Liquid Crystal 5CB Research Articles

Calorimetry of phase transitions in liquid crystal 8CB under shear flow.

A differential scanning calorimeter equipped with a shearing system (shear rate of  < 400 s-1) was developed to elucidate the thermodynamic properties of liquid crystalline phase transitions under shear flow. An analytical method was proposed to accurately estimate the heat flow caused by shear friction to evaluate the transition entropies. The phase transitions of 4'-n-octyl-4-cyano-biphenyl (8CB) under shear flow were investigated using the developed calorimeter. Although several shear-induced transitions for 8CB have been reported in the past using viscosity and small-angle X-ray scattering (SAXS) measurements, only the nematic-isotropic (N-I) and smectic-A-nematic (SA-N) transitions were detected as heat flow peaks. The N-I transition temperature was almost independent of the shear rate. The SA-N transition temperature was also independent of the shear rate, but the transition peak was broadened by applying shear flow. For both transitions, the transition entropies were independent of the shear rate. These results suggest that the thermodynamic properties were not considerably changed by shearing because the molecular alignments in the domains were not substantially changed, whereas shearing changed the LC domain directions, which can be detected by viscosity and SAXS measurements.

Capture of microscale objects using the smectic–isotropic phase interfacial force for soft-manipulation devices

We propose a micromanipulator system that traps microparticles using the interfacial force between liquid crystals and isotropic phases. The maximum interfacial force of a nematic liquid crystal (5CB) was previously shown to be insufficiently strong for use in micromanipulator systems. Because a smectic liquid crystal (12CB) has a higher order of molecular alignment, the smectic-isotropic interfacial force is stronger than the nematic-isotropic interfacial force. We demonstrated in the present study that the smectic-isotropic interface was strong enough to capture tungsten carbide particles. The interfacial force of 12CB was approximately 76.1nN, which was more than 13 times stronger than the nematic-isotropic interfacial force of 5CB and 8CB. Thus, the smectic-isotropic interface of 12CB could be exploited for the development of new manipulation or actuation systems. • The interfacial forces between liquid crystal phase interfaces were evaluated. • The smectic-isotopic interfacial force was much stronger than that of other interfaces. • Micromanipulators can be developed using the smectic-isotropic interface.

INVESTIGATION OF DIELECTRIC PROPERTIES OF MBBA-5CB NEMATIC LIQUID CRYSTAL STRUCTURES

The dielectric anisotropy (Δε') is the response of the LC molecules to an applied electric field. In this study, the dielectric properties of 4-pentyl-4'-cyanobiphenyl (5CB) and N-(4-methoxybenzylidene)-4-butylaniline (MBBA) nematic liquid crystals with positive and negative dielectric anisotropy were investigated in detail. In addition, a structure with zero dielectric anisotropy was obtained by mixing MBBA and 5CB liquid crystals in certain rate (1:0.02). The behavior of the samples under 0V and 20V voltages was investigated in detail. Dielectric measurements were made with dielectric spectroscopy technique, which is a powerful method to investigate the molecular interaction mechanisms of liquid crystal materials, at a frequency of 1 kHz-10 MHz and a voltage of 0-20 V at room temperature. Some important parameters such as dielectric anisotropy (Δε'), relaxation frequency (fR), relaxation time (τ) were determined for the samples. When electric field is applied to nematic liquid crystals, it is seen that, depending on the voltage, the relaxation frequency decreases, and the relaxation time increases.

Effect of Rod-like Nanoparticles on the Dielectric Susceptibility of Nematic Nano-Composites: A Molecular Theory

The effect of rod-like nanoparticles on the high-frequency dielectric susceptibility of the nematic nano-composites has been investigated in the framework of a molecular theory. Analytical expressions for the components of the effective polarizability of a rod-like nanoparticle in the nematic host have been obtained and used in the calculations of the dielectric susceptibility of the composites as functions of the nanoparticle volume fraction. Numerical calculations of the susceptibility have been undertaken using the nematic liquid crystal 5CB as a host doped with either gold or silver particles for different values of the concentration of nanoparticles. It has been shown that the rod-like nanoparticles have a much stronger effect on the components of the dielectric susceptibility of the nano-composites including, in particular, the one with gold nanoparticles in the vicinity of the plasmon resonance. The main conclusion is that at sufficiently large concentration of nanoparticles, the anisotropy of the dielectric susceptibility of the nano-composites may even change the sign with an increasing concentration which may be important for various applications.

