Nematic Emulsions Research Articles

Probing cytoplasmic streaming using liquid crystal droplets

ABSTRACT Giant cells of characean algae provide an excellent platform for testing different techniques to explore cytoplasmic streaming, electrophysiological behaviour, intercellular communication, and drug delivery methods. In this study, we successfully applied nematic emulsions based on 5CB to visualise the shuttle flow in cells’ cytoplasm. Additionally, we demonstrate the compatibility of the liquid crystal material with the intracellular environment.

Transformation between elastic dipoles, quadrupoles, octupoles, and hexadecapoles driven by surfactant self-assembly in nematic emulsion.

Emulsions comprising isotropic fluid drops within a nematic host are of interest for applications ranging from biodetection to smart windows, which rely on changes of molecular alignment structures around the drops in response to chemical, thermal, electric, and other stimuli. We show that absorption or desorption of trace amounts of common surfactants can drive continuous transformations of elastic multipoles induced by the droplets within the uniformly aligned nematic host. Out-of-equilibrium dynamics of director structures emerge from a controlled self-assembly or desorption of different surfactants at the drop-nematic interfaces, with ensuing forward and reverse transformations between elastic dipoles, quadrupoles, octupoles, and hexadecapoles. We characterize intertransformations of droplet-induced surface and bulk defects, probe elastic pair interactions, and discuss emergent prospects for fundamental science and applications of the reconfigurable nematic emulsions.

Linear and nonlinear rheology of oil in liquid crystal emulsions

While the majority of experimental and numerical studies focus on yield stress fluids with short-range repulsive interactions, the effect of long-range interaction is rarely known. In this work, we studied the linear and nonlinear rheological behavior of a model oil in nematic liquid crystal emulsions, which exhibit long-range droplet-droplet interaction. The characteristic of long-range interaction, attractive at large droplet separation and repulsive at a small surface distance, was inferred from the morphology and thixotropy of emulsions. We suggested a model accounting for the plateau modulus purely due to the long-range repulsive interaction. We further illustrated that neither the yield stress nor the relaxation time followed a simple power-law scaling with respect to the distance to jamming. The rescaled steady and dynamic flow curves could not collapse on master curves above jamming transition when the long-range and short-range repulsive interactions contribute simultaneously.

Active nematic emulsions.

The formation of emulsions from multiple immiscible fluids is governed by classical concepts such as surface tension, differential chemical affinity and viscosity, and the action of surface-active agents. Much less is known about emulsification when one of the components is active and thus inherently not constrained by the laws of thermodynamic equilibrium. We demonstrate one such realization consisting in the encapsulation of an active liquid crystal (LC)-like gel, based on microtubules and kinesin molecular motors, into a thermotropic LC. These active nematic emulsions exhibit a variety of dynamic behaviors that arise from the cross-talk between topological defects separately residing in the active and passive components. Using numerical simulations, we show a feedback mechanism by which active flows continuously drive the passive defects that, in response, resolve the otherwise degenerated trajectories of the active defects. Our experiments show that the choice of surfactant, which stabilizes the active/passive interface, allows tuning the regularity of the self-sustained dynamic events. The hybrid active-passive system demonstrated here provides new perspectives for dynamic self-assembly driven by an active material but regulated by the equilibrium properties of the passive component.

2C05 液晶エマルションにおけるマイクロドロップレットの光組織化(フォトニクス・光デバイス)

2C05 液晶エマルションにおけるマイクロドロップレットの光組織化(フォトニクス・光デバイス)

Shear dynamics of an inverted nematic emulsion.

Here we study theoretically the dynamics of a 2D and a 3D isotropic droplet in a nematic liquid crystal under a shear flow. We find a large repertoire of possible nonequilibrium steady states as a function of the shear rate and of the anchoring of the nematic director field at the droplet surface. We first discuss homeotropic anchoring. For weak anchoring, we recover the typical behaviour of a sheared isotropic droplet in a binary fluid, which rotates, stretches and can be broken by the applied flow. For intermediate anchoring, new possibilities arise due to elastic effects in the nematic fluid. We find that in this regime the 2D droplet can tilt and move in the flow, or tumble incessantly at the centre of the channel. For sufficiently strong anchoring, finally, one or both of the topological defects which form close to the surface of the isotropic droplet in equilibrium detach from it and get dragged deep into the nematic state by the flow. In 3D, instead, the Saturn ring associated with the normal anchoring disclination line can be deformed and shifted downstream by the flow, but remains always localized in the proximity of the droplet, at least for the parameter range we explored. Tangential anchoring in 2D leads to a different dynamic response, as the boojum defects characteristic of this situation can unbind from the droplet under a weaker shear with respect to the normal anchoring case. Our results should stimulate further experiments with inverted liquid crystal emulsions under shear, as most of the predictions can be testable in principle by monitoring the evolution of liquid crystalline orientation patterns or by tracking the position and shape of the droplet over time.

