Thermal conductivity of the nematic liquid crystal 4-n-pentyl-4'-cyanobiphenyl

Light-induced bi-directional switching of thermal conductivity in azobenzene-doped liquid crystal mesophases

The development of systems that can be switched between states with different thermal conductivities is one of the current challenges in materials science. Despite their enormous diversity and chemical richness, molecular materials have been only scarcely explored in this regard. Here, we report a reversible, light-triggered thermal conductivity switching of ≈30-40% in mesophases of pure 4,4'-dialkyloxy-3-methylazobenzene. By doping a liquid crystal matrix with the azobenzene molecules, reversible and bidirectional switching of the thermal conductivity can be achieved by UV/Vis-light irradiation. Given the enormous variety of photoactive molecules and chemically compatible liquid crystal mesophases, this approach opens unforeseen possibilities for developing effective thermal switches based on molecular materials.

Silicon-organic hybrid thermo-optic switch based on a slot waveguide directional coupler

We propose and demonstrate a passively biased 2 × 2 thermo-optic switch with high power efficiency and fast response time. The device benefits from the highly concentrated optical field of a slot waveguide mode and the strong thermo-optic effect of a nematic liquid crystal (NLC) cladding. The NLC fills the nano-slot region and is aligned by the subwavelength grating inside. The measured power consumption and thermal time constant are 0.58 mW and 11.8 µs, respectively, corresponding to a figure-of-merit of 6.8 mW µs. The proposed silicon-organic hybrid device provides a new solution to design thermo-optic actuators having low power consumption and fast operation speed.

Spontaneous emission in inertial and dissipative nematic liquid crystals: the role of critical phenomena

We develop a rigorous, field-theoretical approach to the study of spontaneous emission in inertial and dissipative nematic liquid crystals (LCs), disclosing an alternative application of the massive Stückelberg gauge theory to describe critical phenomena in these systems. This approach allows one not only to unveil the role of phase transitions in the spontaneous emission in LCs but also to make quantitative predictions for quantum emission in realistic nematics of current scientific and technological interest in the field of metamaterials. Specifically, we predict that one can switch on and off quantum emission in LCs by varying the temperature in the vicinities of the crystalline-to-nematic phase transition, for both the inertial and dissipative cases. We also predict from first principles the value of the critical exponent that characterizes such a transition, which we show not only to be independent of the inertial or dissipative dynamics, but also to be in good agreement with experiments. We determine the orientation of the dipole moment of the emitter relative to the nematic director that inhibits spontaneous emission, paving the way to achieve directionality of the emitted radiation, a result that could be applied in tuneable photonic devices such as metasurfaces and tuneable light sources.

Thermophoresis of colloids in nematic liquid crystal

Thermophoresis, or the directional motion of colloidal particles in liquids driven by a temperature gradient, is of both fundamental interest and practical use. In this work we explore the thermophoresis of colloids suspended in nematic liquid crystals (LCs). We observe that the motion of these colloids is fundamentally different from that in isotropic systems as a result of elastic distortions in the director fields caused by the colloidal inclusions. In the case of a sufficiently large local temperature and gradient, the elastic energy drives negative thermophoresis of immersed particles, which has a strongly nonlinear dependence on temperature. We develop a theory that incorporates elastic energy minimization into the traditional thermophoretic formulation and demonstrated a good agreement with experimental observations. We also examine the temperature dependence of the effective viscosity of the colloids and highlight the large magnitude of the Soret coefficient (|ST| > 5000), which results from the inherent enhancement in thermophoresis due to elastophoretic considerations and suppression of Brownian diffusion in LC media.

Atomistic Mechanism of Anisotropic Heat Conduction in the Liquid Crystal 4-Heptyl-4'-cyanobiphenyl: All-Atom Molecular Dynamics

All-atom molecular dynamics simulations were performed on 4-heptyl-4'-cyanobiphenyl (7CB) to study the mechanism of heat conduction in this nematic liquid crystal atomistically. To describe 7CB properly, the AMBER-type force field was optimized for the dihedral parameter of biphenyl and the Lennard-Jones parameters. The molecular dynamics calculation using the optimized force field well reproduced the experimental values of the isotropic-nematic phase transition temperature, density, and anisotropy of the thermal conductivity. Furthermore, the contributions of convection, intramolecular interaction, and intermolecular interaction to the thermal conductivity were determined by performing thermal conductivity decomposition analysis. According to the analysis, the contributions of convection, bond stretching, and bond bending interactions were higher in the direction parallel to the nematic director than that perpendicular to the director, which is the origin of the anisotropy in the nematic phase. This result indicates that the anisotropy is caused by well-aligned covalent bonds and high mobility parallel to the director. This quantitative description of the mechanism of heat conduction of 7CB is foreseen to provide new insights toward designing highly thermally conductive liquid-crystalline materials.

