Liquid Crystal Modulator Research Articles

Transmissive Single-Pixel Microscopic Imaging through Scattering Media.

Microscopic imaging is of great significance for medical diagnosis. However, due to the strong scattering and absorption of tissue, the implementation of non-invasive microscopic imaging is very difficult. Traditional single-pixel microscopes, based on reflective optical systems, provide an alternative solution for scattering media imaging. Here, the single-pixel microscope with transmissive liquid crystal modulation is proposed. The microscopic ability of the proposed microscope is calibrated. The multi-spectral microscopic imaging of the object is demonstrated. The transmissive imaging of the object behind the scattering media is analyzed. The proposed prototype of the transmissive single-pixel microscope is expected to be applied in microscopic imaging through scattering media and medical imaging.

Detection of arginase through the optical behaviour of liquid crystals due to the pH-dependent adsorption of stearic acid at the aqueous/liquid crystal interface

The authors here developed a stearic acid-doped liquid crystal (LC)-based sensor for the label-free and real-time detection of arginase (ARG1). This sensor was based on the orientational transition of LCs due to the adsorption of stearic acid at the aqueous/LC interface. The shift in the optical image of LCs from bright to dark was obtained after transferring an aqueous solution of arginase, ʟ-arginine, and urease onto the aqueous/stearic acid-doped LC interface. Arginase catalyzes ʟ-arginine hydrolysis to produce urea, which is further catalyzed by urease to produce ammonium, which in turn increases the pH of the aqueous bulk. This increase in pH causes the deprotonation and adsorption of stearic acid and the formation of a stearate self-assembled monolayer at the interface, which induces a change in the orientation of the LC film from planar to homeotropic anchoring. ʟ-norvaline can inhibit the enzymatic activity of arginase; therefore, the LC optical image stayed bright after adding a mixed solution of arginase and ʟ-norvaline to the optical cell incubated with ʟ-arginine and urea, indicating no orientation change of LCs. ARG1 could be quantitatively detected by the proposed sensor in the range from 1 ng/mL to 100 ng/mL, with the detection limit as low as 0.6 ng/mL. The effect of the human serum matrix on the performance of proposed sensors was also investigated to confirm the applicability in ARG1 detection. The proposed sensor shows great potential for the development of pH-sensitive instruments for the monitoring of enzymatic reactions.

Fabrication and characterization of large aperture adaptive modal liquid crystal lens with a PEDOT:PSS/PVA/DMSO blend used as the modal and rubbing layer

An adaptive modal liquid crystal lens (AMLCL) with a 5 mm aperture and thickness of 20 µm was fabricated and studied. PEDOT:PSS/PVA/DMSO polymer blend film was used as both the modal and rubbing layers simultaneously. Using the modal layer as the rubbing layer facilitates and simplifies cell preparation. An optimal concentration of polymer blend, the 0.1-µm-thick modal layer had a 5 MΩ/□ sheet resistance. AMLCL electrodes were broken down into four parts and the cell placed in the optical setup to study the wavefront shape. It was shown that by applying the trigger voltage to different parts of the electrode and removing parts of the circuit, the cell could function as a spherical, cylindrical, or prismatic lens. Further, the electric power consumption was studied at different voltages and frequencies, showing that the spherical lens requires 0.5 mW to reach its maximum optical power at 1 kHz. Shack-Hartmann wavefront sensor was used to study the AMLCL’s aberrations. Tip aberration in this cell is attributed to the pretilt of the Liquid Crystal (LC) molecules at the surfaces that deflect the transmitting light from its straight path. It was shown that higher-order aberrations are negligible compared to the spherical aberration, which is reduced by raising the frequency to 1 kHz.

