Thickness-dependent phase transition in thin nematic films

Enhanced Orientation of Liquid Crystals Inside Micropores of Metal-Organic Frameworks Having Thermoresponsivity

With the aim of controlling the orientation of liquid crystals (LCs) toward realizing external stimuli-responsive materials with tunable functionalities, we synthesized a composite of LCs and metal-organic frameworks (MOFs) by filling LCs into the pores of MOFs (LC@MOFs) for the first time. The included LCs interact with the MOFs through coordination bonds between the cyano groups of the LCs and the metal ions of the MOFs, enabling the orientation of the LC molecules inside the pores of the MOFs and the realization of birefringence of LC@MOFs. The three-dimensional nanometer interstice frameworks maintained the LC orientation even at temperatures much higher than the isotropic phase transition temperature of bulk LCs. Furthermore, the orientational state changed upon heating or cooling, inducing temperature-dependent birefringence. This study provides a new approach to the development of stimuli-responsive optical materials and stimuli-responsive MOFs.

2D isotropic-nematic transition in colloidal suspensions of ellipsoids

Liquid crystals are important condensed matter systems for technological applications, as well as for fundamental studies. An important unresolved issue is the nature of the phase transition in a two-dimensional (2D) liquid crystal system. In contrast to numerous computational studies reported in the last few decades, there have been no convincing experiments to verify these numerical results. Anisotropic colloids provide an excellent experimental model system to study phase transitions, such as crystallization and glass transition in condensed matter physics with single particle resolution. However, using colloids to probe the two-dimensional liquid crystal transition remains a challenge, since the condensed anisotropic colloids usually become stuck in the metastable glassy state rather than approaching their equilibrium liquid crystal phase. Here we report a method of using an external magnetic field to assist a colloidal system of super-paramagnetic anisotropic particles to overcome the local free energy barriers in the metastable states and approach the equilibrium phase. The experiments demonstrate a 2D isotropic-nematic phase transition with increasing packing density. The effects of the anisotropy of the colloidal particles on the 2D isotropic-nematic transition are explored. Our experimental results are compared with those from previous computational work, and quantitative agreements are reached.

Control of drop shape transformations in cooled emulsions

The general mechanisms of structure and form generation are the keys to understanding the fundamental processes of morphogenesis in living and non-living systems. In our recent study (Denkov et al., Nature 528 (2015) 392) we showed that micrometer sized n-alkane drops, dispersed in aqueous surfactant solutions, can break symmetry upon cooling and "self-shape" into a series of geometric shapes with complex internal structure. This phenomenon is important in two contexts, as it provides: (a) new, highly efficient bottom-up approach for producing particles with complex shapes, and (b) remarkably simple system, from the viewpoint of its chemical composition, which exhibits the basic processes of structure and shape transformations, reminiscent of morphogenesis events in living organisms. In the current study, we show for the first time that drops of other chemical substances, such as long-chain alcohols, triglycerides, alkyl cyclohexanes, and linear alkenes, can also evolve spontaneously into similar non-spherical shapes. We demonstrate that the main factors which control the drop "self-shaping", are the surfactant type and chain length, cooling rate, and initial drop size. The studied surfactants are classified into four distinct groups, with respect to their effect on the "self-shaping" phenomenon. Coherent explanations of the main experimental trends are proposed. The obtained results open new prospects for fundamental and applied research in several fields, as they demonstrate that: (1) very simple chemical systems may show complex structure and shape shifts, similar to those observed in living organisms; (2) the molecular self-assembly in frustrated confinement may result in complex events, governed by the laws of elasto-capillarity and tensegrity; (3) the surfactant type and cooling rate could be used to obtain micro-particles with desired shapes and aspect ratios; and (4) the systems studied serve as a powerful toolbox to investigate systematically these phenomena.

Molecular Dynamics Simulations and their Application to Thin-film Devices

The performance of devices based on organic semiconductors strongly depends on the molecular organisation in thin films. Due to the intrinsic complexity of these systems, a combination of theoretical modelling and experimental techniques is often the key to achieve a full understanding of their inner working. Here, we introduce the modelling of organic semiconductors by means of molecular dynamics simulations. We describe the basic theoretical framework of the technique and review the most popular class of force fields used to model organic materials, paying particular attention to the peculiarities of confined systems like nano-thick films. Representative studies of the organisation of organic functional materials in thin film phases are also reviewed.