Magnetic induced terahertz modulation characteristics based on ferromagnetic nematic liquid crystals

In recent years, solid state terahertz (THz) modulators have obtained rapid progress with the widespread use of two-dimensional (2D) materials in the field of THz; however, challenges remain in preparing flexible THz modulators. In this study, flexible ferromagnetic nematic materials were prepared by doping thermotropic nematic liquid crystals 5CB into magnetic fluids, and the influence of water was reduced by a self-made cyclic olefin copolymer (COC) microfluidic chip. THz modulation characteristics of magnetic fluid and ferromagnetic nematic liquid crystal (FNLC) under the induction of external magnetic field were compared using a THz time domain spectroscopy system. Under the action of a 91 mT magnetic field, the magnetic fluid has a maximum modulation depth (MD) of 54%. Under the same magnetic field, the maximum MD of the FNLC materials increase to 78% because of the rearrangement of Fe3O4 nanoparticles induced by the topological defect of the liquid crystal. We demonstrate that the magneto-optical effect is significantly enhanced in the ferromagnetic nematic liquid crystal hybrid system. This strategy of doping thermotropic nematic liquid crystals to enhance the magneto-optical effect has great potential for THz filtering, modulation, and sensing applications.

Mathematical modelling of angle dependent polarization raman spectroscopy of molybdenum disulfide before and after adding strain agent

Molybdenum disulphide (MoS2) is a layered 2D semiconductor material with a tunable optical bandgap. Its optoelectrical properties make it a good choice for use in 2D devices, such as single-photon emitters. In this study, we investigated the properties of thin layers of MoS2 deposited on a silicon (Si) substrate. MoS2 flakes were passivated and transferred to a particular spot on the substrate by employing a dry mechanical technique. With the assumption that incorporating nematic liquid crystal 5CB on the surface of MoS2 can produce a reconfigurable strain (such as compressive or tensile), then investigating whether 5CB will respond differently or even in the same way with planar aligned molecules. To create a single photon emitter from a 5CB nematic liquid crystal, physical defects in MoS2 must be introduced. The atomic thickness and structural properties of the transferred flake were determined using Raman Spectroscopy. 5CB has increased the angle dependence of A1g and E2g mode in polyvinyl alcohol planar cells. 5CB nematic liquid altered the optical properties of MoS2 and produced compressive strain inside the lattice structure. Our theoretically anticipated model fits the experimental data with reasonable precision.

Nanocalorimetry using microscopic optical wireless integrated circuits

We present in situ calorimetry, thermal conductivity, and thermal diffusivity measurements of materials using temperature-sensing optical wireless integrated circuits (OWiCs). These microscopic and untethered optical sensors eliminate input wires and reduce parasitic effects. Each OWiC has a mass of ∼100 ng, a 100-μm-scale footprint, and a thermal response time of microseconds. We demonstrate that they can measure the thermal properties of nearly any material, from aerogels to metals, on samples as small as 100 ng and over thermal diffusivities covering four orders of magnitude. They also function over a broad temperature range, and we present proof-of-concept measurements of the thermodynamic phase transitions in both liquid crystal 5CB and gadolinium.

From very low to high fields: The dielectric behavior of the liquid crystal 5CB

The liquid crystal 4-cyano-4′-pentylbiphenyl (5CB) is investigated regarding its nonlinear dielectric behavior as reflected in the dependence of the static dielectric constant εs on the magnitude of the electric field. The initial flow-induced homogeneous orientation of nematic domains can be erased by moderate electric fields, yielding states with little to no preferred orientation. Such samples are then subjected to ac and dc electric fields of various amplitudes, covering more than four orders of magnitude, between <10 V cm−1 to 220 kV cm−1. Their regime of linear dielectric response, i.e., field invariant static permittivity, is limited to fields <10 V cm−1, equivalent to a voltage <50 mVrms across a 50 μm thick sample. The Piekara factor, which gauges the sensitivity of εs to E2, is a factor of 108 times larger at low fields than its counterpart at elevated fields, where the nematic domains are highly aligned in homeotropic orientation. After removing the electric field, the nematic system returns to a state with little residual alignment, as judged by the dielectric constant settling near the value of the orientational average, 〈ε〉 = (ε|| + 2ε⊥)/3, and not far from that of the isotropic phase, εiso. The structural recovery associated with a field induced change in alignment is slow compared with dipole reorientation. For a different nematogen, itraconazole, no such extreme sensitivity to electric fields is found.