Coherent random laser fluid of nematic liquid crystal emulsions

Reported here is a coherent (resonant) random laser fluid of nematic liquid crystal (LC) emulsions. It is a low-viscosity fluid like a usual liquid. The coherent laser is developed because of the high LC droplet density (about 32%) and the high refractive index of the LC. The optical loop forms among the LC droplets and its length is evaluated to be approximately 150 µm. The LC emulsion is useful for the investigation of the mechanism of the random laser because it is a fluid and can easily be used to change the scatterer density.

Bivalent defect configurations in inhomogeneous nematic shells

We present a theoretical study of the director fields and energetics of nematic liquid crystal shells with two pairs of surface defects. The pairs of defects can undergo abrupt transitions from a configuration of maximum separation to a state in which the defects are confined to the thinner hemisphere. We construct a phase diagram that maps out the stability and coexistence of these two configurations as a function of shell thickness and thickness inhomogeneity. Our results compare favourably with the experimentally observed transitions in nematic double emulsion droplets and explain their hysteretic character.

Colloid-wall interaction in a nematic liquid crystal: The mirror-image method of colloidal nematostatics

The new area of nematic colloidal systems (or nematic emulsions) has been greatly guided by the fruitful analogy between the colloidal nematostatics and electrostatics. The elastic charge density representation of the colloidal nematostatics [V. M. Pergamenshchik and V. O. Uzunova, Eur. Phys. J. E 23, 161 (2007); Phys. Rev. E 76, 011707 (2007)] develops this analogy at the level of charge density and Coulomb interaction. It shows, however, that the colloidal nematostatics in three dimensions substantially differs from the electrostatics both in its mathematical structure and physical implications: the elastic charge and multipoles are dyads; similar charges attract while opposite charges repel each other, and so on. In this paper we consider the interaction between an elastic charge and elastic dipole with a nematic surface (wall) at which the director alignment is fixed. Using the mirror image method of electrostatics as a guiding idea, we develop the mirror image method in the nematostatics for arbitrary director tilt at the wall. A wall is shown to induce a repulsive 1R{4} force on the elastic dipole which, in general, is accompanied by its reorientation. External torque on the colloid induces an elastic charge therein and triggers switching to the 1R{2} repulsion. The dyadic nature of an elastic dipole is shown to be essential: a particle-wall interaction potential cannot be obtained in phenomenological theories with a single component dipole. In the introductory sections we discuss connection between the director-mediated interaction in two and three dimensions and the electrostatic interaction and consider different symmetries of elastic dipoles. Conservation of the torque components exerted upon colloids is shown to play the role of Gauss' theorem and determines the elastic charge dyad.

Coulomb-like interaction in nematic emulsions induced by external torques exerted on the colloids

An external mechanical torque on colloids immersed in a nematic liquid crystal can induce a Coulomb-like 1/r interaction between them [Lev and Tomchuk, Phys. Rev. E 59, 591 (1999); Lev, ibid. 65, 021709 (2002)]. In this paper we show that the director-mediated Coulomb-like interaction of two colloids is determined by the vectors Gamma perpendicular (1) and Gamma perpendicular (2) of the transverse external torques exerted upon these colloids. We derive the 1/r potential in which the scalar product -(Gamma perpendicular (1) x Gamma perpendicular (2)) of the two torques plays the role of the product of two electrostatic charges. The 1/r interaction is attractive for (Gamma perpendicular (1) x Gamma perpendicular (2))>0 and repulsive for (Gamma perpendicular (1) x Gamma perpendicular (2))<0 ("parallel torques" attract whereas "antiparallel torques" repel each other). The vector of transverse torque determines the two-component "elastic charge" (dyad), which is illustrated by the 1/r2 and 1/r3 terms in the elastic energy (the elastic analogs of the monopole-dipole and dipole-dipole interactions). The general status of the pairwise approach to nematic emulsions is considered in terms of the elastic charge density.

