Nematic-isotropic Phase Transition Research Articles

Random lasing and mode tunability by extraordinary strong and controllable scattering during the nematic-isotropic phase transition

Random lasing emission from the lateral end face of dye doped nematic liquid crystal (DDLC) cells was investigated during the nematic-isotropic phase transition process and abundant lasing behaviors were observed. There was a temperature range of ~10 °C for the phase transition process of DDLCs, in which anisotropic light scattering was investigated. Light scattering of DDLCs was greatly enhanced during the phase transition process and the enhancement of scattering was strongly dependent on the temperature. And a peak scattering coefficient of more than 1000 cm−1 was achieved at a certain temperature, which we called the peak-scattering temperature. Random lasing induced by resonant feedback was observed near the peak-scattering temperature under the appropriate pump energies. In this situation, broad emissive spectra originating from non-resonant feedback or fluorescence were not evident. When the temperature departed from the peak-scattering value and the pump energy was fixed, the hybrid mode caused by both resonant and non-resonant feedback was achieved. And the proportion of the two feedbacks induced lasing modes could be tuned by temperature and pump energy. All the above random lasing phenomena were observed from 6-µm-thick DDLC cells. The strong scattering in the phase transition process was mainly induced by the inhomogeneity of DDLC due to coexistence of both nematic and isotropic phase. The rich lasing behaviors could be used to control lasing modes by temperature or energy of pump pulse.

Ordering behavior and electric response of a ferroelectric nano-doped liquid crystal with ion impurity effects

We present a theoretical study of the simultaneous effects of ferroelectric nanoparticles and ion impurities on the order parameter of liquid crystals. We have focused on the effect of ion impurities on the ordering behavior and electrical response of a ferroelectric nano-doped liquid crystal near the nematic-isotropic phase transition. In this study, a statistical theory based on the orientational distribution of the nanoparticle dopants is used. We have performed the calculations using the Poisson–Boltzmann equation and a modified Landau-De Gennes model. The total density of free energy of the system is obtained within the framework of this theory, and the changes in the concentration of mobile ions due to the presence of ferroelectric nanoparticles are investigated. It is shown that the screening effect of the ion impurities plays a dominant role in the ordering behavior and electrical response of a ferroelectric nano-doped liquid crystal. The ion generation and the consequent changes are also described in some physical properties of a nano-doped system under an applied electric field.

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.

Experimental evidence of continuous isotropic-nematic phase transition in CdS nanowire nanocomposites of a nematic liquid crystal

ABSTRACT The calamitic nematic liquid crystal generally exhibits a discontinuous weakly first-order isotropic-nematic (I-N) phase transition. We found that, on the incorporation of CdS nanowire in the nematic host, the weakly first-order discontinuous I-N phase transition evolved as a continuous phase transition. The parallel and perpendicular components of dielectric permittivity and bulk conductivity of nanocomposites of CdS nanowires of a nematic liquid crystal (NLC) are found to follow continuous I-N phase transition in contrast to the discontinuous weakly first-order transition of the pure host nematic liquid crystal. This was further confirmed from scattered iGntensity measurements on the CdS nanocomposite samples of nematic liquid crystal in homeotropically aligned cells. Such measurements indicate a continuous phase transition for the nanocomposites of NLC having concentrations less than 0.2 wt% of CdS nanowires in NLC. The dielectric anisotropy, birefringence, threshold voltage of nanocomposites of CdS nanowire in a calamitic nematic liquid crystal with varying concentration of the nanowire have been measured. The ionic conductivity of nanocomposites of CdS nanowires of NLC increases with increasing concentration of CdS nanowires.

Temperature-dependent fluorescence emission of 4-cyano-4′-pentylbiphenyl and 4-cyano-4′-hexylbiphenyl liquid crystals and their bulk phase transitions