Molecular Dynamics Simulations on the Flow Behavior of 5CB Liquid Crystal Sandwiched Between Iron Walls

AbstractThe 4‐n‐pentyl‐4′‐cyano biphenyl (5CB) liquid crystal (LC) sandwiched and sheared between atomically flat and grooved iron walls, respectively, is simulated using molecular dynamics, and the flow behavior of the system under confinement, pressure, and shear is investigated. The order parameter P2, velocity, and density of 5CB are profiled as a function of distance from the walls using a block analysis method. The flow‐alignment phenomenon is reproduced, and alignment angles ranging from 11.3° to 13.9° are observed. The competition between the elastic effect of the surface and viscous effect of the flow on the orientation of the 5CB molecules is investigated. The P2 of 5CB located in the vicinity of the grooved walls is higher than that of the flat walls, while this difference is negligible in the case of 5CB distanced from the walls. Instead of boundary slip or Couette flow, the velocity profiles reveal a boundary condition (BC) of central localization, which is very different from the assumed Couette flow BC in most analytical models of LC flow. This work enhances the current molecular‐scale understanding of the flow behavior of confined liquid crystals and provides additional insights into the phenomenon.

P‐12.12: Increasing the Rewriting Speed of Optically Driving Liquid Crystal Display by RM257Doping

The optically driving liquid crystal display (ODLCD) can realize the display function by changing the alignment direction of liquid crystal molecules inside by exposing to the polarized blue light, which has the advantages of low power consumption, non‐contact alignment, easy to fabricate. However, a big issue arises in response time, i.e., rewriting time of the ODLCD. The rewriting time for the ODLCD depending on doping with RM257 was studied. RM257was cured perpendicular to the substrates by applying voltage across the sample that forming a net‐like structure inside liquid crystal 5CB.Among the 9 different RM257 concentrations, 5% RM257 in the mixturewasfound to be the optimal and the minimum rewriting time of 4s had been reached.

A Mesoporous Silica Nanoparticle-Doped Photo-Alignment Layer and Liquid Crystal Layer for Optimizing the Rewriting Speed and the Response Time of Optically Driving Liquid Crystal Displays

Optically driving liquid crystal displays (ODLCDs) are widely applied in display and optical devices due to their long axis of liquid crystal (LC) molecules that can be tuned by a photo-alignment layer under exposure polarized light. However, their use remains challenging due to their long rewriting time and response time. In this work, the rewriting time and the response time of an ODLCD depending on mesoporous silica nanoparticles (MSNs) doped in azo-dye (SD1) and LC 5CB were studied. Among the different concentration ratios of SD1-MSNs (1-0 to 1-0.1), a ratio of 1-0.07 was optimal, decreasing the rewriting time by 40 s (from 69.1 to 29.6 s). Meanwhile, the response time was improved 10 times with MSNs doped into 5CB.

Anisotropic viscous effects of local flow by a rotating microparticle in nematic liquid crystal.

The presented study opens a perspective to investigate the effects of local flow on nematic liquid crystals. A particle rotated in nematic fluids typically generates a rotationally symmetric local flow, which causes a change in the director orientation. The director above the threshold velocity has a particular angle determined by the ratio of Leslie coefficients, α_{2}/α_{3}. In 5CB liquid crystals, this director angle with respect to the flow is approximately 13^{∘}. The angle is calculated through Ericksen-Leslie theory. The angle is not dependent on rotation frequency or particle size but temperature. The area of the influenced region increases with the rotation frequency and particle size. The changes in radius of the influenced region are calculated theoretically using Ericksen number. Further, an interference pattern appears at the edge of the influenced region by the refractive indexes mismatch between the influenced region and the rest. We experimentally obtain the thickness of the influenced region analyzing intervals of the pattern.

