Table of contents

The papers presented in this special issue of Journal of Physics: Condensed Matterform the proceedings of an ESF PESC Exploratory Workshop on Liquid Crystal (LC) Colloid Dispersions held in Bled, Slovenia, in August 2003. This meeting was attended by 50 leading scientists in the field, mainly from Europe but also from the United States and Japan.

The workshop was convened by Chris Care (Sheffield), Slobodan Zumer (Ljubliana) and Philippe Poulin (Bordeaux), in part, to provide a forum for discussion of work performed on LC colloidal suspensions over the past ten years. It was also used to assess how best to coordinate future developments in this field through an international, multi-disciplinary research effort.

The scientific background to the meeting was founded on previous findings that colloidal particles, of the order of a micron in size, experience additional interactions when suspended in a nematic LC. These interactions, which arise from the distortions and defects induced in the nematic elastic field, lead to a range of new self-assembling microstructures with novel optical and mechanical properties.

The presentations made at the meeting fell approximately into the following areas

• Particle assemblies induced by the LC elastic field and/or nematic-isotropic phase separation

• Modelling of colloid–nematic and colloid–colloid interactions and dynamics

• Surface–surface interactions and substrate-induced effects

• Particle assemblies in lamellar/chiral structures

• Anisotropic colloidal particles in nematics

• Effects of external fields on LC colloids

• Theories of compositional structure in LCs

and the papers included in this issue are grouped along similar lines.

The meeting concluded with a round-table discussion at which key areas for future collaboration were identified. These focused, in part, on extending the range of particle sizes and shapes to be used as inclusions in LC solvents. It was agreed that these and other new directions had the potential to yield materials which could be exploited as sensors, actuators, switchable materials, photonic materials, encapsulation media, displays and very high strength, lightweight, materials.

Overall, this workshop proved an excellent forum for an open exchange of ideas between scientists covering a wide range of disciplines (physics, chemistry, mathematics and engineering). The local organization, by Slobodan Zumer and Primoz Ziherl, was excellent, and the scenic setting of Bled was enjoyed by all.

We study the behaviour of colloidal particles when introduced into a liquid crystal matrix. First, we probe the influence of the surface anchoring energy by using various mixtures of surfactant and then, we investigate the influence of the nematogen shape. When the surface energy dominates, a hedgehog defect is formed and, according to an electrostatic analogy, the distortions around the particles exhibit a dipolar character. By contrast, for weaker anchoring, the configuration becomes quadrupolar as evidenced by the observation of Saturn ring defects in thermotropic systems and the structure of latex clusters in lyotropic systems. Also, the shape of the nematogens is shown to control the type of defect and interaction that take place in the system, going from a dipolar symmetry in the discotic phase to a quadrupolar one in the calamitic phase. A simple scaling analysis accounts for our findings.

We present a computer simulation study of a colloidal particle immersed in a solvent comprising liquid crystalline rod-shaped particles and a 10% number concentration of small spherical additives. The presence of the colloidal particle (and its periodic images) is found to induce qualitative changes in the phase behaviour of the rod–sphere mixture. When the colloidal particle favours radial anchoring, it is found that the small spheres spontaneously aggregate to form a droplet which resides in the equatorial plane of the colloidal particle. When the colloidal particle favours tangential anchoring, however, the small spheres aggregate to form droplets at each of the boojums seen experimentally. These findings confirm expectations that small additives to liquid crystalline systems should preferentially reside in disordered regions, whilst also reflecting the competing influence of surface tension effects.

We studied the formation of particle networks in colloid + liquid crystal mixtures cooled below the isotropic–nematic transition temperature by time-resolved laser scanning confocal microscopy. Our observations confirm a recent suggestion that alkane impurities play a crucial role in slowing down the speed of the isotropic–nematic interface. This enables the growing nematic droplets to 'push' particles into increasingly concentrated regions, ultimately resulting in a cellular network solid. We also found that faster cooling rates resulted in increasingly hierarchical cellular structures.

We first review the method of treating nematic wetting of planar surfaces following the approach introduced by Sheng. We present a phase diagram, which in the present form we have not found in the original literature. Then we consider spherical geometries of arbitrary radii, such as colloidal particles, and introduce an appropriate method for handling wetting in curved geometries. We find that a prewetting transition for high curvatures or small particle radii no longer occurs.

We use adaptive finite elements methods to investigate a variety of structures in inverted nematic emulsions numerically. In particular, we study dipolar and quadrupolar interactions between colloidal discs in two-dimensional nematics. The behaviour of colloidal particles near a substrate and at a nematic–isotropic interface are also considered.

A lattice Boltzmann (LB) model of an interface between a nematic and an isotropic fluid is presented. The method is used to study, in two dimensions, the properties of a deformable colloidal droplet of an isotropic fluid suspended in a nematic matrix. The isotropic fluid is modelled by a standard lattice Bhatnagar–Gross–Krook (LBGK) scheme. The LB model of the nematic is a modified LBGK scheme in which a tensor density is used to recover the variable order parameter nemato-dynamics scheme proposed by Qian and Sheng. The interface between the two fluids is modelled by introducing appropriate forcing at the interface. The stress balance is achieved using an extension of a method proposed by Lishchuk et al, and the torque balance is achieved with an appropriate surface molecular field. The resulting interface algorithm recovers the macroscopic equations developed by Rey. Results are presented for the dependence of the shape of the droplet and the nematic defects upon the surface tension and the anchoring strength. A discussion is also presented of the effect of curvature rigidity on the droplet shape.

