Confined Liquid Crystal Research Articles

Functionalization of microfluidic devices for investigation of liquid crystal flows

Systematic studies of thermotropic liquid crystals in confinement, such as liquid crystals in microfluidic channels, require control of the anchoring conditions on the surfaces. Especially for the case of uniform planar anchoring, the standard method involves a mechanical treatment (rubbing) of the surface that is not applicable to microfluidic devices. In the present study, we report methods for the achievement of well-defined anchoring conditions for liquid crystals in microfluidic channels consisting of polydimethylsiloxane and glass. Various physico-chemical techniques enable to establish homeotropic, degenerate planar, uniform planar, and hybrid anchoring conditions on the surface of the channel walls. We characterize the treated surfaces in terms of wettability and liquid crystal anchoring and determine the director field in the microchannels for the different anchoring configurations using polarizing optical microscopy and fluorescence confocal polarization microscopy. The relevance of the surface anchoring for the flow behavior of the liquid crystal in the microchannel is demonstrated by studying the onset of defect-mediated chaotic-like flow at high Ericksen numbers for the different anchoring cases.

Acoustic Torque Acting upon Nematic Liquid Crystals

There is plenty of experimental evidence that the propagation of an ultrasonic wave in a nematic liquid crystal affects the director n, which represents the average molecular orientation, thus producing detectable optical effects. There have been several attempts to explain these observations on the basis of a coherent variational theory. We consider here a general theory for nematoacoustics that incorporates flow effects and that has been recently proposed in E.G. Virga (Phys. Rev. E 80:031705, 2009). An explicit application of the proposed theory to a simple computable case was given in G. De Matteis and E.G. Virga (Phys. Rev. E 83:011703, 2011) by linearizing the corresponding balance equations derived from the basic theory. This study was done in order to estimate phenomenological parameters involved in the theory and by using available experimental data. After reviewing the results previously obtained, as a further application of the governing equations, we consider here an approximate equation for the flowless nematodynamics by introducing a second-order acoustic torque acting upon the nematic and we solve the obtained equation in a standard geometry of confined liquid crystal.

Suppression of Phase Transitions in a Confined Rodlike Liquid Crystal

The nematic-to-isotropic, crystal-to-nematic, and supercooled liquid-to-glass temperatures are studied in the liquid crystal 4-pentyl-4'-cyanobiphenyl (5CB) confined in self-ordered nanoporous alumina. The nematic-to-isotropic and the crystal-to-nematic transition temperatures are reduced linearly with the inverse pore diameter. The finding that the crystalline phase is completely suppressed in pores having diameters of 35 nm and below yields an estimate of the critical nucleus size. The liquid-to-glass temperature is reduced in confinement as anticipated by the model of rotational diffusion within a cavity. These results provide the pertinent phase diagram for a confined liquid crystal and are of technological relevance for the design of liquid crystal-based devices with tunable optical, thermal, and dielectric properties.

G-1-1 ナノ隙間における液体の粘弾性と剪断配向特性の同時計測法に関する研究(G-1 マイクロナノ理工学,口頭発表:マイクロナノ理工学)

Liquids confined in nanometer-sized gap width have characteristic viscoelastic properties. In our previous study, we have developed highly-sensitive shear force measuring method, which we called the "fiber wobbling method (FWM)", and revealed that liquid polymers exhibit enhanced viscoelasticity when they are sheared. Although mechanisms to explain these phenomena must involve molecular orientation confined in the gap, details are not clarified. In this study, we succeeded in simultaneously measuring viscoelasticity and molecular orientation of confined liquid crystal using FWM combined with birefringence measurement. Our experimental results showed that the relationship between nanometer-sized gap widths and viscoelasticity strongly depends on the molecular orientation.

Density functional theory of liquid crystals and surface anchoring: Hard Gaussian overlap-sphere and hard Gaussian overlap-surface potentials