Real-time liquid crystal-based creatinine sensor using a micro-patterned flexible substrate

ABSTRACT We propose a novel, simple, cost-effective sensor based on orientational transitions of nematic liquid crystals (LC) for real-time, quantitative, and sensitive detection of creatinine. A micro-well array structure on a polydimethylsiloxane (PDMS) substrate was fabricated to replace traditional copper grids stabilising a thin LC film, which contains 4-cyano-4ʹ-pentylbiphenyl (5CB) LC material and 4ʹ-hexylbiphenyl-4-carboxylic acid (HBA). The principle of the sensor is based on the ordering transition of LCs induced by enzymatic reactions of creatinine and creatinine deaminase (CD), resulting in an optical signal change. The orientational response of LCs was quantitatively monitored in the presence of different amounts of creatinine by cross-polarised optical microscopy and light transmittance measurement system in real time. Concentrations from 50 to 1100 μM of creatinine were successfully detected.

Translational and rotational phase ordering of symmetric dimer mesogens: Molecular rigidity effect

A theoretical approach towards molecular rigidity and mesogenic behaviour of symmetric dimer molecules viz., α,ω-bis (4-cyanobiphenyl-4′-yloxycarbonyl) propane (3dimer) and α,ω-bis (4-cyanobiphenyl-4′-yloxycarbonyl) hexane (6dimer) has been analysed with regard to orientational and translational motions. The crystallographic information files of these molecules have been used to report the structures. The formalism of CNDO/2 approach has been executed to estimate the charges and dipole moment components of all atoms. Total configurational interaction energies have been estimated using Rayleigh-Schrodinger perturbation theory. These energy data have been utilized to estimate the configurational probability using the formula given by Maxwell-Boltzmann at phase transition temperature. The translational/ rotational ordering of different configurations has been analysed. The mesogenic behaviour of these compounds has been explained according to their relative order that helps in establishing molecular mesogenic models. As the manipulation of a molecular configuration is significant from the view point of photonics, this manuscript is expected to provide a route in understanding in this feature depending on the parameters reported in this manuscript. The impact of rigidity and aspect ratio upon the liquid crystal transitions has been discussed.

Real-space renormalization-group treatment of the Maier-Saupe-Zwanzig model for biaxial nematic structures.

The Maier-Saupe-Zwanzig model for the nematic phase transitions in liquid crystals is investigated in a diamond hierarchical lattice. The model takes into account a parameter to describe the biaxiality of the microscopic units. Also, a suitably chosen external field is added to the Hamiltonian to allow the determination of critical parameters associated with the nematic phase transitions. Using the transfer-matrix technique, the free energy and its derivatives are obtained in terms of recursion relations between successive generations of the hierarchical lattice. In addition, a real-space renormalization-group approach is developed to obtain the critical parameters of the same model system. Results of both methods are in excellent agreement. There are indications of two continuous phase transitions. One of them corresponds to a uniaxial-isotropic transition, in the class of universality of the three-state Potts model on the diamond hierarchical lattice. The transition between the biaxial and the uniaxial phases is in the universality class of the Ising model on the same lattice.

Photocontrollable Resistivity Change in Nanoparticle-Doped Liquid Crystal Alignment Layer: Voltage Holding and Discharging Properties of Fringe-Field Switching Liquid Crystal Modes

In liquid crystal (LC) displays, deriving an optimum resistance level of an LC alignment polyimide (PI) layer is important because of the trade-off between the voltage holding and surface-discharging properties. In particular, to apply a power-saving low-frequency operation scheme to fringe-field switching (FFS) LC modes with negative dielectric LC (n-LC), delicate material engineering is required to avoid surface-charge-dependent image flickering and sticking problems, which severely degrade with lowering operation frequency. Therefore, this paper proposes a photocontrolled variable-resistivity PI layer in order to systematically investigate the voltage holding and discharging properties of the FFS n-LC modes, according to the PI resistivity (ρ) levels. By doping fullerene into the high-ρ PI as the photoexcited charge-generating nanoparticles, the ρ levels of the PI were continuously controllable with a wide tunable range (0.95 × 1015 Ω∙cm to 5.36 × 1013 Ω∙cm) through Ar laser irradiation under the same LC and LC alignment conditions. The frequency-dependent voltage holding and discharge behaviors were analyzed with photocontrolled ρ variation. Thus, the proposed experimental scheme is a feasible approach in PI engineering for a power-saving low-frequency FFS n-LC mode without the image flickering and image sticking issues.