A new method is presented for mesoscopic simulations of particle dispersions in liquid crystal solvents. It allows efficient first principle simulations of the dispersions involving many particles with many body interactions mediated by the solvents. Demonstrations have been performed for the aggregation of colloid dispersions in two dimensional nematic and smectic-C* solvents neglecting hydrodynamic effects, which will be taken into account in the near future.

We present the results of our numerical calculations that focus on the dynamics of a nematic liquid crystal around a spherical particle imposing strong homeotropic anchoring at the surface. The first part of this article is devoted to the discussion of the effect of an external magnetic or electric field on the director configuration of a nematic liquid crystal. With the aid of an adaptive mesh refinement scheme, together with the tensor description of the orientational order, for the first time in numerical calculations we successfully reproduce the transition from a hyperbolic hedgehog defect to a Saturn ring defect, which was observed in a recent experiment. We also find that the trajectories of the defect core sensitively depend on the field strength. In the second part we investigate how a hydrodynamic flow influences the orientational order of a nematic liquid crystal around a particle carrying a hyperbolic hedgehog defect. We observe that for an intermediate Ericksen number, which characterizes the ratio of the viscous force to the elastic force of a nematic liquid crystal, the liquid crystal is strongly convected by the flow, which results in a considerable elastic distortion.

A simple model is proposed for the liquid crystal matrix surrounding 'soft' colloidal particles whose separation is much smaller than their radii. We use our implementation of the Onsager approximation of density-functional theory (Chrzanowska et al 2001 J. Phys.: Condens. Matter13 4715) to calculate the structure of a nanometrically thin film of hard Gaussian overlap particles of elongations κ = 3 and 5, confined between two solid walls. The penetrability of either substrate can be tuned independently to yield symmetric or hybrid alignment. Comparison with Monte Carlo simulations of the same system (Cleaver and Teixeira 2001 Chem. Phys. Lett.338 1, Barmes and Cleaver 2004 in preparation) reveals good agreement in the symmetric case.

Metallic nanoparticles dispersed in a cholesteric liquid crystal can order in accordance with the helical structure of the chiral phase. Since the liquid crystals we used have a glassy state, the nanostructures may be examined by transmission electron microscopy. The platinum nanoparticles form periodic ribbons which mimic the well-known fingerprint cholesteric texture. The particles do not decorate the pristine texture but create a novel structure with a larger periodicity. The distance between the ribbons is directly correlated to the helical pitch which therefore becomes a simple control parameter to tune the structuring of nanoparticles. Investigations of cross-sections show how the particles are arranged in the volume; a selective segregation proceeds at the periphery of the film and the particle ordering is localized close to the film–air interface. On the fingerprint patterning of nanoparticles, we do an analogy with the positive staining of polymer films with heavy-metal-containing compounds for transmission electron microscopy investigations and we discuss the accumulation of particles in the sites with the highest energy of director distortions.

We report x-ray scattering studies of the smectic liquid crystal octylcyano-biphenol (8CB) confined by strained colloidal silica gels. The gels, comprised of aerosil particles, possess an anisotropic structure that stabilizes long-range nematic order in the liquid crystal while introducing random field effects that disrupt the smectic transition. The short-range smectic correlations that form within this environment are inconsistent with the presence of a topologically ordered state predicted for 3D random field XY systems and are quantitatively like the correlations of smectics confined by isotropic gels. Detailed analysis reveals that the quenched disorder suppresses the anisotropic scaling of the smectic correlation lengths observed in the pure liquid crystal. These results and additional measurements of the smectic-A to smectic-C transition in 4-n-pentylphenylthiol--n-octyloxybenzoate () indicate that the observed smectic behaviour is dictated by random fields coupling directly to the smectic order while fields coupling to the nematic director play a subordinate role.

Using an extension of the Parsons–Lee density-functional theory, we have calculated the phase behaviour of mixtures of hard bodies, focusing on the formation of smectic phases. The interactions are represented by hard spherocylinders, and the mixtures have two components of lengths L₁, L₂ and widths D₁, D₂, respectively, with D₁ = D₂. The special case where one of the components is a hard sphere (L₁ = 0, ) is also studied. Particular emphasis is put on the interplay between smectic-phase formation and smectic–smectic segregation. In general, smectic–smectic segregation is seen to occur in a wide range of compositions and pressures, except when the length ratio is relatively close to unity, i.e. particles have similar lengths; in this case segregation appears only at high pressure. Finally, in the case where q is very different from unity and the composition is such that there is a small fraction of long molecules, even when the mixture is macroscopically homogeneous, there appears a microsegregated phase where the minority component is expelled to the interstitial regions between the smectic layers.

An overview is given of the isotropic and nematic phase behaviour in binary mixtures of hard rods or plates with different lengths or diameters within the framework of the Onsager theory. On the basis of Gaussian trial functions the relative importance of different entropic contributions in the demixing of the isotropic and nematic phases is explained for different mixtures. Modifications of this theory are discussed and new results are given for mixtures of plates differing only in diameter or thickness.

We report on a Monte Carlo study of the pathway for crystal nucleation in a fluid of short, hard, colloidal rods. In the earliest stages of nucleation, a single-layered lamellar crystallite forms. Subsequent thickening of this lamella is hampered by the fact that the top and bottom surfaces of the crystallite are preferentially covered by rods that align parallel to the surface. As a single lamella is thermodynamically not stable, subsequent growth of individual crystals is stunted. Recently experimental evidence for such stunted crystal growth has been reported by Maeda and Maeda (2003 Phys. Rev. Lett.90 018303) for experiments on suspensions of colloidal rods.