This article applies the density functional theory to confined liquid crystals, comprised of ellipsoidal shaped particles interacting through the hard Gaussian overlap (HGO) potential. The extended restricted orientation model proposed by Moradi and co-workers [J. Phys.: Condens. Matter 17, 5625 (2005)] is used to study the surface anchoring. The excess free energy is calculated as a functional expansion of density around a reference homogeneous fluid. The pair direct correlation function (DCF) of a homogeneous HGO fluid is approximated, based on the optimized sum of Percus-Yevick and Roth DCF for hard spheres; the anisotropy introduced by means of the closest approach parameter, the expression proposed by Marko [Physica B 392, 242 (2007)] for DCF of HGO, and hard ellipsoids were used. In this study we extend an our previous work [Phys. Rev. E 72, 061706 (2005)] on the anchoring behavior of hard particle liquid crystal model, by studying the effect of changing the particle-substrate contact function instead of hard needle-wall potentials. We use the two particle-surface potentials: the HGO-sphere and the HGO-surface potentials. The average number density and order parameter profiles of a confined HGO fluid are obtained using the two particle-wall potentials. For bulk isotropic liquid, the results are in agreement with the Monte Carlo simulation of Barmes and Cleaver [Phys. Rev. E 71, 021705 (2005)]. Also, for the bulk nematic phase, the theory gives the correct density profile and order parameter between the walls.

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.

Influence of confinement on molecular and director reorientational dynamics of liquid crystal

We present the qualitative results of investigations of influence of confinement of liquid crystal in random porous medium on relaxation due to molecular reorientations investigated by dielectric spectroscopy and fluctuations of director orientations (collective relaxation) investigated by photon correlation spectroscopy. The relaxation times of the process due to the molecular rotations in deeply supercooled state of confined liquid crystal were slower than at the temperatures corresponding to nematic phase by a factor of 10 6. This slowing down was accompanied by anomalous broadening of the dielectric spectra. We attribute this qualitative change in molecular relaxation to coupling of molecules to the pore walls and among themselves via molecule-wall interactions. In the nematic phase of bulk liquid crystals, the relaxation process due to director orientation fluctuations (collective mode) was single exponential. The dynamics of this process was also drastically modified by random confinement. The slow relaxation process, which does not exist in the bulk liquid crystal, and a broad spectrum of relaxation times, appear for confined liquid crystal. These results on the influence of confinement on relaxation properties of liquid crystal were complimented by differential scanning calorimetry experiments.

Platform for Controlled Supramolecular Nanoassembly

We here present a two-dimensional (2D) micro/nano-fluidic technique where reactant-doped liquid-crystal films spread and mix on micro- and nanopatterned substrates. Surface-supported phospholipid monolayers are individually doped with complementary DNA molecules which hybridize when these lipid films mix. Using lipid films to convey reactants reduces the dimensionality of traditional 3D chemistry to 2D, and possibly to 1D by confining the lipid film to nanometer-sized lanes. The hybridization event was observed by FRET using single-molecule-sensitive confocal fluorescence detection. We could successfully detect hybridization in lipid streams on 250 nm wide lanes. Our results show that the number and density of reactants as well as sequence of reactant addition can be controlled within confined liquid crystal films, providing a platform for nanochemistry with potential for kinetic control.

Continuous Paranematic-to-Nematic Ordering Transitions of Liquid Crystals in Tubular Silica Nanochannels

The optical birefringence of rodlike nematogens (7CB, 8CB), imbibed in parallel silica channels with 10 nm diameter and 300 microm length, is measured and compared to the thermotropic bulk behavior. The orientational order of the confined liquid crystals, quantified by the uniaxial nematic ordering parameter, evolves continuously between paranematic and nematic states, in contrast to the discontinuous isotropic-to-nematic bulk phase transitions. A Landau-de Gennes model reveals that the strength of the orientational ordering fields, imposed by the silica walls, is beyond a critical threshold, that separates discontinuous from continuous paranematic-to-nematic behavior. Quenched disorder effects, attributable to wall irregularities, leave the transition temperatures affected only marginally, despite the strong ordering fields in the channels.

Composites containing confined n-octyl-cyanobiphenyl: Monomer and dimer species in the surface layer by in situ FTIR spectroscopy

Confinement of 4- n-octyl-4′-cyanobiphenyl (8CB) to nanoporous molecular sieves with hexagonal structure of cylindrical pores (4.6 nm diameter) is studied. Thermogravimetric investigations have indicated that the pores are completely filled. Several surface species inside the pores and onto the external surface of the grains were demonstrated by differential thermal analysis and by in situ infrared spectroscopy. Arguments are given that bulk-like monomer and dimer species along with hydrogen bonded ones might coexist in the so-called surface layer, but their population varies drastically as function of the temperature. In addition, chemical changes of the confined liquid crystal are quite possible inside these nanopores, at temperatures lower than for the bulk.

