Nematic Droplets Research Articles

Self-assembly of fractal liquid crystal colloids

Nematic liquid crystals are anisotropic fluids that self-assemble into vector fields, which are governed by geometrical and topological laws. Consequently, particulate or droplet inclusions self-assemble in nematic domains through a balance of topological defects. Here, we use double emulsions of water droplets inside radial nematic liquid crystal droplets to form various structures, ranging from linear chains to three-dimensional fractal structures. The system is modeled as a formation of satellite droplets, distributed around a larger, central core droplet and we extend the problem to explain the formation of fractal structures. We show that a distribution of droplet sizes plays a key role in determining the symmetry properties of the resulting geometric structures. The results are relevant to a variety of inclusions, ranging from colloids suspensions to multi-emulsion systems. Such systems have potential applications for novel switchable photonic structures as well as providing wider insights into the packing of self-assembled structures.

A preliminary study on liquid crystalline epoxy curatives from natural abietic acid

Abietic acid was used for preparing two thermotropic liquid crystalline curing agents with different mesogens (LCC1 and LCC2). The chemical structure of the synthesized LCCs was investigated by FTIR and 1HNMR spectroscopic techniques. Their LC phase transition temperatures and texture were identified by differential scanning calorimeter (DSC) and polarized optical microscope (POM), respectively. Both LCCs show nematic droplet texture -as revealed by POM micrographs- but different isotropization temperatures. Eventually, they were used for producing epoxy coatings using a multifunctional abietic acid based epoxy resin; prepared in our previous study. Cured coats were evaluated by standard coating testing and compared with a commercial curing agent. Dynamic mechanical (DMA) and thermogravimetric (TGA) analyses show that the fully bio-based cured epoxies by either LCCs possess high glass transition temperature (Tg), high modulus (G′) and improved thermal stability.

A Finite Element Method for a Phase Field Model of Nematic Liquid Crystal Droplets

We develop a novel finite element method for a phase field model of nematic liquid crystal droplets. The continuous model considers a free energy comprised of three components: the Ericksen's energy for liquid crystals, the Cahn-Hilliard energy representing the interfacial energy of the droplet, and a weak anchoring energy representing the interaction of the liquid crystal molecules with the surface tension on the interface (i.e. anisotropic surface tension). Applications of the model are for finding minimizers of the free energy and exploring gradient flow dynamics. We present a finite element method that utilizes a special discretization of the liquid crystal elastic energy, as well as mass-lumping to discretize the coupling terms for the anisotropic surface tension part. Next, we present a discrete gradient flow method and show that it is monotone energy decreasing. Furthermore, we show that global discrete energy minimizers $\Gamma$-converge to global minimizers of the continuous energy. We conclude with numerical experiments illustrating different gradient flow dynamics, including droplet coalescence and break-up.

Orientational instability and spontaneous rotation of active nematic droplets.

In experiments, an individual chemically-active liquid crystal (LC) droplet submerged in the bulk of a surfactant solution may self-propel along a straight, helical, or random trajectory. In this paper, we develop a minimal model capturing all three types of self-propulsion trajectories of a drop in the case of a nematic LC with homeotropic anchoring at the LC-fluid interface. We emulate the director field within the drop by a single preferred polarization vector that is subject of two reorientation mechanisms, namely, the internal flow-induced displacement of the hedgehog defect and the droplet's rotation. Within this reduced-order model, the coupling between the nematic ordering of the drop and the surfactant transport is represented by variations of the droplet's interfacial properties with nematic polarization. Our analysis reveals that a novel mode of orientational instability emerges from the competition of the two reorientation mechanisms and is characterized by a spontaneous rotation of the self-propelling drop responsible for helical self-propulsion trajectories. In turn, we also show that random trajectories in isotropic and nematic drops alike stem from the advection-driven transition to chaos. The succession of the different propulsion modes is consistent with experimentally-reported transitions in the shape of droplet trajectories as the drop size is varied.

Spontaneous Sequestration of Proteins into Liquid Crystalline Microdroplets

AbstractIn this work a facile method for fabricating protein‐sequestered liquid crystal (LC) emulsion droplets based on the uptake of surface‐engineered protein–polymer surfactant (PS) core–shell bioconjugates is described. Uptake of myoglobin‐PS, bovine serum albumin‐PS, Zn‐porphyrin myoglobin‐PS, horseradish peroxidase‐PS, and glucose oxidase‐PS occurs without structural or functional degradation, and gives rise to sequestration within the interior or at the surface of 4‐cyano‐4′‐pentylbiphenyl (5CB) nematic droplets depending on the surface modification of the protein‐PS bioconjugates. Differences in uptake behavior are used to achieve the spontaneous positional assembly of multiple proteins in the LC phase, and the use of spatially separated glucose oxidase‐PS and horseradish peroxidase‐PS bioconjugates is demonstrated to produce 5CB based droplets capable of housing an enzyme cascade reaction. This method opens a pathway for the development of bioactive liquid crystal droplets and can have potential applications in the optical sensing of biomolecular substrates.