Activity-induced isotropic-polar transition in active liquid crystals

Active fluids are intrinsically out-of-equilibrium systems due to the internal energy injection of the active constituents. We show here that a transition from a motion-less isotropic state towards a flowing polar one can be possibly driven by the sole active injection through the action of polar-hydrodynamic interactions in absence of an ad hoc free energy which favors the development of an ordered phase. In particular, we propose an analytical argument and we perform lattice Boltzmann simulations where the appearance of large temporal fluctuations in the polar fraction of the system is observed at the transition point. We show that elastic absorption plays a relevant role in energy transfer dynamics, contrary to the case of the usual active gel theory where this term can be factually neglected.

Economic design of a CEV x̄ control chart for determining optimal right-censored times

ABSTRACT Lifetime is a crucial quality characteristic of electronic products that must be monitored using control charts. Lifetime may provide right-censored data. A conditional expected value (CEV) x̄ control chart was developed to monitor type I right-censored data. Researchers had always assumed that the censored time is a known constant when designing control charts. This study provides an economic design model for optimizing CEV x̄ charts and type I right-censored time for reducing costs associated with quality control and life tests. This method was employed on a practical case of a liquid crystal display module. A sensitivity analysis was also conducted on the influence of the model parameters on the design and censored time of the CEV x̄ chart.

Selective Liquid Crystal Driving Mode Achieved by Controlling the Pretilt Angle via a Nanopatterned Organic/Inorganic Hybrid Thin Film

AbstractBehavior of liquid crystal (LC) is a result of interaction between the geometrical shape restrictions of the adjacent surface and molecular forces among LCs or adjacent surface. For years, continuous efforts have been made to control LC orientation and anchoring with pretilt angle for modulating the electro‐optical characteristics. For now, diverse driving modes have been developed including twisted nematic, optically compensated bend, electrically controlled birefringence, and vertical alignment. However, it has the limitation that different fabrication process should be adopted in different driving mode such as materials of alignment layer and techniques for aligning the LCs. Herein, selective LC modes are achieved by controlling the LC pretilt angle using nanopatterned organic/inorganic hybrid thin films composed of polyimide (PI) and tin oxide (SnO). It is possible to control the surface wettability according to the composition ratio between PI and SnO, thereby adjusting the pretilt angle of the LCs. Fabrication of SnO combined with PI applied via embossing allows for the large‐scale replication for LC alignment and based on consumer demand, devices can be manufactured in various modes through simple configuration changes. Therefore, an inorganic compound combined with an organic one permits designing addressable LC driving modes.

Remotely Controlled, Reversible, On-Demand Assembly and Reconfiguration of 3D Mesostructures via Liquid Crystal Elastomer Platforms.

Three-dimensional (3D) mesostructures are gaining rapidly growing interest due to their potential applications in a broad range of areas. Despite intensive studies, remotely controlled, reversible, on-demand assembly and reconfiguration of 3D mesostructures, which are desired for many applications, including robotics, minimally invasive biomedical devices, and deployable systems, remain a challenge. Here, we introduce a facile strategy to utilize liquid crystal elastomers (LCEs), a soft polymer capable of large, reversible shape changes, as a platform for reversible assembly and programming of 3D mesostructures via compressive buckling of two-dimensional (2D) precursors in a remote and on-demand fashion. The highly stretchable, reversible shape-switching behavior of the LCE substrate, resulting from the soft elasticity of the material and the reversible nematic-isotropic transition of liquid crystal (LC) molecules upon remote thermal stimuli, provides deterministic thermal-mechanical control over the reversible assembly and reconfiguration processes. Demonstrations include experimental results and finite element simulations of 3D mesostructures with diverse geometries and material compositions, showing the versatility and reliability of the approach. Furthermore, a reconfigurable light-emitting system is assembled and morphed between its "on" and "off" status via the LCE platform. These results provide many exciting opportunities for areas from remotely programmable 3D mesostructures to tunable electronic systems.