Computer simulation of bistable switching in a nematic device containing pear-shaped particles

We study the microscopic basis of bistable switching of a confined liquid crystal via Monte Carlo simulations of hard pear-shaped particles. Using both dielectric and dipolar field couplings to this intrinsically flexoelectric fluid, it is shown that pulsed fields of opposing polarity can be used to switch between the vertical and hybrid aligned states. Further, it is shown that the field-susceptibility of the surface polarisation, rather than the bulk flexoelectricity, is the main driver of this switching behaviour.

Density functional theory of liquid crystals and surface anchoring

This paper applies the density functional theory to confined liquid crystals comprising ellipsoidal shaped particles interacting through the hard Gaussian overlap (HGO) potential. The restricted orientation model proposed by Rickayzen [Mol. Phys. 95, 393 (1998)] is extended to study the surface anchoring. The excess free energy is calculated as a functional expansion of density around a reference homogeneous fluid. The pair direct correlation function (DCF) of a homogeneous HGO fluid is approximated, based on the Percus-Yevick DCF for hard spheres; the anisotropy is introduced by means of the closest approach parameter. The average number density and orientational order parameter profiles of a HGO fluid confined in between planar walls are obtained using a hard needle-wall potential to represent the particle-wall interactions. For short and long needle lengths, the homeotropic and planar anchoring are observed, respectively. For the bulk isotropic phase the calculated density and order parameter profiles are in agreement with the Monte Carlo simulation of Barmes and Cleaver [Phys. Rev. E 69, 61705 (2004)]. However, for the bulk nematic phase the theory gives the correct density profile between the walls. The correct order parameters are obtained close to the walls whereas for the region in the middle of the walls, the agreement is less satisfactory.

Isotropic–nematic transition in liquid crystals confined between rough walls

The effect of rough walls on the phase behaviour of a confined liquid crystal (LC) fluid is studied using constant pressure Monte Carlo simulations. The LC is modelled as a fluid of soft ellipsoidal molecules and the rough walls are represented as a hard wall with a number of molecules randomly embedded in them. It is found that the isotropic–nematic (IN) transition is shifted to higher pressures for rougher walls.

Dielectric relaxation in thin liquid crystal layers formed on cylindrical pore walls

Dielectric spectroscopy was used to study the influence of layer thickness of confined liquid crystal (with different orientation of molecules at pore walls) on relaxations due to surface induced polarization and due to molecular reorientations. Low frequency measurements provided information on the relaxation of surface polarization that arose at the liquid crystal–pore wall interface. The dynamics of molecular reorientations were investigated in high frequency experiments. The relaxation of surface induced polarization was slower for confined liquid crystal with axial boundary conditions than for that with a radial orientation, suggesting that the mobility of molecules in surface layers was strongly affected by boundary conditions. The radial alignment of molecules facilitated the investigation of the librational mode. Temperature dependence of relaxation time of this mode was different from that of reorientations of molecules around their short axes.

Atomic force microscope study of presmectic modulation in the nematic and isotropic phases of the liquid crystal octylcyanobiphenyl using piezoresistive force detection

Using a temperature controlled atomic force microscope (AFM), we have studied surface induced pre-smectic order in the nematic and isotropic phases of 4-cyano- 4'-n -octylbiphenyl. A modified AFM head with piezoresitive cantilevers has been used to measure the structural force between a flat BK7 glass plate and a 10 microm glass sphere, both being treated to induce homeotropic alignment of the confined liquid crystal layer in between. We have observed surface-induced presmectic force not only in the isotropic, but also in the nematic phase. We have measured the temperature dependencies of the presmectic force, the smectic correlation length xi and the smectic order parameter psi at the surface. The correlation length xi(T) shows a power-law temperature dependence with a critical exponent of nu=0.67 +/- 0.03 and the bare correlation length of xi(0) = (0.39 +/- 0.08) nm, in good agreement with x-ray data. The smectic density at the surface is psi(2)(S) =0.4 in the nematic phase and decreases in the isotropic phase.

Capillary Smectization and Layering in a Confined Liquid Crystal

Using density-functional theory, we have analyzed the phase behavior of a model liquid crystal confined between two parallel, planar surfaces (i.e., the so-called slit pore). As a result of confinement, a rich phase behavior arises. The complete liquid-crystal phase diagram of the confined fluid is mapped out as a function of wall separation and chemical potential. Strong commensuration effects in the film with respect to wall separation lead to enhanced smectic ordering, which gives capillary smectization (i.e., formation of a smectic phase in the pore), or frustrated smectic ordering, which suppresses capillary smectization. These effects also produce layering transitions. Our nonlocal density-functional-based analysis provides a unified picture of all the above phenomena.