The structural transformations of polymer dispersed liquid crystalline films in an external deformation field

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. The experiments were performed on PDLC films with the bipolar nematic director configuration in the droplets, where the film transmittance, microscopic structure, and birefringence the polymer matrix were studied. It is shown that the orientation 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.

Study the effect of different terminal substituents on the liquid crystalline properties of the new synthesized 1,2,4-oxadiazole derivatives

A new series of 4-(5-(p-tolyl)-1,2,4-oxadiazol-3-yl)phenyl substitutedbenzoate, (IIIx) incorporating ester linkage with various substituted groups in various positions on the phenyl ring were synthesized and investigated by FT-IR and 1H-NMR spectroscopies in addition to the CHN analysis, their mesophase behaviors were examined by optical polarized microscopy (OPM) and differential scanning calorimetry (DSC). Only the derivatives that have terminal substituents in the para position showed liquid crystalline properties with a wide range of nematic phase and various textures (Schlieren, thread-like, marble and nematic droplets) both in heating and/or cooling runs. The relationship between nature and position of the substituent groups, central core and ester linkage of these derivatives with their liquid crystalline properties were studied and the results were analyzed, discussed and compared with other mesogenic materials have a chemical composition similar to the compounds prepared in this study.

Light-Responsive Microstructures in Droplets of the Twist-Bend Nematic Phase.

We report on the structure and optical manipulation of the director configurations in emulsions of liquid-crystalline droplets of a compound exhibiting the nematic (N) and the twist-bend nematic (NTB) phases. We demonstrate a decrease in the ratio of the bent elastic constant K33 to the splay constant K11 by nearly 2 orders of magnitude with decreasing temperature in the N phase. The director structures in liquid-crystal droplets doped with a photoswitchable surfactant without and under ultraviolet (UV) light are discussed in light of the strong elastic anisotropy of the investigated compound. We also compare our findings with the results obtained in doped nematic droplets of a conventional 4-cyano-4'-pentylbiphenyl (5CB) liquid crystal. The dynamics of droplets in the NTB phase by UV light irradiation are also studied using polarizing and confocal microscopies.

Azodendrimers as a photoactive interface for liquid crystals

ABSTRACTDendrimers with azo moieties in their peripherals (azodendrimers), which we developed, are useful as a photoactive interface for liquid crystal (LC) systems. The azodendrimer spontaneously adsorbs on a variety of interfaces, so that we can obtain photoactive surfaces without pretreatment only by introducing a small amount of the azodendrimers in LCs. We first describe our previous studies applied for a variety of LC systems such as sandwich cells, LC droplets and shells in a solvent and microparticles in LCs. Some unpublished results on nematic droplets of bent-shaped molecules demonstrate current developments in the research of the interactions between the photoswitchable dendrimer and complex liquid crystalline structures.

Self-propelled motion switching in nematic liquid crystal droplets in aqueous surfactant solutions.

The self-propelled motions of micron-sized nematic liquid crystal droplets in an aqueous surfactant solution have been studied by tracking individual droplets over long time periods. Switching between self-propelled modes is observed as the droplet size decreases at a nearly constant dissolution rate: from random to helical and then straight motion. The velocity of the droplet decreases with its size for straight and helical motions but is independent of size for random motion. The switching between helical and straight motions is found to be governed by the self-propelled velocity, and is confirmed by experiments at various surfactant concentrations. The helical motion appears along with a shifting of a point defect from the self-propelled direction of the droplet. The critical velocity for this shift of the defect position is found to be related with the Ericksen number, which is defined by the ratio of the viscous and elastic stresses. In a thin cell whose thickness is smaller than that of the initial droplet size, the droplets show more complex trajectories, including "figure-8s" and zigzags. The appearance of those characteristic motions is attributed to autochemotaxis of the droplet.

Cascade “Melting” of a Linear Disclination in Chiral Nematic Droplets

A new structure of the elastic field of a chiral nematic liquid crystal in a cylindrical capillary with a singularity on the axis of the capillary in the form of a “melted” helical disclination has been obtained experimentally. It has been shown theoretically that the number of cylindrical surfaces, cascades at which a jump of the direction of the director orientation field n by π occurs, can be arbitrary. The particular number of cascades depends on the radius R of the cylinder and boundary conditions. An expression for the free energy of the elastic field has been obtained in the one-constant approximation of the continuum theory of liquid crystals.

Formation of radial aligned and uniform nematic liquid crystal droplets via drop-on-demand inkjet printing into a partially-wet polymer layer

This paper investigates the drop-on-demand inkjet printing of a nematic liquid crystal (LC) onto a variety of substrates. Achieving both a well-defined droplet boundary and uniformity of the LC director in printed droplets can be challenging when traditional alignment surfaces are employed. Despite the increasing popularity of inkjet printing LCs, the mechanisms that are involved during the deposition process such as drop impact, wetting and spreading have received very little attention, in the way of experiments, as viable routes for promoting alignment of the resultant LC droplets. In this work, radial alignment of the director and uniformity of the droplet boundary are achieved in combination via the use of a partially-wet polymer substrate, which makes use of the forces and flow generated during droplet impact and subsequent wetting process. Our findings could have important consequences for future LC inkjet applications, including the development of smart inks, printable sensors and lasers.