Thermo-Optical Generation of Particle-Like Structures in Frustrated Chiral Nematic Film

The creation of metastable particle-like structures in frustrated (unwound) chiral nematic film containing light-absorbing additive is studied. It is shown that such localized structures can be generated by the thermo-optical action of a focused laser beam or arise spontaneously at a phase transition from an isotropic to a liquid crystal state. Observed axisymmetric patterns resemble cholesteric spherulites with toroidal double-twisted director-field configuration.

Temperature-Dependent Thermoelastic Anisotropy of the Phenyl Pyrimidine Liquid Crystal

Controlling thermoelastic anisotropy of liquid crystals (LCs) is important for achieving reliable structural stability and efficient heat dissipation, especially for high-performance LC devices. A solid understanding of the thermoelastic anisotropy and its relation with the LC molecular structure is, however, still missing. Here, we studied the direction-dependent mechanical and thermal properties of 5-n-octyl-2-(4-n-octyloxy-phenyl)-pyrimidine (PYP8O8) in a wide temperature range, covering five phases (i.e., crystalline, smectic C, smectic A, nematic, and liquid), by Brillouin light spectroscopy and temperature wave analysis, respectively. We found that the mechanical anisotropy is much smaller than the thermal anisotropy at LC phases; both anisotropies show strong phase dependence, with the biggest change occurring at the crystalline to LC phase transition; and the anisotropy of the phonon mean-free path correlates with the structural anisotropy of the rigid core of the LC molecule. The analysis of the temperature-dependent thermoelastic anisotropy of LCs yields insights into structure-based phonon engineering.

Concurrent guiding of light and heat by transformation optics and transformation thermodynamics via soft matter

Controlling light and heat via metamaterials has presented interesting technological applications using transformation optics (TO) and transformation thermodynamics (TT). However, such devices are commonly mono-physics and mono-purpose, because the used metamaterial is designed to deal with one type of physical mechanisms. Here we demonstrate, for the first time, how to connect TO and TT via the liquid crystal 4-Cyano-4'-pentylbiphenyl (5CB) and, to exemplify such link, we present a multiphysics, multi-purpose device that simultaneously controls light and heat using such material. The anisotropic multiphysics properties of 5CB bond TO and TT, expanding the usage of these theories. The device, composed by 5CB confined between two right circular concentric cylinders, concentrates light (as a converging lens) and simultaneously repels heat from the inner cylinder when the molecules are along the direction [Formula: see text] and it disperses light (as a diverging lens) and concurrently concentrates heat to the inner cylinder, without disturbing the external temperature field, when the molecules are along the direction [Formula: see text], contributing for saving materials and designing miniaturized multiphysics systems.

Thermal and shape topological robustness of heat switchers using nematic liquid crystals

One interesting way to control heat is to use devices designed by transformation thermics, where artificial media are used. However, once manufactured (either repelling or concentrating heat, for example), besides being mono-purpose, such devices are designed according to a specific geometric boundary conditions. Another problem is the temperature dependence of the materials employed, since their properties are sometimes considered temperature-invariant. In this paper, we show that a previously proposed bi-objective heat switcher (Phys. Rev. E 89, 020501(R) (2014)) is in fact robust against temperature and geometric deformations, due to the topological properties of the molecular nematic orientation. Using a geometrical approach for heat propagation, by performing finite element simulations, we show that a device made by concentric cylinders with thermotropic nematic liquid crystal between them, sustains its functionality even with their molecular thermal conductivities depending on the temperature, achieving a 60% increase and a 44% decrease in the heat flux for each mode. Utilizing topological arguments we show that deformations on the surface of the outer cylinder do not break the operating mode (repeller or concentrator). We present a comparison between our geometrical approach and the transformation thermodynamics to give an additional explanation for the obtained results. We hope the presented device is useful for heat control under mechanical and thermal influence of the external environment.