This study examined the fluorescence emission properties of bulk 4-Cyano-4′-pentylbiphenyl (5CB) and 4-Cyano-4′-hexylbiphenyl (6CB) and their variation with temperature. The temperature-dependent fluorescence emission spectra of these two thermotropic liquid crystals (LCs) were measured between 4 and 80 °C and the changes in fluorescence intensities and wavelengths were monitored. The fluorescence spectra of both liquid crystals are characterized by emissions of two structurally different excimers centred around 380–386 nm and 412 nm. The results show that the specific changes in intensity and wavelength differ between the two LCs and suggest presence of two different molecular conformers as the temperature changes. During nematic-isotropic phase transition, the fluorescence spectra show significant intensity increase, which can be exploited for sensitive analytical applications. The crystalline-nematic phase transition of 5CB was found to be 22 °C and the nematic-isotropic liquid transition occurred at 35.5 °C. On the other hand, the crystalline-nematic phase transition of 6CB occurred at 16 °C, while its nematic-isotropic phase transition was observed at 29 °C. The dramatic temperature-dependent phase changes observed in the bulk LCs were completely absent in a solution of 6CB. The observed temperature-dependent spectral evolutions are discussed in the context of possible excimer conformations that exist in different mesophases of the two liquid crystals as well as change in the angle between the two benzene rings, which can contribute to excimer behaviour during phase transitions.

Quasi-Stationary States in Ionic Liquid-Liquid Crystal Mixtures at the Nematic-Isotropic Phase Transition

An open system is a system driven away from equilibrium by a source that supplies an inflow of energy and a sink to maintain an outflow. A typical example of an open system is a system close to its phase transition temperature under irradiation by a laser. This provides a steady flow of energy through a photon flux. The sink in that case is the environment to which energy is lost in the form of heat dissipation. Creation of such a thermodynamically open state suggests that we can expect generation of exotic spatio-temporal structures length-scale independent correlation maintained under the global dissipative forces provided by the surroundings. Internal long-range forces can bring in additional spatio-temporal correlations, giving rise to states with a very long lifetime, the ‘quasistationary states’ (QSS). In this communication, we report evolution of quasistationary states, in a mixture of the well-known liquid crystal (N-(4-methoxybenzylidene)-4-butylaniline, MBBA) and an iron-based room temperature ionic liquid (RTIL), namely, 1-ethyl-3-methylimidazolium tetrachloroferrate (EMIF) at the Nematic-Isotropic phase transition, when focused radiation with 532 nm wavelength from a Nd:YAG laser (200-300 mW optical power) is incident on the sample. We explain the QSS by invoking a sharp negative thermal gradient due to the laser photon flux and dipolar interactions. In our model, the dipoles are the charge transfer complexes (CTCs) formed in the RTIL by resonant laser pumping, which create an orientational ordering and balance the fluctuating force of the thermal gradient to create the QSS. In the absence of such CTCs in a mixture of MBBA and a Gallium-based RTIL (1-ethyl-3-methylimidazolium tetrachlorogallate, EMIG), the QSS was not observed.

Ordering of adsorbed rigid rods mediated by the Boussinesq interaction on a soft substrate.

Orientational ordering driven by mechanical distortion of soft substrates plays a major role in material transformation processes such as elastocapillarity and surface anchoring. We present a theoretical model of the orientational response of anisotropic rods deposited onto a surface of a soft, elastic substrate of finite thickness. We show that anisotropic rods exhibit a continuous isotropic-nematic phase transition, driven by orientational interactions between surface deposited rods. This interaction is mediated by the deformation of the underlying elastic substrate and is quantified by the Boussinesq solution adapted to the case of slender, surface deposited rods. From the microscopic rod-rod interactions, we derive the appropriate Maier-Saupe mean-field description, which includes the Boussinesq elastic free energy contribution due to the substrate elasticity, derive the conditions for the existence of a continuous orientational ordering transition, and discuss the implication of results in the soft (bio)system context.

Inhomogeneous nematic-isotropic phase transition of a thermotropic liquid crystal doped with iron oxide nanoparticles

We elucidate the local distribution and aggregated structure of iron oxide nanoparticles (IONPs; 6 nm in diameter) doped in the matrix of a nematic liquid crystal (LC), 4-cyano-4'-pentylbiphenyl, utilizing in situ cryogenic transmission electron microscopy and polarized light microscopy. We show that tens of IONPs aggregate into a sphere-like morphology, and the aggregates combine into elongated clusters with a length of hundreds of nm. With the IONP-doped LC matrix confined to a thin glass cell, we study the nematic-isotropic (N-I) phase transition, and suggest that local heterogeneity of LC textures as seen in polarized microscopy is caused by the existence of IONP aggregate clusters. These clusters act also as nuclei for the formation of isotropic domains upon heating.