Transverse-mode control in VCSELs by electrically tunable liquid crystal mode filters

A distinctive method is used to achieve transverse-mode control by building and integrating a tunable liquid crystal (LC) mode filter on top of traditional vertical-cavity surface-emitting lasers (VCSELs). An LC cell is fabricated by injecting a type of nematic LC material (E7) in an annular groove patterned by SiO2. By electrically tuning the refractive index of the LC, a spatially dependent reflectivity profile can be achieved and optimized, which directly influences the threshold modal gain of each transverse mode provided by oxide-confined VCSELs. The flexible and accurate control performance of the LC mode filter structure have been demonstrated by the simulation results and our analysis. This design is a completely novel method to realize VCSELs with an accurate and real-time controllable transverse-mode, and it will probably play a significant role in the future.

Impurity-induced nematic-isotropic transition of liquid crystals.

Complex fluids made of liquid crystals (LCs) and small molecules, surfactants, nanoparticles or 1D/2D nanomaterials show novel and interesting features, making them suitable materials for various applications starting from optoelectronics to biosensing. While these additives (impurities) introduce new features in the complex fluids, they may also alter the phase transition behaviour of LCs depending on the physiochemical properties of the added impurity. This article reports on the phase transition of 4-cyano-4'-alkylbiphenyl (nCB) LCs in the presence of an associative impurity, i.e., water and a non-associative impurity, i.e., hexane employing computational methods and experiments. In particular, all-atom (AA) simulations and coarse-grained (CG) models were designed for two complex systems, i.e., 6CB + water and 6CB + hexane and corresponding spectrophotometry experiments were performed using a homologous LC, i.e., 5CB. Results from the simulations and experiments elucidate that the phase transition of LCs depends on the mixing/demixing phenomenon of the impurity in the LC. While associative liquids like water which do not mix with LCs do not influence the nematic-to-isotropic phase transition of LCs, hexane, being a non-associative liquid, mixes well with LCs and induces a sharp impurity-induced nematic-to-isotropic phase transition. Upon application of both AA and CG simulations, we could reach the conclusion that the mixing/demixing phenomenon in an LC + impurity system influences the entropy of the system and hence the observed phase transitions are entropy-driven.

Effect of Single/Mixed Surfactant Systems on Orientations of Liquid Crystals and Interaction of Proteins with Surfactants at Fluid Interfaces

A series of single surfactant systems, i.e, quaternary ammonium-based gemini surfactants with different spacers and alkyl chain lengths (m-n-m; m = 12, n = 2, 3, 4, 6; n = 3, m = 12, 14, 16), halogen-free surface-active ionic liquid (HF-SAILs) with different symmetries ([Cnmim][C12H25SO4]; n = 6, 8, 10, 12), and single-chain cationic surfactants including 1-dodecyl-3-methylimidazolium bromide ([C12mim]Br) and dodecyltrimethylammonium bromide (DTAB), along with certain combinations of different surfactants (12-3-12/[C12mim]Br and 12-3-12/DTAB) were applied to an aqueous/liquid crystal interface (ALI). All the surfactants could induce an orientational transition of liquid crystals (LCs) from a planar to homeotropic state, which caused a bright-to-dark optical shift. It was proved that double-chain surfactants and the mixed surfactants inclined to adsorb at the ALI triggering the orientational transition. Inspiringly, a quicker and more sensitive dark-to-bright optical response was observed for mixed surfactant system-decorated interfaces in contact with proteins (such as bovine serum albumin (BSA), lysozyme, and trypsin) as opposed to the single surfactant systems. The ALI decorated by the 12-3-12/[C12mim]Br system was particularly efficient and exhibited the most sensitive optical response for BSA (0.01 ng mL−1). The order parameters (SCD) of surfactants tails at the interface and the free energy of proteins with 12-3-12 and [C12mim]Br were calculated, respectively. The results explain that the 12-3-12/[C12mim]Br-laden ALI shows a quicker and more sensitive optical response for BSA. This work inspired us to study mixed surfactant systems-decorated LC interfaces and further provides new insights for different chemical and biological applications.