Liquid crystal alignment at macroscopically isotropic polymer surfaces: Effect of an isotropic-nematic phase transition

We study the effect of an isotropic-nematic (I-N) phase transition on the liquid crystal alignment at untreated polymer surfaces. We demonstrate that the pattern at the untreated substrate in the planar cell where the other substrate is uniformly rubbed strongly depends on the temperature gradient across the cell during the I-N phase transition, being macroscopically isotropic if the untreated substrate is cooled faster, but becoming almost homogeneous along the rubbing direction in the opposite temperature gradient. We interpret the observed effect using complementary models of heat transfer and nematic elasticity. Based on the heat transfer model we show that the asymmetric temperature conditions in our experiments provide unidirectional propagation of the I-N interface during the phase transition and determine the initial director orientation pattern at the test's untreated surface. Using the Frank-Oseen model of nematic elasticity, we represent the three-dimensional director field in the nematic cell as a two-dimensional (2D) pattern at the untreated surface and perform 2D numeric simulations. The simulations explain the experimental results: Different initial director orientations at the untreated surface evolve into different stationary patterns.

Thermally Functional Liquid Crystal Networks by Magnetic Field Driven Molecular Orientation

Aligned liquid crystal networks were synthesized by photopolymerization of liquid crystal monomers in the presence of magnetic fields. Grazing incident wide-angle X-ray scattering was used to characterize the degree of molecular alignment of mesogen chains and time-domain thermoreflectance was used to measure thermal conductivity. Liquid crystal networks with mesogenic units aligned perpendicular and parallel to the substrate exhibit thermal conductivity of 0.34 W m^-1 K^-1 and 0.22 W m^-1 K^-1, respectively. The thermal conductivity and orientational order of liquid crystal networks vary as a function of temperature. The thermal conductivity of liquid crystal networks can be manipulated by a magnetic field at above the glass transition temperature (65 °C) where the reduced viscosity enables molecular reorientation on the time scale of 10 min.

Photonic vortices induced in a single-component phototropic liquid crystal

Using the direct coupling mechanism of light with a liquid via molecular absorption, i.e. the opto-thermal effect, we demonstrate the formation of well-controlled three-dimensional circular flows, i.e. a toroidal vortex, inside the liquid crystal (LC) droplet placed on a glass plate in its isotropic phase. We investigated the behavior of a droplet formed of a phototropic liquid crystal and composed of a mesogenic azobenzene derivative under the Gaussian beam light illumination in four different geometries. The light-induced liquid flows in the isotropic phase of the LC were visualized by dispersing carbon micro-particles in the volume of the LC. Movements of the particles could be observed under an optical microscope from the top and side views, respectively. The formation of the stable in time toroidal vortex (the photonic vortex) is dependent on laser light illumination geometry, properties of the liquid and substrate but does not depend on gravitational forces being similar for droplets situated either above or below the glass plate. The main mechanism of the indirect conversion of light into mechanical work is related to the temperature induced gradient of surface tension known as the Marangoni effect.

Photo-thermal effects in gold nanoparticles dispersed in thermotropic nematic liquid crystals

The last few years have seen a growing interest in the ability of metallic nanoparticles (MNPs) to control temperature at the nanoscale. Under a suitable optical radiation, MNPs feature an enhanced light absorption/scattering, thus turning into an ideal nano-source of heat, remotely controllable by means of light. In this framework, we report our recent efforts on modeling and characterizing the photo-thermal effects observed in gold nanoparticles (GNPs) dispersed in thermotropic Liquid Crystals (LCs). Photo-induced temperature variations in GNPs dispersed in Nematic LCs (NLCs) have been studied by implementing an ad hoc theoretical model based on the thermal heating equation applied to an anisotropic medium. Theoretical predictions have been verified by performing photo-heating experiments on a sample containing a small percentage of GNPs dispersed in NLCs. Both theory and experiments represent an important achievement in understanding the physics of heat transfer at the nanoscale, with applications ranging from photonics to nanomedicine.

