Slobodan Zumer

Reconfigurable knots and links in chiral nematic colloids.

Colloidal particles inside a liquid crystalline solvent can be manipulated to create knots of arbitrary shape and complexity. Tying knots and linking microscopic loops of polymers, macromolecules, or defect lines in complex materials is a challenging task for material scientists. We demonstrate the knotting of microscopic topological defect lines in chiral nematic liquid-crystal colloids into knots and links of arbitrary complexity by using laser tweezers as a micromanipulation tool. All knots and links with up to six crossings, including the Hopf link, the Star of David, and the Borromean rings, are demonstrated, stabilizing colloidal particles into an unusual soft matter. The knots in chiral nematic colloids are classified by the quantized self-linking number, a direct measure of the geometric, or Berry’s, phase. Forming arbitrary microscopic knots and links in chiral nematic colloids is a demonstration of how relevant the topology can be for the material engineering of soft matter.

Polymer networks formed in liquid crystals

Abstract Monomers with reactive double bonds were mixed with liquid crystals and polymerized under UV irradiation. The polymer networks formed are anisotropic and consist of fibrils. The orientation of the polymer networks depend on the orientation of the liquid crystals during polymerization. Optical and scanning electron microscopy (SEM) were used to study the polymer networks.

Mesoscopic modelling of colloids in chiral nematics

We present numerical modelling of colloidal particles in chiral nematics with cubic symmetry (blue phases) within the framework of the Landau-de Gennes free energy. The interaction potential of a single, nano-sized colloidal particle with a -1/2 disclination line is calculated as a generic trapping mechanism for particles within the cholesteric blue phases. The interaction potential is shown to be highly anisotropic and have threefold rotational symmetry. We discuss the equilibration of the colloidal texture with respect to particle positions and the unit cell size of the blue phase. We also describe how preservation of the liquid crystal volume and the number of particles allows blue phase colloidal structures with different unit cell sizes and configurations to be compared numerically.

Flexoelectro-optic effect in a hybrid nematic liquid crystal cell

W e analyse the influence of charged impurities and flexoelectric polarization on the optical transmission of a hybrid aligned nematic liquid crystal cell. The theoretical results obtained within the framework of the Poisson-Boltzmann equation and Frank elastic theory are compared with the observed optical response [N. V. Madhusudana and G. Durand,J. Phys.Lett. 46, L-195 (1985)]. We show that impurities can be very important for the behaviour of the system in the low field regime where the flexoelectric effect is relevant, and we determine the flexoelectric coefficient, the anchoring strength, and the concentration of impurities in the sample previously studied by Madhusudana and Durand.

Effect of flexoelectricity and order electricity on defect cores in nematic droplets

Macroscopic polarization, induced in a strongly deformed nematic director field, is shown to strongly affect the stability of defect structures. We present a numerical study which addresses the role of flexoelectricity and order electricity in confined nematic liquid crystals, more specifically in nematic droplets with planar and homeotropic surface anchoring. Electrostatic potential induced by flexo- and order electricity are calculated in the droplets taking into the account full anisotropy of the nematic director profile. Potentials as high as 1 V are found in 200 nm droplets with flexoelectric constant e = 40 pC m−1. Spatial profiles of contributions to the total free energy are calculated to identify flexo- and order elasticity relevant regions. For different flexoelectric coupling strengths and different anchoring strengths, changes in the core of the defects are examined. Their stability is discussed. Finally, flexo- and order electricity driven transitions between point defect, ring defect and axial structure are demonstrated in homeotropic droplets.

Proton spin-lattice relaxation by critical polarization fluctuations in KH2PO4

The observed anomalous decrease in the proton spin-lattice relaxation timeT1 on approaching the Curie point in a rather pure KH2PO4 single crystal is explained by magnetic dipolar coupling to the ferroelectric mode. The isolated “non-interacting” O−H...O proton flipping time is estimated from theT1 data as τ=0.66·10−12 sec for the paraelectric phase and τ=2.24·10−12 sec for the ferroelectric phase, in good agreement with the results obtained from other methods.

Holographic diffraction gratings using polymer-dispersed ferroelectric liquid crystals

By controlling the morphology of holographic polymer-dispersed ferroelectric liquid crystals (FLCs), highly aligned FLC domains are obtained for diffractive optical applications. Rapid, thresholdless switching is observed for various grating pitch sizes between approximately 3 and approximately 12 microm. A simple phenomenological model is presented encompassing a distribution of domain sizes and an effective field that stabilizes the FLC domains to reflect the observed thresholdless switching and optical behavior.

Complex field-stabilised nematic defect structures in Laguerre–Gaussian optical tweezers

Novel optical field-stabilised defect structures are presented in nematic liquid crystals as produced by Laguerre–Gaussian optical beams. Our modelling study is based on the phenomenological free energy approach. To mimic generic experiments, Laguerre–Gaussian optical beams of various helical indices are applied to a nematic cell. The symmetry of these field-stabilised nematic structures is directly determined by the beam indices. We show that the structures can also be tuned by the beam intensity and the strength of the nematic elastic constant. Finally, local intensity-induced control of the nematic order via absorption of the beam is demonstrated as a complementary mechanism for producing and tuning the nematic field-stabilised structures.

External and intrinsic anchoring in nematic liquid crystals: A Monte Carlo study

We present a Monte Carlo study of external surface anchoring in nematic cells with partially disordered solid substrates, as well as of intrinsic anchoring at free nematic interfaces. The simulations are based on the simple hexagonal lattice model with a spatially anisotropic intermolecular potential. We estimate the corresponding extrapolation length b by imposing an elastic deformation in a hybrid cell-like nematic sample. Our estimates for b increase with increasing surface disorder and are essentially temperature independent. Experimental values of b are approached only when both the coupling of nematic molecules with the substrate and the anisotropy of nematic-nematic interactions are weak.

Liquid Crystal Order in a Highly Restrictive Porous Glass

Abstract The thermodynamic and structural properties of octylcyanobiphenyl (8CB) confined to the highly restrictive and randomly interconnected pores of Vycor glass were studied via AC calorimetry, DSC, and small angle neutron scattering. The weakly first order nematic to isotropic phase transition is absent and replaced by a continuous evolution of local orientational ordering with decreasing temperature. The results are consistent with predictions from a model that treats the host media as a collection of noninteracting pore segments. The continuous smectic-A to nematic phase transition is also absent with no evidence of any smectic ordering. In addition, crystallization is replaced by a glass-like melting transition with indications of another melting transition at much lower temperatures.