Self-organized arrays of dislocations in thin smectic liquid crystal films

Combining optical microscopy, synchrotron X-ray diffraction and ellipsometry, we studied the internal structure of linear defect domains (oily streaks) in films of a smectic liquid crystal 8CB with thicknesses in the range of 100-300 nm. These films are confined between air and a rubbed PVA polymer substrate which imposes hybrid anchoring conditions (normal and unidirectional planar, respectively). We show how the presence or absence of dislocations controls the structure of highly deformed thin smectic films. Each domain contains smectic layers curved in the shape of flattened hemicylinders to satisfy both anchoring conditions, together with grain boundaries whose size and shape are controlled by the presence of dislocation lines. A flat grain boundary normal to the interface connects neighboring hemicylinders, while a rotating grain boundary (RGB) is located near the axis of curvature of the cylinders. The RGB shape appears such that dislocation lines are concentrated at its summit close to the air interface. The smectic layers reach the polymer substrate via a transition region where the smectic layer orientation satisfies the planar anchoring conditions over the entire polymer substrate and whose thickness does not depend on that of the film. The strength of planar anchoring appears to be high, larger than 10(-2) mJ m(-2), compensating for the high energy cost of creating an additional 2D defect between a horizontal smectic layer and perpendicular ones of the transition region. This 2D defect may be melted, in order to avoid the creation of a transition region structure composed of a large number of dislocations. As a result, linear defect domains can be considered as arrays of oriented defects, straight dislocations of various Burger vectors, whose location is now known, and 2D nematic defects. The possibility of easy variation between the present structure with a moderate amount of dislocations and a structure with a large number of dislocations is also demonstrated.

Crossover from a Kosterlitz-Thouless phase transition to a discontinuous phase transition in two-dimensional liquid crystals

Liquid crystals in two dimensions do not support long-range nematic order, but a quasinematic phase where the orientational correlations decay algebraically is possible. The transition from the isotropic to the quasinematic phase can be continuous and of the Kosterlitz-Thouless type, or it can be first order. We report here on a liquid-crystal model where the nature of the isotropic to quasinematic transition can be tuned via a single parameter p in the pair potential. For p<p(t), the transition is of the Kosterlitz-Thouless type, while for p>p(t), it is first order. Precisely at p=p(t), there is a tricritical point where, in addition to the orientational correlations, also the positional correlations decay algebraically. The tricritical behavior is analyzed in detail, including an accurate estimate of p(t). The results follow from extensive Monte Carlo simulations combined with a finite-size scaling analysis. Paramount in the analysis is a scheme to facilitate the extrapolation of simulation data in parameters that are not necessarily field variables (in this case, the parameter p), the details of which are also provided. This scheme provides a simple and powerful alternative for situations where standard histogram reweighting cannot be applied.

NMR of guest-host systems: 8CB in chiral nematic porous glasses

Liquid crystals confined to porous materials often have different critical phenomena and ordering than in the bulk. Through the selection of pore size, structure and guest liquid crystal, these systems could enable a variety of functional materials for applications such as sensors and displays. A recent example of such a system is chiral nematic mesoporous films infiltrated with liquid crystal 4-cyano-4'-n-octylbiphenyl (8CB), which has reversible thermal switching of its optical bandgap. The optical bandgap is lost when the ordered 8CB guests are heated above ∼50 °C, where the 8CB becomes isotropic. In this study, we have used NMR cryoporometry and pulsed-field gradient diffusion measurements to determine the pore sizes and structures of various chiral nematic mesoporous silica and organosilica films. Temperature and orientation-dependent wideline (15)N NMR spectra of films infiltrated with (15)N-labelled 8CB guests show that the ordering of the 8CB mesogens is consistent with an average orientation parallel to the chiral nematic pore axes. Inclusion of a large, orientation-dependent shift was necessary to fit the spectra, probably due to susceptibility differences between the 8CB guests and the organosilica host.

Thermal switching of the reflection in chiral nematic mesoporous organosilica films infiltrated with liquid crystals

Materials that undergo stimulus-induced optical changes are important for many new technologies. In this paper, we describe a new free-standing silica-based composite film that exhibits reversible thermochromic reflection, induced by a liquid crystalline guest in the pores of iridescent mesoporous films. We demonstrate that selective reflection from the novel mesoporous organosilica material with chiral nematic organization can be reversibly switched by thermal cycling of the 8CB guest between its isotropic and liquid crystalline states, which was proven by solid-state NMR experiments. The switching of the optical properties of the chiral solid-state host by stimulus-induced transitions of the guest opens the possibility of applications for these novel materials in sensors and displays.

Predicting the anchoring of liquid crystals at a solid surface: 5-cyanobiphenyl on cristobalite and glassy silica surfaces of increasing roughness

We employ atomistic molecular dynamics simulations to predict the alignment and anchoring strength of a typical nematic liquid crystal, 4-n-pentyl-4'-cyano biphenyl (5CB), on different forms of silica. In particular, we study a thin (~20 nm) film of 5CB supported on surfaces of crystalline (cristobalite) and amorphous silica of different roughness. We find that the orientational order at the surface and the anchoring strength depend on the morphology of the silica surface and its roughness. Cristobalite yields a uniform planar orientation and increases the order at the surface with respect to the bulk whereas amorphous glass has a disordering effect. Despite the low order at the amorphous surfaces, a planar orientation is established with a persistence length into the film higher than the one obtained for cristobalite.

