Polar electro-optic switching in droplets of an achiral nematic liquid crystal

A polar electro-optic response is observed in droplets of an achiral nematic liquid crystal in coexistence with the isotropic phase. Between crossed polarizers each pancake-shaped droplet shows extinction brushes in the form of a centred cross aligned with the polarizer axes. An applied electric field E induces a rotation of the crosses about the field direction, with about half the droplets switching clockwise and the other half anticlockwise. The sense of rotation in each droplet changes when E is reversed. We propose that a twisted bipolar director structure is stabilized in the droplets by a relatively large splay elastic constant and tangential boundary conditions. The molecules twist along the diameter of the droplets, perpendicular to the applied field, which results in a linear rotation of the director by the inverse flexoelectric effect. Since the molecules are achiral, the handedness of the twist, and hence the sense of the switching, in any droplet is arbitrary.     

Polar electro-optic switching in droplets of an achiral nematic liquid crystal

A polar electro-optic response is observed in droplets of an achiral nematic liquid crystal in coexistence with the isotropic phase. Between crossed polarizers each pancake-shaped droplet shows extinction brushes in the form of a centred cross aligned with the polarizer axes. An applied electric field E induces a rotation of the crosses about the field direction, with about half the droplets switching clockwise and the other half anticlockwise. The sense of rotation in each droplet changes when E is reversed. We propose that a twisted bipolar director structure is stabilized in the droplets by a relatively large splay elastic constant and tangential boundary conditions. The molecules twist along the diameter of the droplets, perpendicular to the applied field, which results in a linear rotation of the director by the inverse flexoelectric effect. Since the molecules are achiral, the handedness of the twist, and hence the sense of the switching, in any droplet is arbitrary.     

Dye-dependent studies on droplet pattern and electro-optic behaviour of polymer dispersed liquid crystal

In order to study the droplet pattern and electro-optic (EO) behaviour of polymer dispersed liquid crystal (PDLC) with the addition of dye, dichroic polymer dispersed liquid crystal (DPDLC) films were prepared using a nematic liquid crystal (NLC), photo-curable polymer (NOA 65) and anthraquinone blue dichroic dye (B2), in equal ratio (1:1) of polymer and liquid crystal (LC) by polymerisation induced phase separation (PIPS) technique. Dichroic dye was taken in different concentration (wt./wt. ratio) as 0.0625%, 0.125%, 0.25%, 0.5% and 1% of the LC mixture in DPDLC films. Initially, in an open circuit when there is no proviso for external electric field (0 V), LC droplets in polymer matrix exhibited bipolar pattern, though on closing the circuit with the increase of electric field pattern of droplets starts changing, LC molecules align along the direction of applied electric field and aligned completely relatively at higher field (30 V), which illustrate vertical radial pattern. Further, results show that the DPDLC film containing 0.0625% dye concentration with consistent average droplet size ~4.30 μm, exhibits the best transmission at lower operating voltage.     

Droplet shape and reorientation fields in nematic droplet/polymer films

Polymer films containing droplets of nematic liquid crystal form an important class of new electro-optic light valves and displays. While previous work has shown that the nematic droplet size is an important factor in the electro-optic properties of these films, here we report that the droplet shape is equally important in determing the electro-optics of the film. Electron micrographs show that for films using polyvinyl alcohol as the polymeric binder the cavities formed by the polymer matrix are oblate in nature, and aligned with the minor axis perpendicular to the film plane. In oblate cavities the elastic-deformation free energy is minimized when the director field in the droplet is aligned along a major axis of the spheroid; the electric field performs work on the nematic in reorienting the nematic into a higher-energy state, equal to the elastic-free-energy difference between the two configurations. Calculations and experiment are used to estimate the elastic and electric field free-energy-density changes that occur upon reorientation of the nematic droplet. The general agreement between these two values is used to indicate that droplet shape anisotropy is a major factor in determining the electrooptic properties of these films.     

Droplet shape and reorientation fields in nematic droplet/polymer films

Polymer films containing droplets of nematic liquid crystal form an important class of new electro-optic light valves and displays. While previous work has shown that the nematic droplet size is an important factor in the electro-optic properties of these films, here we report that the droplet shape is equally important in determing the electro-optics of the film. Electron micrographs show that for films using polyvinyl alcohol as the polymeric binder the cavities formed by the polymer matrix are oblate in nature, and aligned with the minor axis perpendicular to the film plane. In oblate cavities the elastic-deformation free energy is minimized when the director field in the droplet is aligned along a major axis of the spheroid; the electric field performs work on the nematic in reorienting the nematic into a higher-energy state, equal to the elastic-free-energy difference between the two configurations. Calculations and experiment are used to estimate the elastic and electric field free-energy-density changes that occur upon reorientation of the nematic droplet. The general agreement between these two values is used to indicate that droplet shape anisotropy is a major factor in determining the electrooptic properties of these films.     