Nematic and Smectic Phases: Dynamics and Phase Transition

We study in this paper the dynamics of molecules leading to the formation of nematic and smectic phases using a mobile 6-state Potts spin model with Monte Carlo simulation. Each Potts state represents a molecular orientation. We show that, with the choice of an appropriate microscopic Hamiltonian describing the interaction between individual molecules modeled by 6-state Potts spins, we obtain the structure of the smectic phase by cooling the molecules from the isotropic phase to low temperatures: molecules are ordered in independent equidistant layers. The isotropic-smectic phase transition is found to have a first-order character. The nematic phase is also obtained with the choice of another microscopic Hamiltonian. The isotropic-nematic phase transition is a second-order one. The real-time dynamics of the molecules leading to the liquid-crystal ordering in each case is shown by a video.

Quadrupole modelling of dual lock-in method for the simultaneous measurements of thermal diffusivity and thermal effusivity

The simultaneous measurement of thermal diffusivity and thermal effusivity based on the temperature wave analysis (TWA) is proposed by introducing the dual lock-in temperature sensor with a photo-thermal ac heat source. The thermal effect from each component in the multilayered experimental system is simulated by the quadrupole modeling adopted to the pure periodic heating of the temperature wave. The frequency dependency of the thermal effect from the thermal contact resistance and capacitance in the sensor were analyzed and minimized by taking the optimal frequency range of the temperature wave. The proposed method is applied to examine the nematic-isotropic phase transition of the liquid crystal 4′-(pentyloxy)-4-biphenyl carbonitrile (5OCB). The results were compared with the previous results measured with a single sensor and the advantage of the dual lock-in temperature sensor was demonstrated.

Prediction of the density–pressure–temperature behavior in the isotropic–nematic phase transition of MBBA liquid crystal

In this work, it is proposed a perturbation theory in conjunction with the Convex Peg HERSW model and an adjustable intermolecular potential. The perturbation theory developed can quantitatively predict the density–pressure–temperature experimental behavior at the isotropic–nematic phase transition of MBBA at 1 atm.

Photomechanical coupling in photoactive nematic elastomers

Photoactive nematic elastomers are soft rubbery solids that undergo deformation when illuminated. They are made by incorporating photoactive molecules like azobenzene into nematic liquid crystal elastomers. Since its initial demonstration in 2001, it has received increasing interest with many recent studies of periodic and buckling behavior. However, theoretical models developed have focused on describing specific deformation modes (e.g., beam bending and uniaxial contraction) in the absence of mechanical loads, with only limited attention to the interplay between mechanical stress and light-induced deformation. This paper explores photomechanical coupling in a photoactive nematic elastomer under both light illumination and mechanical stress. We begin with a continuum framework built on the free energy developed by Corbett and Warner (Phys. Rev. Lett. 2006). Mechanical stress leads to nematic alignment parallel to a uniaxial tensile stress. In the absence of mechanical stress, in the photo-stationary state where the system reaches equilibrium, the nematic director tends to align perpendicular to the polarization of a linearly polarized light. However, sufficient illumination can destroy nematic order through a first-order nematic-isotropic phase transition which is accompanied by a snap through deformation. Combined illumination and mechanical stress can lead to an exchange of stability accompanied by stripe domains. Finally, the stress-intensity phase diagram shows a critical point that may be of interest for energy conversion.

Dielectric, electrooptic and viscoelastic properties in cybotactic nematic phase doped with ferroelectric nanoparticles

Hydrogen bonded liquid crystal, possessing a wide temperature range of cybotactic phase was used as a matrix to disperse ferroelectric LiNbO3 nanoparticles at the concentrations from 0.5 wt.% to 3.0 wt.%. The influence of nanoparticles on the physical properties has been studied with the help of polarizing optical microscopy (POM), impedance spectroscopy and electro-optic measurements. POM investigation revealed a nematic droplet dispersed in the isotropic phase due to the addition of ferroelectric nanoparticles. The suppression of the polarization has been observed and explained by the occurrence of antiparallel dipole configuration. The isotropic-nematic phase transition temperature was decreased due to the reduction in the parameter order. The decreasing in dielectric anisotropy (∆ε), response time (τ) and in threshold Freedericksz voltage (Vth) was associated to the decrease in splay constant elastic (K11) and in the rotational viscosity (γ). The curves of ∆ε, Vth, τ, K11 and γ when NPs concentration increases, showed a minimum at the same concentration (c = 1 wt.%) allowing the determination of the optimal NPs concentration.