Direct nanomechanical measurement of an anchoring transition in a nematic liquid crystal subject to hybrid anchoring conditions

We have used a surface forces apparatus to measure the normal force between two solid curved surfaces confining a film of nematic liquid crystal (5CB, 4'-n-pentyl-4-cyanobiphenyl) under hybrid planar-homeotropic anchoring conditions. Upon reduction of the surface separation D, we measured an increasingly repulsive force in the range D = 35-80 nm, reaching a plateau in the range D = 10-35 nm, followed by a short-range oscillatory force at D < 5 nm. The oscillation period was comparable to the cross-sectional diameter of the liquid crystal molecule and characteristic of a configuration with the molecules parallel to the surfaces. These results show that the director field underwent a confinement-induced transition from a splay-bend distorted configuration at large D, which produces elastic repulsive forces, to a uniform planar nondegenerate configuration with broken homeotropic anchoring, which does not produce additional elastic forces as D is decreased. These findings, supported by measurements of the birefringence of the confined film at different film thicknesses, provide the first direct observation of an anchoring transition on the nanometer scale.

Experimental study of hybrid nematic wetting films

Liquid crystal layers, with thickness less than 1 μm, are deposited on isotropic - solid or liquid - substrates and investigated in the bulk nematic range of temperatures. The boundary conditions at interfaces are antagonist ones, therefore the layers are distorted due to nematic elasticity. These films are referred to as "hybrid nematics". The consequences are complex. First, a forbidden range of film thickness is observed, depending only on temperature. Second, the anisotropy of the elastic response gives rise to striking stripe patterns in the thicker films. This behavior is common to several members of the series of n-cyanobiphenyls deposited on oxidized silicon wafers, water and glycerol. The aim of the study is to collect data, and determine which ones find a place within a common theoretical framework.

Competition between capillarity, layering and biaxiality in a confined liquid crystal

The effect of confinement on the phase behaviour and structure of fluids made of biaxial hard particles (cuboids) is examined theoretically by means of Onsager second-order virial theory in the limit where the long particle axes are frozen in a mutually parallel configuration. Confinement is induced by two parallel planar hard walls (slit-pore geometry), with particle long axes perpendicular to the walls (perfect homeotropic anchoring). In bulk, a continuous nematic-to-smectic transition takes place, while shape anisotropy in the (rectangular) particle cross-section induces biaxial ordering. As a consequence, four bulk phases, uniaxial and biaxial nematic and smectic phases, can be stabilised as the cross-sectional aspect ratio is varied. On confining the fluid, the nematic-to-smectic transition is suppressed, and either uniaxial or biaxial phases, separated by a continuous transition, can be present. Smectic ordering develops continuously from the walls for increasing particle concentration (in agreement with the supression of nematic-smectic second-order transition at confinement), but first-order layering transitions, involving structures with n and n + 1 layers, arise in the confined fluid at high concentration. Competition between layering and uniaxial-biaxial ordering leads to three different types of layering transitions, at which the two coexisting structures can be both uniaxial, one uniaxial and another biaxial, or both biaxial. Also, the interplay between molecular biaxiality and wall interactions is very subtle: while the hard wall disfavours the formation of the biaxial phase, biaxiality is against the layering transitions, as we have shown by comparing the confined phase behaviour of cylinders and cuboids. The predictive power of Onsager theory is checked and confirmed by performing some calculations based on fundamental-measure theory.

Isotropic-to-nematic transition in confined liquid crystals: an essentially nonuniversal phenomenon

Computer simulations are presented of the isotropic-to-nematic transition in a liquid crystal confined between two parallel plates a distance H apart. The plates are neutral and do not impose any anchoring on the particles. Depending on the shape of the pair potential acting between the particles, we find that the transition either changes from first order to continuous at a critical film thickness H=H(x) , or that the transition remains first order irrespective of H . This demonstrates that the isotropic-to-nematic transition in confined geometry is not characterized by any universality class, but rather that its fate is determined by microscopic details. The resulting capillary phase diagrams can thus assume two topologies: one where the isotropic and nematic branches of the binodal meet at H=H(x), and one where they remain separated. For values of H where the transition is strongly first order the shift Deltaepsilon of the transition temperature is in excellent agreement with the Kelvin equation. Not only is the relation Deltaepsilon proportional, variant 1/H recovered but also the prefactor of the shift is in quantitative agreement with the independently measured bulk latent heat and interfacial tension.