Optical properties of stretched polymer dispersed liquid crystal films

The basic mechanisms determining the formation of optical anisotropy in stretched, thin polymer dispersed liquid crystal (PDLC) films with micron sized nematic droplets have been studied experimentally and the results analysed in terms of a proposed theoretical model. The experiments were performed on PDLC films with the bipolar nematic director configuration in the droplets, where the film transmittance, microscopic structure, and birefringence of the polymer matrix were studied. It is shown that the orientational ordering of bipolar nematic droplets, introducing the main contribution to the ability of stretched PDLC film to polarize the transmitted light, is strongly dependent upon initial droplet shape and the elastic properties of the polymer matrix. The 'anomalous' nematic director orientation is also observed in a portion of elongated droplets where the axes of bipolar configurations do not coincide with the major axes of the droplet cavities due to the presence of inclusions at the cavity walls. The effect of alternation of droplet size and shape upon stretching and the influence of optical anisotropy of the polymer matrix on film transmittance are analysed. On the basis of the results obtained, simple criteria for optimization of main PDLC polarizer performance are formulated.     

Computer simulation of elongated bipolar nematic droplets II. External field aligned normal to the droplet axis of symmetry

Numerical results from the modelling and computer simulation of the magnetic-induced director reorientation dynamics in elongated bipolar nematic droplets are presented in this paper. The magnetic field is applied normally to the droplet axis-of-symmetry direction, which is one possible scenario found in applications of polymer dispersed liquid crystal (PDLC) films. This case has not yet been studied numerically, and its understanding is far from complete. The model is composed of the Leslie-Ericksen and Frank continuum theories and is solved in two dimensions since bipolar nematic droplets exhibit mirror symmetry in certain planes. The numerical results replicate frequently reported experimental observations on the performance of PDLC films. These observations include the ubiquitous exponential increase followed by saturation in light transmittance as the external applied field increases, and the exponential increase (decrease) followed by saturation as time increases in the on (off)-state. Furthermore, in contrast to current understanding for both the on- and off-states, the model predicts that the directors in the centre (surface) region of the droplet exhibit a dead time (no dead time) before reorientation. The numerical results presented in this paper provide a better understanding of the director reorientation dynamics in elongated bipolar nematic droplets; this can be used to optimize the design and performance of devices using PDLC films.     

Computer simulation of elongated bipolar nematic droplets II. External field aligned normal to the droplet axis of symmetry

Numerical results from the modelling and computer simulation of the magnetic-induced director reorientation dynamics in elongated bipolar nematic droplets are presented in this paper. The magnetic field is applied normally to the droplet axis-of-symmetry direction, which is one possible scenario found in applications of polymer dispersed liquid crystal (PDLC) films. This case has not yet been studied numerically, and its understanding is far from complete. The model is composed of the Leslie-Ericksen and Frank continuum theories and is solved in two dimensions since bipolar nematic droplets exhibit mirror symmetry in certain planes. The numerical results replicate frequently reported experimental observations on the performance of PDLC films. These observations include the ubiquitous exponential increase followed by saturation in light transmittance as the external applied field increases, and the exponential increase (decrease) followed by saturation as time increases in the on (off)-state. Furthermore, in contrast to current understanding for both the on- and off-states, the model predicts that the directors in the centre (surface) region of the droplet exhibit a dead time (no dead time) before reorientation. The numerical results presented in this paper provide a better understanding of the director reorientation dynamics in elongated bipolar nematic droplets; this can be used to optimize the design and performance of devices using PDLC films.     

Texture formation under phase ordering and phase separation in polymer-liquid crystal mixtures

Computational modeling of texture formation in coupled phase separation-phase ordering processes in polymer/liquid crystal mixtures is performed using a unified model based on the nematic tensor order parameter and gradient orientation elasticity. The computational methods are able to resolve defect nucleation, defect-defect interactions, and defect-particle interactions, as well as global and local morphological features in the concentration and order parameter spatiotemporal behavior. Biphasic structures corresponding to polymer dispersed liquid crystals (PDLCs), crystalline filled nematic (CFNs), and random filled nematics (RFNs) are captured and analyzed using liquid crystal defect physics and structure factors. Under spinodal decomposition due to concentration fluctuations, the PDLC structure emerges, and the nucleation and repulsive interaction of defects within nematic droplets leads to bipolar nematic droplets. Under spinodal decomposition due to ordering fluctuations, the CFNs structure emerges, and the stable polymer droplet crystal is pinned by a lattice of topological defects. For intermediate cases, where the mixture is unstable to both concentration and nematic order fluctuations, the RFN structure emerges, and polymer droplets and fibrils are pinned by a defect network, whose density increases with the curvature of the polymer-liquid crystal interface. The simulations provide an information of the role of topological defects on phase separation-phase ordering processes in polymer-liquid crystal mixtures.     