Spontaneous Alignment of Self‐Assembled Cationic and Amphiphilic β‐Sheet Peptides

AbstractAligned peptide assemblies are of interest for both fundamental understanding of biological systems as well as development of novel biomaterials. Herein, Pro‐Lys‐(Phe‐Lys)5‐Pro (PFK), a cationic and amphiphilic β‐sheet synthetic peptide that self‐assembles into fibrils, capable of undergoing spontaneous isotropic to nematic phase transition, that intrinsically aligns in solution, is presented. Upon dissolution PFK forms isotropically dispersed fibrils due to delicate balance between attractive H bonds and hydrophobic interactions to electrostatic repulsive forces. The transition from isotropic to nematic phase alignment occurring over a period of several days is monitored in situ by both small‐angle X‐ray scattering and cryogenic transmission electron microscopy. The degree of alignment and the time required to achieve a complete isotropic–nematic phase transition of PFK fibrils in aqueous solution is found to be concentration dependent as predicted by Onsager's excluded volume theory. Remarkably, PFK nematic phases remain aligned over the course of several months without the application of any external stimuli. Aligned PFK gels can be formed by mild shearing through a salty medium. This system provides a novel route for producing nanoscale aligned materials for potential biomedical applications.

Light-Driven Dynamic Adhesion on Photosensitized Nematic Liquid Crystalline Elastomers.

In liquid crystal elastomers (LCEs), the internal mechanical loss increases around the nematic-isotropic phase transition and remains high all through the nematic phase, originating from the internal orientational relaxation related to the so-called "soft elasticity". Because the viscoelastic dissipation of the materials affects their adhesion properties, the nematic-isotropic phase transition can cause dramatic changes in the adhesion strength. Although the phase transitions can generally be induced by heat, here, we demonstrate the light-driven transition in dynamic adhesion in dye-doped nematic LCE. The special dye is chosen to efficiently generate local heat on light absorption. The adhesion strength is lowered with fine tunability depending on the light power, which governs the effective local temperature and through that the viscoelastic damping of the system. We demonstrate the light-assisted dynamic control of adhesion in a 90°-peel test and in pick-and-release of objects, which may lead to the development of stimuli-responsive adhesive systems with fine spatio-temporal controls.

Surface chemistry driven dynamic rheology, microstructure of fumed and colloidal silica particles in nematic liquid crystals

We investigate the influence of surface chemistry of silica particles on the dynamic rheology and underlying microstructure of silica in nematic liquid crystal (NLC), 4-cyano-4-pentylbiphenyl (5CB) suspensions. The hydrophilic fumed silica (Aerosil 200), synthesized (uncoated) and N-methyl-3-aminopropyltrimethoxysilane (MAP) coated colloidal silica particles in 5CB suspensions form gels at a critical particle concentration of 1.5 wt% (φ = 0.007), 15 wt% (φ = 0.075) and 20 wt% (φ = 0.103) respectively. On the other hand hydrophobic fumed silica (Aerosil RX300) and Dimethyl octadecyl [3-(trimethoxysilyl) propyl] ammonium chloride (DMOAP) coated colloidal silica particles in 5CB suspensions form gels at relatively higher particle loading of 3.5 wt% (φ = 0.016) and 50 wt% (φ = 0.314) respectively. The storage modulus (G′) of the Aerosil 200/5CB, uncoated silica/5CB and MAP coated silica/5CB suspensions follow power-law scaling G′ ~ φm, where m is 4.74 ± 0.5 and 5.89 ± 0.3 and 6.28 ± 0.5 respectively. In contrast, Aerosil RX300/5CB suspensions have a higher value of m (6.53 ± 0.4). Dynamic rheology data of solid gel-like Aerosil 200/5CB, uncoated silica/5CB and MAP coated silica/5CB suspensions exhibit signatures of soft glassy materials, exemplified by (a) an almost frequency (ω) independent G′ and a minima in loss modulus (G″) and (b) strain overshoot in G″, whereas Aerosil RX300/5CB and DMOAP coated silica/5CB suspensions show signatures of flocculated gels with strong dependence of G′ on ω, an absence of minima in G″ and no strain overshoot in G″. Furthermore, G′ of gel-like Aerosil 200/5CB suspension is insensitive to the nematic-isotropic phase transition (TNI) of 5CB, while the Aerosil RX300/5CB suspension shows a liquid-like behavior (G″ > G′) at temperature (T) > TNI of 5CB. We believe that the observed differences in the dynamic rheology are dictated by the surface anchoring or interactions of 5CB molecules with the –OH groups of hydrophilic silica. The microscopy results show that at T < TNI, the 5CB in Aerosil 200/5CB suspensions gets restricted in the network of Aerosil 200 nanoparticles, whereas the flocs of Aerosil RX300 nanoparticles are expelled by the nucleating 5CB domains to form a particulate network in Aerosil RX300/5CB suspensions. Moreover, the uncoated, MAP and DMOAP coated colloidal silica/5CB suspensions do not exhibit the prominent effect of T on rheology except the discontinuity in G′ and G″ at TNI of 5CB.