Nanoscale Thermotropic Phase Transitions Enhancing Photothermal Microscopy Signals

The photothermal heterodyne imaging technique enables studies of individual weakly absorbing nano-objects in various environments. It uses a photoinduced change in the refractive index of the environment. Taking advantage of the dramatic index of refraction change occurring around a thermotropic liquid-crystalline phase transition, we demonstrate a 40-fold signal-to-noise ratio enhancement for gold nanoparticles imaged in 4-cyano-4'-pentylbiphenyl (5CB) liquid crystals over those in a water environment. We studied the photothermal signal as a function of probe laser polarization, heating power, and sample temperature quantifying the optimal enhancement. This study established photothermal microscopy as a valuable technique for inducing and/or detecting local phase transitions at the nanometer scales.

The heat conductivity of liquid crystal phases of a soft ellipsoid string-fluid evaluated by molecular dynamics simulation

We have applied a nonequilibrium molecular dynamics heat flow algorithm to calculate the heat conductivity of a molecular model system, which forms uniaxial and biaxial nematic liquid crystals. The model system consists of a soft ellipsoid string-fluid where the ellipsoids interact according to a repulsive version of the Gay-Berne potential. On compression, this system forms discotic or calamitic uniaxial nematic phases depending on the dimensions of the molecules, and on further compression a biaxial nematic phase is formed. In the discotic nematic phase, the heat conductivity has two components, one parallel and one perpendicular to the director, where the last mentioned component is the largest one. This order of magnitudes is reversed in the calamitic nematic phase. In the biaxial nematic phase there are three components of the heat conductivity, one in the direction around which the long axes of the molecules are oriented, this is the largest component, another one in the direction around which the normals of the broadsides of the molecules are oriented, this is the smallest component, and one in the direction perpendicular to these two directions with a magnitude in between those of the first mentioned components. The relative magnitudes of the components of the heat conductivity span a fairly wide interval so it should be possible to use the model to parameterise experimental data.

Onset of Rayleigh-Bénard convection in cylindrical containers

We determined the critical Rayleigh numbers Ra{c} for the onset of convection in cylindrical containers with aspect ratios 1 approximately <Gamma [ triple bond] D/L approximately <9 ( D is the diameter and L the height) and the patterns that form just above Ra{c}, both from experiment and by direct numerical simulation (DNS). Results for Ra{c} agree well with the linear stability analysis by Buell and Catton for containers with finite sidewall conductivity. For Gamma<or=1.58+/-0.10 , we found that the patterns correspond to an azimuthal Fourier mode with mode number m=1 , corresponding to a single convection roll. For 1.58 approximately < Gamma approximately <3.26+/-0.02 , the pattern was a concentric roll, corresponding to m=0 . For 3.26<or=Gamma approximately >4, an m=1 mode was found again, but near Gamma=4 either m=1 or m=2 was observed in different runs. These results are consistent with the marginal stability curves calculated by Buell and Catton in the sense that the mode that is the first as a function of Ra to acquire a positive growth rate is the one that is observed. For Gamma approximately >4, the theoretical marginal curves for the four lowest modes lie very close together. There we found patterns near onset that corresponded to various modes, including m=2 and 4. At relatively large Gamma approximately > 6, we observed parallel straight rolls quite close to onset. Our patterns agree with several DNS investigations by others, but at some Gamma values differ from those observed experimentally by Stork and Müller. Some results for the pattern evolution with increasing Ra are reported as well.

Sliding planar anchoring and viscous surface torque in a cholesteric liquid crystal

We propose a surface treatment allowing one to obtain a sliding planar anchoring of nematic (or cholesteric) liquid crystals. It consists of depositing a thin layer of the polymercaptan hardener of an epoxy resin on an isotropic substrate (bare or ITO-coated glass plates). Microscopic observations of defect annihilations and capacitance measurements show that the molecules align parallel to the surface and slide viscously on it when they change orientation, which implies a zero (or extremely small) azimuthal anchoring energy. In contrast, the zenithal anchoring energy W theta is found to be larger than 3 x 10(-5)J/m2. We also measured the liquid crystal rotational surface viscosity gammaS by a thermo-optical method using the large temperature variation of the pitch of a compensated cholesteric mixture. We found that the sliding length gammaS/gamma1 (where gamma1 is the bulk rotational viscosity) is very large in comparison with the length of a liquid crystal molecule. This result is explained by a simple model which takes into account the diffusion of the liquid crystal within the polymer layer.