Thin-thick coexistence behavior of 8CB liquid crystalline films on silicon

The wetting behavior of thin films of 4-n-octyl-4'-cyanobiphenyl (8CB) on Si is investigated via optical and x-ray reflectivity measurement. An experimental phase diagram is obtained showing a broad thick-thin coexistence region spanning the bulk isotropic-to-nematic (T(IN)) and the nematic-to-smectic-A (T(NA)) temperatures. For Si surfaces with coverages between 47 and 72 +/- 3 nm, reentrant wetting behavior is observed twice as we increase the temperature, with separate coexistence behaviors near T(IN) and T(NA). For coverages less than 47 nm, however, the two coexistence behaviors merge into a single coexistence region. The observed thin-thick coexistence near the second-order NA transition is not anticipated by any previous theory or experiment. Nevertheless, the behavior of the thin and thick phases within the coexistence regions is consistent with this being an equilibrium phenomenon.

Isotropic-to-nematic phase transition in a liquid-crystal droplet

In this paper, we focus on the isotropic-to-nematic phase transition in a liquid-crystal droplet. We present the results of an experiment to measure the growth of the nematic phase within an isotropic phase liquid-crystal droplet. Experimentally, we observe two primary phase transition regimes. At short time scales, our experimental results (R(t) approximately t0.51) show good agreement with a Stefan-type model of the evolution of the nematic phase within the isotropic phase of a liquid crystal. As time progresses, the growth of the nematic phase is restricted by increased confinement of the droplet boundary. During this stage of growth, the nematic phase grows at a slower rate of R(t) approximately t0.31. The slower growth at later stages might be due to a variety of factors such as confinement-induced latent heat reduction; a change of defect strength during its evolution; or interactions between the defect and the interface between the liquid crystal and oil or between adjacent defects. The presence of two growth regimes is also consistent with the molecular simulations of Bradac et al. (Bradac, Z.; Kralj, S.; Zumer, S. Phys. Rev. E 2002, 65, 021705) who identify an early stage domain regime and a late stage confinement regime. For the domain and confinement regimes, Bradac et al. (Bradac, Z.; Kralj, S.; Zumer, S. Phys. Rev. E 2002, 65, 021705) obtained growth exponents of 0.49 +/- 0.05 and 0.25 +/- 0.05. These are remarkably close to the values 0.51 and 0.31 observed in our experiments.

Planar anchoring and surface melting in the smectic-A phase

We study ultrathin films of 8CB in planar anchoring on a MoS2 inorganic substrate. We evidence an anchoring breakage for 60-nm-thick films, in favor of the homeotropic anchoring at the air interface. This allows one to determine the 8CB-MoS2 smectic anchoring energy. We then demonstrate for films thinner than 60 nm that, under the homeotropic bulk, an intermediate film remains in planar anchoring, associated with a melting of the smectic layers close to the substrate. Such a melting could be general for planar or tilted anchorings and we show that, for strong anchorings, the anchoring energy can be driven by the deformations of this intermediate nematic film.

Spherical particle immersed in a nematic liquid crystal: effects of confinement on the director field configurations

The effects of confinement on the director field configurations are studied for a spherical particle immersed in a nematic liquid crystal. The liquid crystal is confined in a cylindrical geometry and the particle is located on the axis of symmetry. A finite element method is used to minimize the Frank free energy for various sizes of the system. The liquid crystal is assumed to possess strong anchoring at all the surfaces in the system. Two structures are examined for strong homeotropic anchoring at the surface of the particle: configuration with a Saturn ring disclination line and configuration with a satellite point defect (hedgehog defect). It is shown that the equilibrium locations of the Saturn ring and of the hedgehog point defect change with confinement. It is also found that confinement induces an increase in the elastic free energy that differs substantially with the type of topological defect under consideration. In particular, the evaluation of the total free energy that includes an approximate contribution for the core defect shows that, for micrometer-sized particles in confined systems, the Saturn ring configuration appears to be more stable than the hedgehog defect. This result is in contrast to the bulk situation, where the hedgehog is more stable than the Saturn ring, and it helps explain recent experimental observations of Saturn ring defects around confined micrometer-sized solid particles.

Binary separation in very thin nematic films: thickness and phase coexistence

The behavior as a function of temperature of very thin films (10 to 200 nm) of pentylcyanobiphenyl on silicon substrates is reported. In the vicinity of the nematic-isotropic transition we observe a coexistence of two regions of different thicknesses: thick regions are in the nematic state while thin ones are in the isotropic state. Moreover, the transition temperature is shifted downward following a 1/h(2) law ( h is the film thickness). Microscope observations and small-angle x-ray scattering allowed us to draw a phase diagram which is explained in terms of a binary first-order phase transition where thickness plays the role of an order parameter.