Dynamics of Director Fluctuations in Confined Liquid Crystals

AbstractDynamic light scattering was applied to study the influence of randomness, boundary conditions (planar-axial and homeotropic-radial) and layer thickness (at nanoscale) of 5CB and 8CB confined to cylindrical pores and filled with Aerosil particles on relaxation of director orientational fluctuations. For confined 8CB in the nematic phase two well-defined relaxation processes were observed for both axial and radial orientations of the liquid crystal. The first process is qualitatively associated with bulk-like nematic director fluctuations. The second relaxation process (with slower relaxation time than the first one) is most likely due to the fluctuations in layers nearest the wall surface. The separation between the first and the second (slow) processes is clearer for thinner layers and the amplitude of the slow process is greater for thinner layers. This suggests that the slow process is surface related relaxation. The mode due to relaxation of fluctuations of director orientation in the vicinity of the surface of Aerosil particles was observed in filled liquid crystals as well.

Potential of mean force between a spherical particle suspended in a nematic liquid crystal and a substrate: sphere size effects.

The expanded ensemble density of states method (ExEDOS) is used to investigate the effective interaction of a spherical colloidal particle suspended in a confined liquid crystal (LC) with a substrate. The potential of mean force (PMF) is determined as a function of the normal distance between the particle and the substrate's surface. The presence of the substrate induces a layered structure of the LC, which in turn greatly influences the PMF. We analyze the structure of the Saturn ring defect that accompanies the colloidal sphere, and find that the ring is displaced slightly towards the surface when the sphere is within the first LC surface layer. A transition occurs from an overall attraction of the colloid to the substrate to a global repulsion when the sphere's radius is roughly twice the length of the LC molecules.

Controlling disorder in liquid crystal aerosil dispersions.

The effect of disorder in the behavior of liquid crystal (LC) is assessed and controlled by dispersing known amounts of silica aerosil in the liquid crystal material. Using deuteron nuclear magnetic resonance, the director configuration and the orientational order was determined for hydrophilic aerosil dispersions in octylcyanobiphenyl. The confined liquid crystal exhibits a well-defined alignment as the silica spheres stabilize the molecular configuration. At low silica densities, a silica network is eventually established, forming a soft gel. When the sample orientation in the magnetic field is changed, a few silica strands links are broken and a fraction of the LC molecules is realigned. The field anneals the random disorder introduced by the aerosil up to a certain density beyond which, in the so-called stiff-gel regime, disordering effects completely dominate. At a fixed temperature in the isotropic phase, there is surface-induced order that is linearly proportional to the silica density.

Effects of wetting and anchoring on capillary phenomena in a confined liquid crystal.

A fluid of hard spherocylinders of length-to-breadth ratio L/D=5 confined between two identical planar, parallel walls--forming a pore of slit geometry--has been studied using a version of the Onsager density-functional theory. The walls impose an exclusion boundary condition over the particle's centers of mass, while at the same time favoring a particular anchoring at the walls, either parallel or perpendicular to the substrate. We observe the occurrence of a capillary transition, i.e., a phase transition associated with the formation of a nematic film inside the pore at a chemical potential different from micro(b)-the chemical potential at the bulk isotropic-nematic transition. This transition terminates at an Ising-type surface critical point. In line with previous studies based on the macroscopic Kelvin equation and the mesoscopic Landau-de Gennes approach, our microscopic model indicates that the capillary transition is greatly affected by the wetting and anchoring properties of the semi-infinite system, i.e., when the fluid is in contact with a single wall or, equivalently, the walls are at a very large distance. Specifically, in a situation where the walls are preferentially wetted by the nematic phase in the semi-infinite system, one has the standard scenario with the capillary transition taking place at chemical potentials less than micro(b) (capillary nematization transition or capillary ordering transition). By contrast, if the walls tend to orientationally disorder the fluid, the capillary transition may occur at chemical potentials larger than micro(b), in what may be called a capillary isotropization transition or capillary disordering transition. Moreover, the anchoring transition that occurs in the semi-infinite system may affect very decisively the confinement properties of the liquid crystal and the capillary transitions may become considerably more complicated.