The electro‐optical response of nematic emulsions

Switchable nematic emulsions are micron‐sized droplets of nematic liquid crystal, floating in isotropic fluid matrices. Such droplets can be switched from an opaque (off) to a transparent (on) state by application of very low electric fields. It is known that the electro‐optical properties of liquid crystal dispersions are affected by several parameters, including the liquid crystal loading. The electro‐optical response of nematic emulsions has been investigated as a function of liquid crystal weight percentage. Almost transparent films with a reduced contrast ratio are obtained with lower liquid crystal contents. A macroscopic phase separation is observed when liquid crystal content exceeds 45 wt %. On the contrary, large contrast ratios and very low switching fields can be obtained if liquid crystal ranges from 25 to 35 wt %. Consequently, nematic emulsions prepared in this liquid crystal range can be used as promising systems for electro‐optical applications. In addition to technological developments, these results can help computational and basic studies of phase separation in novel multiphase liquid crystalline materials.

Elastic charge density representation of the interaction via the nematic director field

The interaction between particle-like sources of the nematic director distortions (e.g., colloids, point defects, macromolecules in nematic emulsions) allows for a useful analogy with the electrostatic multipole interaction between charged bodies. In this paper we develop this analogy to the level corresponding to the charge density and consider the general status of the pairwise approach to the nematic emulsions with finite-size colloids. It is shown that the elastic analog of the surface electric charge density is represented by the two transverse director components on the surface imposing the director distortions. The elastic multipoles of a particle are expressed as integrals over the charge density distribution on this surface. Because of the difference between the scalar electrostatics and vector nematostatics, the number of elastic multipoles of each order is doubled compared to that in the electrostatics: there are two elastic charges, two vectors of dipole moments, two quadrupolar tensors, and so on. The two-component elastic charge is expressed via the vector of external mechanical torque applied on the particle. As a result, the elastic Coulomb-like coupling between two particles is found to be proportional to the scalar product of the two external torques and does not directly depend on the particles' form and anchoring. The real-space Green function method is used to develop the pairwise approach to nematic emulsions and determine its form and restrictions. The pairwise potentials are obtained in the familiar form, but, in contrast to the electrostatics, they describe the interaction between pairs (dyads) of the elastic multipole moments. The multipole moments are shown to be uniquely determined by the single-particle director field, unperturbed by other particles. The pairwise approximation is applicable only in the leading order in the small ratio particle size-to-interparticle distance as the next order contains irreducible three-body terms.

Photoelectrochromic switchable nematic emulsions

Switchable nematic emulsions are composite systems formed by liquid-crystal droplets dispersed in a fluid, homogeneous, monomer matrix. These systems can be switched from an opaque to a transparent state by application of a suitable alternating electric field. An electrochromic device provides a reversible and visible change in its transmittance as a result of either oxidizing or reducing electrochemical processes. A photochromic device provides a reversible and visible change in its transmittance as the result of a single chemical transformation of the species induced by suitable electromagnetic radiation. These devices have been proven to be useful for a variety of electro-optical applications as switchable windows, electromagnetic shutters, and displays. Our article reports preliminary results on trifunctional devices based on nematic emulsions that host electrochromic and photochromic guest molecules. These trifunctional films are able to modulate the light transmission (with an external alternating electric field) and to change their color (with static field or UV light) by means of suitable electric fields or electromagnetic radiation.

Improvement of response times in electrochromic switchable nematic emulsions

Electrochromic nematic emulsion can be obtained by fast cooling of isotropic mixtures of electrochromic mixture, nematic liquid crystal and monomers. Films obtained with this procedure show very low saturation fields but rather long relaxation times. In this paper we have doped nematic electrochromic emulsions with surfactant agents, in order to obtain a good compromise between low saturation fields and fast relaxation times. The decay time decreases in a significant way and the saturation field is slightly affected by the addition of small amounts of surfactant.

Tailoring the Interfaces between Nematic Liquid Crystal Emulsions and Aqueous Phases via Layer-by-Layer Assembly

We report the assembly of polyelectrolyte multilayer (PEM) films at the interfaces of thermotropic liquid crystal (LC) droplets dispersed in an aqueous phase. Exposure of PEM-coated droplets to surfactant slowed the bipolar-to-radial ordering transition of the LCs by 2 orders of magnitude relative to naked droplets. This shows that PEMs can be used to influence the interactions of analytes with the LC cores of the droplets, allowing tuning of the LC emulsion sensing properties.