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.

Absence of nematic quasi-long-range order in two-dimensional liquid crystals with three director components

The Lebwohl–Lasher model describes the isotropic–nematic transition in liquid crystals. In two dimensions, where its continuous symmetry cannot break spontaneously, it is investigated numerically since decades to verify, in particular, the conjecture of a topological transition leading to a nematic phase with quasi-long-range order. We use scale invariant scattering theory to exactly determine the renormalization group fixed points in the general case of N director components (RP N−1 model), which yields the Lebwohl–Lasher model for N = 3. For N > 2 we show the absence of quasi-long-range order and the presence of a zero temperature critical point in the universality class of the O(N(N + 1)/2 − 1) model. For N = 2 the fixed point equations yield the Berezinskii–Kosterlitz–Thouless transition required by the correspondence RP 1 ∼ O(2).

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.

Kinetic singularities at transition points from equilibrium to metastable states of the Lennard-Jones particle system

The equilibrium and metastable states of the Lennard-Jones vapor, liquid, and crystal, are studied in the vicinity of the phase transition points. Vapor-liquid, liquid-vapor, liquid-crystal, and crystal-liquid transitions are modeled within the molecular dynamics method. As a research tool, a four-point correlation function is used for the qualitative detection of collective motions of atoms in the equilibrium and metastable states. We use its modification, a correlation coefficient, and find singularities at the transition from equilibrium to metastable states. The nature of these changes, as precursors of nucleation, is discussed.

Ultrasensitive and Selective Detection of SARS-CoV-2 Using Thermotropic Liquid Crystals and Image-Based Machine Learning

Rapid, robust virus-detection techniques with ultrahigh sensitivity and selectivity are required for the outbreak of the pandemic coronavirus disease 2019 (COVID-19) caused by the severe acute respiratory syndrome-coronavirus-2 (SARS-CoV-2). Here, we report that the femtomolar concentrations of single-stranded ribonucleic acid (ssRNA) of SARS-CoV-2 trigger ordering transitions in liquid crystal (LC) films decorated with cationic surfactant and complementary 15-mer single-stranded deoxyribonucleic acid (ssDNA) probe. More importantly, the sensitivity of the LC to the SARS ssRNA, with a 3-bp mismatch compared to the SARS-CoV-2 ssRNA, is measured to decrease by seven orders of magnitude, suggesting that the LC ordering transitions depend strongly on the targeted oligonucleotide sequence. Finally, we design a LC-based diagnostic kit and a smartphone-based application (app) to enable automatic detection of SARS-CoV-2 ssRNA, which could be used for reliable self-test of SARS-CoV-2 at home without the need for complex equipment or procedures.

Stimuli-Responsive Dynamic Metaholographic Displays with Designer Liquid Crystal Modulators.

Flat optics, realized by the artificially created 2D material platform called optical metasurfaces, is currently undergoing a science-to-technology transition. However, "real-time" active operations of such flat optical devices remain yet unresolved. Here, liquid crystals (LCs)-integrated metaholograms for ultracompact dynamic holographic displays are proposed. The anisotropic nature of the LCs allows facile and repeatable manipulation of the polarization of light. Specifically designed ("designer") LCs and efficient helicity-encoded metaholograms are combined to realize stimuli-responsive dynamic displays. The designer LC modulators are used as switches that enable a variety of external stimuli (e.g., electric field, heat, surface pressure) to operate holographic images in real-time. Such a dynamic metaholographic platform will provide a path to external stimuli-driven "smart" sensing and display applications such as hologram labels for temperature/pressure/touch monitoring and interactive holographic displays with haptic motion recognition.