Impact of ferroelectric nanoparticles on the dielectric constant of nematic liquid crystals

ABSTRACT The influence of ferroelectric BaTiO3 nanoparticles on the dielectric constant in the isotropic phase of the isotropic-nematic phase transition is studied within Landau phenomenological theory. The ferroelectric BaTiO3 nanoparticles dependence of transition temperature and dielectric constant in the isotropic phase is calculated. We compare our theoretical results with experimental data available in the literature.

Self-assembled monolayer formation of pentamers-like molecules onto FCC(111) surfaces: the case of curcuminoids onto Au(111) surface

The adsorption of rigid straight electrically polarized pentamers over a FCC(111) surface is studied. The model was inspired by the deposition of 2-thiophene molecules over the Au(111) surface, which was previously characterized by experimental techniques and simulated under the frame of the density functional theory. We now obtain and report the charge distribution of the molecule which allows to propose a deposition model followed by Monte Carlo simulations over an ad-hoc lattice gas model. We show that for a certain value of the chemical potential there exists an isotropic-nematic phase transition which can explain the formation of a self-assembled monolayer like the one observed in the transmission electron microscopy images. An order parameter is defined to characterize the transition which presents a step-like behavior at a critical chemical potential value. The possible nature of the nematic transition in conjunction with an ergodicity breakdown is discussed as future work by means of statistical physics techniques.

‘Quasi-Tricritical’ and Glassy Dielectric Properties of a Nematic Liquid Crystalline Material

Results of dielectric studies in the nematic and isotropic liquid phases of pentylcyanobiphenyl (5CB), a model rod-like liquid crystalline material, are shown. They are based on the discussion of the evolution of dielectric constant ( ε ), its changes under the strong electric field (nonlinear dielectric effect, NDE), and finally, the primary relaxation time. It is shown that changes in ε T and NDE are entirely dominated by the impact of pretransitional fluctuations (pre-nematic and pre-isotropic, respectively) which are associated with the weakly discontinuous character of the isotropic–nematic phase transition. This influence also extends for the low-frequency, ionic species dominated, region. Notable, that the derivative-based and distortions sensitive analysis revealed the tricritical nature of the I-N transition. Although the glass transition in 5CB occurs in the deeply supercooled state at T g ≈ − 68 ° C , the dynamics (changes of the primary relaxation time) follow a previtreous pattern both in the isotropic and in the nematic phase. Finally, the discussion of the ’molecular’ vs. ‘quasi-critical’ characterizations of the isotropic and nematic phases is presented. It shows the evident prevalence of the ‘quasi-critical-picture’, which offers the consistent temperature parameterization in the total tested temperature range.

Исследование диэлектрических параметров нематических жидких кристаллов в приграничных слоях

This article presents the results of a study of the dielectric properties of thin liquid crystal layer systems. To study the dielectric parameters of nematic liquid crystals in the boundary layers, the frequency and temperature dependences of the real (ε') and imaginary (ε'') parts of the dielectric constant were determined. We studied samples that are in macroscopic layers and samples located in flat capillary layers (d ~ 0.1 μm). In both cases, the measurement is performed by the same method, namely, the method of temporary dielectric spectroscopy. The entire procedure for measuring, recording, accumulating and processing data was carried out automatically. In this case, the results of dielectric measurements could be presented both in the frequency and time domains. For measurements, a plane-parallel measuring cell made of brass and titanium electrodes was used. It should be noted that the surface of a solid body has a strong influence on the phase state of liquids and liquid crystals in the boundary layer. For example, in such a layer, liquid molecules acquire a mesophase ordering. In the case of NLC, the nematic potential increases and, as a result, the temperature of the nematic-isotropic phase transition increases. However, a shift in the bleaching temperature in the NLC mixture studied by us did not appear due to the smallness of the effect. The substance enclosed in the microspace between the surfaces of the mica has a higher dielectric constant than the corresponding bulk sample. Moreover, such a difference is observed both in the isotropic and in the nematic phases. This can be explained as follows. In a thick layer, a dipole-dipole intermolecular interaction takes place, which reduces the effective dipole moment of the molecules.