Non-Oberbeck-Boussinesq effects in turbulent thermal convection in ethane close to the critical point

As shown in earlier work [Ahlers, J. Fluid Mech. 569, 409 (2006)], non-Oberbeck-Boussinesq (NOB) corrections to the center temperature in turbulent Rayleigh-Bénard convection in water and also in glycerol are governed by the temperature dependences of the kinematic viscosity and the thermal diffusion coefficient. If the working fluid is ethane close to the critical point, the origin of non-Oberbeck-Boussinesq corrections is very different, as will be shown in the present paper. Namely, the main origin of NOB corrections then lies in the strong temperature dependence of the isobaric thermal expansion coefficient beta(T). More precisely, it is the nonlinear T dependence of the density rho(T) in the buoyancy force that causes another type of NOB effect. We demonstrate this through a combination of experimental, numerical, and theoretical work, the last in the framework of the extended Prandtl-Blasius boundary-layer theory developed by Ahlers as cited above. The theory comes to its limits if the temperature dependence of the thermal expension coefficient beta(T) is significant. The measurements reported here cover the ranges 2.1<or similar to Pr<or similar to 3.9 and 5x10(9)<or similar to Ra<or similar to 2x10(12) and are for cylindrical samples of aspect ratios 1.0 and 0.5.

Thermal diffusivity and the conformal transformation on nematic liquid crystals

In this paper, we will study theoretically and experimentally the anisotropy of the thermal diffusivity in nematic liquid crystals. We will show that the Baalss-Hess conformal transformation [D. Baalss and S. Hess, Phys. Rev. Lett. 57, 86 (1986)], can be used to obtain a free of adjustable parameters equation that describes the anisotropy of the heat diffusion in these materials. The results of this theory will be compared with experimental data. This study will partially confirm the widely known experimental evidences that indicate that the thermal diffusivity is larger in the director direction than in the one perpendicular to it. For calamitic nematic phases this is true; for discotic nematic ones the reverse situation would be found; the diffusivity would become larger in the direction perpendicular to the director. We will also present experimental data supporting this theoretical prediction. The data comprehend the calamitic and discotic nematic lyotropic phases and a nematic thermotropic phase.

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.

Wave-number selection by target patterns and sidewalls in Rayleigh-Bénard convection

We present experimental results for patterns of Rayleigh-Bénard convection in a cylindrical container with static sidewall forcing. The fluid used was methanol, with a Prandlt number sigma=7.17 , and the aspect ratio was Gamma identical withR/d approximately 19 ( R is the radius and d the thickness of the fluid layer). In the presence of a small heat input along the sidewall, a sudden jump of the temperature difference DeltaT from below to slightly above a critical value Delta T(c) produced a stable pattern of concentric rolls (a target pattern) with the central roll (the umbilicus) at the center of the cell. A quasistatic increase of epsilon identical withDeltaT/Delta T(c) -1 beyond epsilon(1,c) approximately 0.8 caused the umbilicus of the pattern to move off center. As observed by others, a further quasistatic increase of epsilon up to epsilon=15.6 caused a sequence of transitions at epsilon(i,b) ,i=1,...,8 , each associated with the loss of one convection roll at the umbilicus. Each loss of a roll was preceded by the displacement of the umbilicus away from the center of the cell. After each transition the umbilicus moved back toward but never quite reached the center. With decreasing epsilon new rolls formed at the umbilicus when epsilon was reduced below epsilon(i,a) < epsilon(i,b) . When decreasing epsilon , large umbilicus displacements did not occur. In addition to quantitative measurements of the umbilicus displacement, we determined and analyzed the entire wave-director field of each image. The wave numbers varied in the axial direction, with minima at the umbilicus and at the cell wall and a maximum at a radial position close to 2Gamma/3 . The wave numbers at the maximum showed hysteretic jumps at epsilon(i,b) and epsilon(i,a) , but on average agreed well with the theoretical predictions for the wave numbers selected in the far field of an infinitely extended target pattern. To our knowledge there is as yet no prediction for the wave number selected by the umbilicus itself, or by the cell wall of the finite experimental system.

