordering and nematic–isotropic phase transition. We show that spherically isotropic nanoparticles effectively dilute the liquid crystal medium and decrease the nematic–isotropic transition temperature. At the same time, anisotropic nanoparticles become aligned by the nematic host and, reciprocally, improve the liquid crystal alignment. The theory clarifies the microscopic origin of the experimentally observed shift of the isotropic–nematic phase transition and an improvement of the nematic order in composite materials. A considerable softening of the first order nematic–isotropic transition caused by strongly anisotropic nanoparticles is also predicted