The effect of voltage controlled orientation order of liquid crystals in non-acrylic polymer dispersed liquid crystals films

The nematic liquid crystals (LCs) are randomly dispersed material with random orientation order in polymer dispersed liquid crystal (PDLC) films. The LCs change their orientation from random to vertical as electric field is applied. This transformation of orientation order of nematic liquid crystals in the PDLC films is controlled by many factors operating simultaneously. For instance, some factors like the internal forces of attractions among the neighboring LC molecules, anchoring with polymeric matrix, ITO glass boundaries, and chemical structures of the materials are less studied. The learning of extent of vertical orientation of liquid crystal droplets in an electric field is essential to attain optimum electro optical properties of PDLCs. In this finding, bipolar and radial LCs droplets with random orientation have been observed in non-acrylic polymeric media. It is learned that with small increase of contents of external material, the extent of vertical orientation has been varied intensely. The extent of vertical orientation of LCs molecules increases as the contents of external non-acrylic polymeric material decreased. For this study, the orientations of LCs with respect to material type/contents, external applied force, and restoration of electric filed as hysteresis have been studied in details.     

Proton N.M.R. lineshape in nematic microdroplets

The dependence of the proton N.M.R. absorption spectrum on nematic director configuration and molecular self-diffusion in nematic submicrondroplets is analysed. The lineshape is evaluated numerically for radial and bipolar director configuration. The motional averaging is taken into account by means of a numerical simulation of the molecular diffusion which induces slow molecular reorientations due to non-uniform orientational ordering in the droplet. This diffusion process strongly affects the absorption spectra of the radial configuration, whereas spectra of the bipolar configuration are only slightly influenced. The possibility of determinating the submicrometre nematic droplet structures using the proton N.M.R. lineshape is discussed.     

Proton N.M.R. lineshape in nematic microdroplets

The dependence of the proton N.M.R. absorption spectrum on nematic director configuration and molecular self-diffusion in nematic submicrondroplets is analysed. The lineshape is evaluated numerically for radial and bipolar director configuration. The motional averaging is taken into account by means of a numerical simulation of the molecular diffusion which induces slow molecular reorientations due to non-uniform orientational ordering in the droplet. This diffusion process strongly affects the absorption spectra of the radial configuration, whereas spectra of the bipolar configuration are only slightly influenced. The possibility of determinating the submicrometre nematic droplet structures using the proton N.M.R. lineshape is discussed.     

Molecular dynamics simulation of polymer dispersed liquid crystal droplets under competing boundary conditions

This paper is devoted to the molecular dynamics simulation of structural organization inside a polydispersed liquid crystal (LC) droplet under competing boundary conditions. The droplet is assumed to be placed at the liquid crystal interface between two different regions of the solid polymer matrix, which accordingly separates the droplet into two hemispheres: the first of these is under radial boundary conditions; the second hemisphere is under bipolar boundary conditions. The droplet is considered as a jagged sphere filled with LC molecules, modelled as classical spins (unit vectors), whose centres of mass are associated with sites of a cubic lattice inside the cavity. The orienting action of the polymer matrix, and hence the resulting boundary conditions, are modelled by the interaction between the internal LC molecules (possessing only orientational degrees of freedom), and those of a delimiting surface layer (a jagged spherical shell), whose orientations are fixed, radial or bipolar, respectively. All interactions are modelled by the short range McMillan pair potential. The molecular orientation inside the LC droplet has been determined for various anchoring strengths of the interaction between internal spins and boundary layers. We have investigated the structure of the spherical defect resulting in the central region of the droplet, as well as of the boojum ‐ like defects existing near the poles of the droplet. It has been found that a change of relative radial and bipolar anchoring strengths can affect both central and boojum ‐ like defects. The effect of an external field on the molecular orientation inside the droplet has also been investigated. It has been found that a sufficiently strong external field increases the radius of the spherical defect placed in the central region of the droplet.     

Molecular dynamics simulation of polymer dispersed liquid crystal droplets under competing boundary conditions

This paper is devoted to the molecular dynamics simulation of structural organization inside a polydispersed liquid crystal (LC) droplet under competing boundary conditions. The droplet is assumed to be placed at the liquid crystal interface between two different regions of the solid polymer matrix, which accordingly separates the droplet into two hemispheres: the first of these is under radial boundary conditions; the second hemisphere is under bipolar boundary conditions. The droplet is considered as a jagged sphere filled with LC molecules, modelled as classical spins (unit vectors), whose centres of mass are associated with sites of a cubic lattice inside the cavity. The orienting action of the polymer matrix, and hence the resulting boundary conditions, are modelled by the interaction between the internal LC molecules (possessing only orientational degrees of freedom), and those of a delimiting surface layer (a jagged spherical shell), whose orientations are fixed, radial or bipolar, respectively. All interactions are modelled by the short range McMillan pair potential. The molecular orientation inside the LC droplet has been determined for various anchoring strengths of the interaction between internal spins and boundary layers. We have investigated the structure of the spherical defect resulting in the central region of the droplet, as well as of the boojum - like defects existing near the poles of the droplet. It has been found that a change of relative radial and bipolar anchoring strengths can affect both central and boojum - like defects. The effect of an external field on the molecular orientation inside the droplet has also been investigated. It has been found that a sufficiently strong external field increases the radius of the spherical defect placed in the central region of the droplet.     