Magnetic Field‐Induced Shape Transitions in Multiphase Polymer‐Liquid Crystal Blends

AbstractSummary: This paper presents a computational study of phase separation‐phase ordering‐texturing in blends of polymer coils and rod‐like nematic liquid crystals under the presence of magnetic fields, using an extended version of the Matsuyama‐Evans‐Cates model (Phys. Rev. E 2000, 61, 2977). This work demonstrates that demixing in these blends leads to droplet morphologies with tunable droplet shapes and director textures. In contrast to filled nematics, where solids are suspended in a nematic liquid crystal matrix, demixing in coil‐mesogenic rods blends leads to nematic emulsions, in which the deformable viscoelastic polymer drops are suspended in a nematic matrix. Under strong anchoring conditions, the imposition of a magnetic field leads to a director re‐orientation that due to strong anchoring produces a droplet shape change. Magnetic field‐induced shape transitions in these blends are shown to be second order with a finite critical field threshold that diverges as anchoring strength vanishes. A morphological‐texture diagram summarizes the magnetic field‐anchoring conditions that promote anisotropic shapes. This work presents additional material processing routes to design and control bi‐phasic morphologies in polymer‐liquid crystal blend.Computed morphology phase diagram in terms of magnetic field strength ΛM and anchoring strength. ΛϕQ.magnified imageComputed morphology phase diagram in terms of magnetic field strength ΛM and anchoring strength. ΛϕQ.

Spontaneous Size Selection in Cholesteric and Nematic Emulsions

We study the spontaneous size selection in lyotropic cholesteric (W/O) and thermotropic nematic (O/W) liquid crystal emulsions. The droplet sizes have been characterized by dynamic light scattering, which indicates a narrow monomodal distribution of droplets achieved spontaneously even without emulsion filtration. Anchoring of the director, provided by the chosen surfactant on the interface, may generate a topological defect inside the droplet. Below the critical radius R = K/W, determined by the ratio of Frank elastic and the surface anchoring constants, the effective anchoring strength is weak and droplets are not topologically charged; this allows them to coalesce freely, depleting the size distribution in this range. Large droplets possess a topological charge of +1 and present a high elastic energy barrier for pair coalescence; the resulting size distribution is skewed, with R > R, and effectively frozen.

Colloidal interactions in two-dimensional nematic emulsions

We review theoretical and experimental work on colloidal interactions in two-dimensional (2D) nematic emulsions. We pay particular attention to the effects of (i) the nematic elastic constants, (ii) the size of the colloids, and (iii) the boundary conditions at the particles and the container. We consider the interactions between colloids and fluid (deformable) interfaces and the shape of fluid colloids in smectic-C films.

Electrochromism in switchable nematic emulsions

Switchable nematic emulsions are composite systems formed by liquid-crystal droplets dispersed in a fluid, homogeneous, monomer matrix, which can be turned from an opaque to an optically transparent state by application of a suitable ac electric field. An electrochromic device provides a reversible and visible change in its transmittance and/or reflectance as the result of either oxidation or reduction electrochemical processes. Both devices have been proven to be useful for a variety of electro-optical applications as switchable windows, electromagnetic shutters, and displays. This letter reports preliminary results on a bifunctional device based on a switchable nematic emulsion, which hosts electrochemical reactions. The presence of a liquid-crystal dispersion ensures the switching from a scattering and opaque state to a transmissive and transparent state, while the oxidation–reduction reactions allow a contemporary and independent change in color.

Liquid crystal orientation in elliptic droplets in nematic emulsions

Elliptic droplets of nematic liquid crystal dispersed in a fluid organic monomer were obtained by phase separation from an isotropic mixture consisting of an organic monomer and a nematic liquid crystal contained in a poly(ethylene terephthalate) cell with inner surfaces treated with rubbed polyimide. The elliptic shape is a consequence of the constraint upon droplet growth along the direction perpendicular to the cell surfaces owing to the small thickness. Then, the resulting droplets will have a contact area with the inner surfaces of the cell treated with polyimide, which will impart a planar orientation on the liquid crystal in the droplet. By means of an optical microscope, using a simple pin hole of 5 μ m, we have selected single droplets for a series of samples having different contact areas. By polarized infrared spectroscopy we have also studied the liquid crystal orientation in selected areas of the droplets. We then report the dependence of the order parameter of the liquid crystal on different contact areas with the alignment surface of the cell. The good degree of planar alignment of the liquid crystal in the elliptic droplets allows the use of such a technique for realizing electro-optical films operating in the reverse mode. We report the electro-optical transmission of reverse mode films with different sizes of elliptic droplet.