Cooling rate effects on luminescence and structure properties of the 5CB liquid crystal, doped by different nanoparticle dispersions

The effect of the 5CB liquid crystal cooling rate on its luminescence spectra at the low temperature (T = 4.2 K) has been found and studied for two types of nanocomposites: 5CB liquid crystal, doped by carbon nanotubes (NT) or so-called hybrid nanoparticles (NT + OMMT). Hybrid nanoparticles were prepared from organomodified layered montmorillonite minerals (OMMT) with the addition of carbon nanotubes. Experiments have been carried out at two cooling rates, 2 and 10 K/min. Structure of the luminescence spectral bands is associated with 5CB crystallization peculiarities and formation of its glass-like state under different cooling regimes of the isotropic phase. The reasons for the short-wavelength shifts of the luminescence peaks for the 5CB–(NT + OMMT) sample compared to the 5CB–NT one are discussed.

Thermotropic liquid crystal doped with ferroelectric nanoparticles: Electrical behavior and ion trapping phenomenon

Ionic behavior in liquid crystal is a topic with intense interest in many fields of material science, such as dielectric materials. In this work ferroelectric nanoparticles Lithium Cobalt Molybdenum Oxide(Li2Co2(MoO4)3) are synthesized, characterized and dispersed in 4′-octyl-4-biphenylcarbonitrile (8CB) liquid crystal. It is found that the synthesized nanoparticles have a very large impact on the dielectric and impedance spectra. By fitting a suitable equivalent electric circuit model to the experimental data we demonstrate the change of electro–physical parameters of the liquid crystal in the bulk and at the liquid crystal–metal electrode interface. The dopants have an effect on the conductivity, reduces the viscosity and the response time, which can be correlate to the capturing of impurity ions. Temperature dependence of ionic behavior such as ion concentration, diffusion constant and the mobility are investigated. According to our experimental investigation, suppress of impurity ions by ferroelectric nanoparticles is achieved.

Sign reversal of Casimir-Lifshitz torque with separation distance: A theoretical guide to experimentation

The Casimir-Lifshitz torque between a pair of birefringent materials was recently shown to exhibit a change in sign with respect to the separation distance owing to the planar dielectric properties of the interacting materials. The recent experiment that verified the existence of the Casimir-Lifshitz torque precluded a sign reversal between the liquid crystal 5CB $(4\text{\ensuremath{-}}\mathrm{cyano}\text{\ensuremath{-}}4\text{\ensuremath{-}}n\text{\ensuremath{-}}\mathrm{pentylbiphenyl})$ and calcite [Somers et al., Nature (London) 564, 386 (2018)]. Here, we will show that the sign reversal was not observed in the experiment because it would happen at a larger distance than was investigated for the pairs of materials considered. The purpose of this paper is to identify appropriate choices of combinations of materials that will facilitate experimental observation of the distance-dependent sign reversal of the torque in the experimentally feasible distance regime. Specifically, we propose a setup comprising biaxial black phosphorus and the liquid crystal 5CB or 5PCH $(4\text{\ensuremath{-}}\mathrm{cyano}\text{\ensuremath{-}}4\text{\ensuremath{-}}n\text{\ensuremath{-}}\mathrm{pentylcyclohexane}\text{\ensuremath{-}}\mathrm{phenyl})$ as a particularly viable option. The feature of sign reversal will be an interesting addition to the control mechanism of actuation and switch devices that are applicable for the Casimir-Lifshitz torque.

Orientation Distribution of Molecules in a Smectic Liquid Crystal with a Distorted Director Geometry.

Quantitative determination of the molecular orientation distribution function in samples of liquid crystals with a complex director geometry was performed using the numerical simulation of electron paramagnetic resonance (EPR) spectra of the spin probes in a liquid-crystalline medium. To achieve the quantitative agreement of experimental and simulated EPR spectra, the hierarchy of the orientation order was explicitly taken into account, namely, the local ordering of liquid crystal molecules by the mean-field potential of surrounding molecules, and the partial disordering of local directors within the sample. The samples under study are planar and twist LC cells with liquid crystal 8CB cooled from the nematic into the smectic A phase in the magnetic field. The presence of the magnetic field perpendicular to the cell director leads to distortion of the orientation of the liquid crystal in the cell. The spin probe technique was successfully employed for the reliable measurement of orientation distribution functions of the low nonorthorhombic symmetry. Orientation order parameters up to 12th rank were measured, including nonaxial and nonorthorhombic order parameters. It is shown that the presence of several contradicting aligning forces leads to the tilt of the preferential director toward the direction, which is a compromise between the orienting forces.