Director configurations in nematic droplets with tilted surface anchoring

The polymer dispersed nematic liquid crystal (LC) with the tilted surface anchoring has been studied. The droplet orientational structures with two point surface defects – boojums and the surface ring defect – are formed within the films. The director tilt angle α = 40° ± 4° at the droplet interface and LC surface anchoring strength W_s ~ 10^–6 (J m^?2) have been estimated. The bipolar axes within the studied droplets of oblate ellipsoidal form can be randomly oriented are oriented randomly relatively to the ellipsoid axes as opposed to the droplets with homeotropic and tangential anchoring.     

Numerical simulation of droplet coalescence behavior in gas phase under the coupling of electric field and flow field

The coalescence behavior of droplets in an electric field belongs to the important research contents of electrohydrodynamics. Based on the phase field method of the Cahn–Hilliard equation, the electric field and the flow field are coupled to establish the numerical model of twin droplet coalescence in a coupled field. The effects of flow rate, electric field strength, droplet diameter, and interfacial tension on the coalescence behavior of droplets during the coalescence process were investigated. The results show that the dynamic behavior of the droplets is divided into coalescence, after coalescence rupture, and no coalescence under the coupling of electric field and flow field. The proper increase of the electric field strength will accelerate the coalescence of the droplets, and the high electric field strength causes the droplets to burst after coalescence. Excessive flow rates make droplets less prone to coalescence. Under the coupling field, the larger the droplet interface tension, the smaller the droplet diameter, the smaller the flow rate, and the shorter the droplet coalescence time. The results provide a theoretical basis for the application of electrostatic coalescence in gas–liquid separation technology.     

Computer simulation of elongated bipolar nematic droplets 1. External field aligned parallel to the droplet axis of symmetry

The magnetically-induced transient nematic director reorientation dynamics, confined in elongated bipolar droplets, is studied in this paper. Numerical results are obtained by solving the Leslie-Ericksen continuum theory in ellipses. The aspect ratio is varied to determine the effect of droplet shape on director reorientation dynamics. The magnetic field is restricted to the droplet axis of symmetry direction, which has not yet been studied but is fundamentally important in polymer dispersed liquid crystal (PDLC) film operation. The numerical results replicate frequently-reported experimental observations on the performance of PDLC films. These observations include the familiar exponential increases followed by saturation in light transmittance as the external applied field increases and the exponential increase (decrease) followed by saturation as time increases in the on- (off-) state. In addition, the experimental observation that switching field strength increases while decay time decreases as the droplet becomes more elongated, are also exhibited by the numerical results.     

Computer simulation of elongated bipolar nematic droplets 1. External field aligned parallel to the droplet axis of symmetry

The magnetically-induced transient nematic director reorientation dynamics, confined in elongated bipolar droplets, is studied in this paper. Numerical results are obtained by solving the Leslie-Ericksen continuum theory in ellipses. The aspect ratio is varied to determine the effect of droplet shape on director reorientation dynamics. The magnetic field is restricted to the droplet axis of symmetry direction, which has not yet been studied but is fundamentally important in polymer dispersed liquid crystal (PDLC) film operation. The numerical results replicate frequently-reported experimental observations on the performance of PDLC films. These observations include the familiar exponential increases followed by saturation in light transmittance as the external applied field increases and the exponential increase (decrease) followed by saturation as time increases in the on- (off-) state. In addition, the experimental observation that switching field strength increases while decay time decreases as the droplet becomes more elongated, are also exhibited by the numerical results.     

A case of mistaken identity: spontaneous formation of twisted bipolar droplets from achiral nematic materials

New observations of the structure of nematic droplets containing material with a low K33/K11 ratio indicate that these droplets adopt a twisted bipolar, rather than concentric, orientation. The data supporting this new interpretation are presented and discussed. These data provide verification for a prediction of Williams regarding the relative stabilities of bipolar, twisted bipolar, and concentric droplets. Nematic liquid crystals confined to spheres and cylinders form a general class of structures in which achiral molecules can form chiral structures due to the combination of external boundary conditions and the minimization of the free energy near defect structures.