Rayleigh-Bénard convection in elliptic and stadium-shaped containers

We report on defect formation in convection patterns of stadium-shaped and elliptical horizontal layers of fluid heated from below (Rayleigh-Bénard convection). The fluid was ethanol with a Prandtl number sigma=14.2. The outermost convection roll was forced to be parallel to the sidewall by a supplementary wall heater. The major- and minor-axis aspect ratios Gamma(i)=D(i)/2d, i=1, 2 (D(i) are the major and minor diameter and d the thickness) were 19.4 and 13.0, respectively. For the stadium shape, we found a stable pattern that was reflection-symmetric about the major diameter and had a downflow roll of length L(s) along a large part of this diameter. This roll terminated in two convex disclinations, as expected from theory. No other patterns with the outermost roll parallel to the sidewall were found. The wave numbers of the rolls in the curved sections and L(s) decreased with increasing epsilon identical with DeltaT/DeltaT(c)-1, consistent with a prediction for wave-number selection by curved rolls in an infinite system. At large epsilon, the roll adjacent to the sidewall became unstable due to the cross-roll instability. For the elliptical shape, wave-director frustration yielded a new defect structure predicted by Ercolani et al. Depending on the sample history, three different patterns with the outermost roll parallel to the wall were found. For one, the central downflow roll seen in the stadium was shortened to the point where it resembled a single convection cell. Along much of the major diameter there existed an upflow roll. The new defect structure occurred where the two downflow rolls surrounding the central upflow roll joined. This joint, instead of being smooth as in the stadium case, was angular and created a protuberance pointing outward along the major diameter. We also found a pattern with an upflow roll along the major diameter without the central downflow cell. A third pattern contained a downflow cell, but this cell was displaced by a roll width from the center along a minor diameter. As epsilon increased, the length L(e) between the two protuberances and the wave numbers along the outer parts of the major diameter decreased for all three patterns, analogous to what was found for the stadium. The upper stability limit of these patterns was also set by the cross-roll instability.

Traveling concentric-roll patterns in Rayleigh-Bénard convection with modulated rotation

We present experimental results for pattern formation in Rayleigh-Bénard convection with modulated rotation about a vertical axis. The dimensionless rotation rate Omega was varied as Omega(m)=Omega[1+delta cos(phi Omega t)] (time is scaled by the vertical viscous diffusion time of the cell). We used a cylindrical cell of aspect ratio (radius/height) Gamma=11.8 and varied Omega, delta, phi, and epsilon identical with R/R(c)(Omega)-1 (R is the Rayleigh number). The fluid was water with a Prandtl number of 4.5. Sufficiently far above onset even a small delta greater than approximately 0.02 stabilized a concentric-roll (target) pattern. Multiarmed spirals were observed close to onset. The rolls of the target patterns traveled radially inward independent of the sense of rotation. The radial speed v was nearly independent of epsilon for fixed Omega, delta, and phi. However, v increased with any one of Omega, delta, and phi when all the other parameters were held fixed.

Convection in the presence of a first-order phase change

We report experimental and theoretical results for two-phase convection in a thin horizontal layer of a fluid with a first-order phase change and heated from below. A top layer of the nematic phase of a liquid crystal is located above the bottom layer of the isotropic phase of the same substance. A horizontal field of 1000 G is applied in order to align the director of the nematic phase. Over some ranges of the thickness of the isotropic phase, and in sufficiently large thermal gradients, the more dense nematic phase can be stably stratified above the less dense isotropic one, with a stable interface between them. Based on the equations of motion derived for this problem by Busse and Schubert [J. Fluid Mech. 46, 801 (1971)], we evaluate the bifurcation lines between the quiescent and convecting states and the corresponding critical wave vectors as a function of the interface position. We report experimental measurements based on Nusselt-number determinations for the locations of the bifurcation lines. They are in good agreement with the theoretical results. We also report approximate determinations of the critical wave numbers which are semiquantitatively consistent with the theory. A great diversity of patterns is observed in the convecting states, including normal and parallel rolls, rolls with defects and disorder, target patterns and spirals, and cellular flow with upflow or downflow at the cell center. These patterns are discussed in terms of the breaking of the mirror symmetry at the horizontal midplane by the interface, and in terms of the orienting effects of the magnetic field.