Insertion of the Liquid Crystal 5CB into Monovacancy Graphene.

Interfacial interactions between liquid crystal (LC) and two-dimensional (2D) materials provide a platform to facilitate novel optical and electronic material properties. These interactions are uniquely sensitive to the local energy landscape of the atomically thick 2D surface, which can be strongly influenced by defects that are introduced, either by design or as a byproduct of fabrication processes. Herein, we present density functional theory (DFT) calculations of the LC mesogen 4-cyan-4′-pentylbiphenyl (5CB) on graphene in the presence of a monovacancy (MV-G). We find that the monovacancy strengthens the binding of 5CB in the planar alignment and that the structure is lower in energy than the corresponding homeotropic structure. However, if the molecule is able to approach the monovacancy homeotropically, 5CB undergoes a chemical reaction, releasing 4.5 eV in the process. This reaction follows a step-by-step process gradually adding bonds, inserting the 5CB cyano group into MV-G. We conclude that this irreversible insertion reaction is likely spontaneous, potentially providing a new avenue for controlling both LC behavior and graphene properties.

Analysis of the relationship between roughness parameters of wear surface and tribology performance of 5CB liquid crystal

The aim of this work was to investigate the relationship between roughness parameters of wear surface and lubricating performance of 4-Cyano-4′-pentylbiphenyl (5CB) liquid crystal. After running-in with acetylacetone (Hacac), lubricating with 5CB realizes a significant friction reduction and generates a groove-structured wear surface with larger Sku, more negative Ssk, smaller Str, and a higher Svk/Spk, which are in good agreement with the roughness parameters that achieving lower friction. Chemical reactions in the running-in process with Hacac changes the morphology of wear debris and decreases its particle size and Si content, which are believed to be reasons for the formation of the groove-structured wear surface. Damaging the groove-structured wear surface, by introducing a process of washing up wear surfaces or lubricating with ethanol briefly between running-in with Hacac and lubricating with 5CB, changes roughness parameters of wear surface and induces higher friction coefficient. We hope this study will provide a new perspective to understand the effect of roughness parameters on tribological performance, and to provide references for surface designing.

Comparative dielectric study of bent-core liquid crystal doped 5CB

Bent-core liquid crystal (LC) is used to dope the (4-cyano-4′-pentylbiphenyl) liquid crystal (5CB). The polarizing microscopy results show that 5CB and LC exhibit Nematic (N) and Banana-Shaped (B1) mesophases, respectively. In this paper, we use dielectric spectroscopy to determine the dielectric properties of the 1% LC doped with 5CB liquid crystal. The dielectric parameters of these materials have been determined by different applied voltage. In this respect, dielectric properties of 5CB + 1% LC have been compared with 5CB. The dielectrical anisotropy of 5CB and 5CB + 1% LC liquid crystal composite changes from the positive dielectric anisotropy to negative with the frequency of the applied voltage. The critical frequency fc values of dielectrical anisotropy for 5CB and 5CB + 1% LC composite were found to be 688 and 534 kHz, respectively. The real (ε′) and imaginary components (ε′′) of dielectric constant have been calculated by capacitance method. It has been also determined that adding LC to 5CB decreases the voltage dependent dielectric strength and increases threshold voltages.

Hexagonal boron nitride (h-BN) nanosheets as lubricant additive to 5CB liquid crystal for friction and wear reduction

The h-BN nanosheets (h-BNn) was used as an additive to 4-n-pentyl-4′-cyanobiphyl liquid crystal (5CB), and its influence on tribological performance was investigated on a ball-on-disc tribometer for steel/steel contacts. Micron h-BN (h-BNμ) was used as a reference additive, and it is found that h-BNn has superior lubricating effects than h-BNμ. When h-BNn concentration is 0.01 wt%, compared to pure 5CB, the friction coefficient and the wear scar diameter on the ball reduced by 15.5% and 33.8%, respectively. Raman spectra analysis shows that h-BNn entered sliding interface, forming a low shear resistance protective layer on wear surfaces.