Optical orientation of nematic liquid crystal droplets via photoisomerization of an azodendrimer dopant

Two sequential transformations of the orientational structure in nematic liquid crystal droplets containing a dendrimer additive (nanosized macromolecules with light-absorbing azobenzene terminal moieties) under light irradiation in the UV–blue spectral range were investigated. The origin of these transitions is in the change of the boundary conditions due to photoisomerization of the dendrimer adsorbed onto the liquid crystal–glycerol interface. It was shown that the photoisomerization processes of dendrimer molecules in a liquid crystal are accompanied by a spatial rearrangement of their azobenzene moieties, which is the key point in the explanation of the observed effects.

On the Defect Structure of Biaxial Nematic Droplets

We present a detailed Monte Carlo study of the effects of molecular biaxiality on the defect created at the centre of a nematic droplet with radial anchoring at the surface. We have studied a lattice model based on a dispersive potential for biaxial mesogens [Luckhurst et al., Mol. Phys. 30, 1345 (1975)] to investigate how increasing the biaxiality influences the molecular organisation inside the confined system. The results are compared with those obtained from a continuum theory approach. We find from both approaches that the defect core size increases by increasing the molecular biaxiality, hinting at a non universal behaviour previously not reported.

Mechanisms of Rotational Dynamics of Chiral Liquid Crystal Droplets in an Electric Field

The dynamics of the orientational structure of chiral nematic (CN) droplets in an isotropic medium in dc and ac electric fields is investigated by the polarized light microscopy technique. It is shown theoretically that the dynamics of rotational processes in these kinds of systems is determined by electroconvective processes developing due to the flexoelectric polarization associated with the initial configuration of the director field in droplets. It is established experimentally that the linear and quadratic regions of dependence of the rotational velocity of droplets on the electric field strength are explained by the above-mentioned mechanisms. Numerical simulation on the basis of the approach developed gives good agreement with experimental data.

Photoinduced orientational structures of nematic liquid crystal droplets in contact with polyimide coated surface

Orientational structure transitions due to a photoinduced surface anchoring modulation were achieved and investigated in the case of NLC microdroplets placed between glycerol and solid surface. The boundary conditions at NLC-glycerol varied under the light action on the dendromer dopant containing azobenzene terminal fragments. At the same time, the NLC had a strong planar anchoring with the polyimide treated substrate. Numerical simulation of the observed orientational structures in NLC microdroplets were carried out.

Photogalvanic/pyroelectric power conversion and self-pulsing of electro-wetting of LC droplet on lithium niobate (LN)-crystal

Photogalvanic current and voltage photoinduced by CW laser illumination of Fe-doped LN reveals quasi-periodic pulsing due to microplasmas discharges. This transformation of constant external influence into periodic response is interesting as example of self-excited systems, resembling pulsations of biological systems. Microplasmas discharges are also interesting for applications in various fields, such as surface treatment, sterilization, water splitting. We analyze self-pulsing of photogalvanic current visualized by dynamic electrowetting in a simple LC-cell, formed by a nematic LC droplet placed on Fe-doped LN crystal. In modeling contributions from both photogalvanic (bulk photovoltaic) and pyroelectric (charging by heating-cooling) effect are included. The analysis of dynamic regime reveals that a current pulse (in microsecond range) has asymmetric shape with extremely sharp rise with longer decay time. Reflected interference patterned was modulated by the dynamic electro-wetting effect. For high light intensity power conversion efficiency is enhanced due to nonlinear photogalvanic effect and microplasmas discharges.

Can elastic constants and surface alignment be obtained from polarized microscopy images of nematic droplets? A Monte Carlo study

We present a Monte Carlo study of the effects of elastic anisotropy on the textures of nematic droplets with different boundary conditions. Simulated polarized optical microscopy (POM) images are analysed for uniaxial nematics with radial, bipolar and toroidal boundaries and for numerous combinations of the splay, twist and bend elastic constant values. An atlas of simulated textures is presented as an aid to understanding new experimental results.

Chiral symmetry-breaking dynamics in the phase transformation of nematic droplets

ABSTRACTDynamic simulations of the isotropic–nematic phase transformation of liquid crystal droplets with homeotropic surface anchoring are found to predict chiral symmetry-breaking dynamics. These observations occur when using material parameters for pentyl-cyanobiphenyl (5CB) but not with the single elastic constant approximation for this material, which is frequently used in simulations. The twisting dynamic process occurs during the relaxation of the domain from an unstable radial texture to a stable uniform texture and involves simultaneous defect loop motion and twisting of the bulk nematic texture.

Electrically induced structure transition in nematic liquid crystal droplets with conical boundary conditions.

Polymer-dispersed liquid crystal composites have been a focus of study for a long time for their unique electro-optical properties and manufacturing by "bottom-up" techniques at large scales. In this paper, nematic liquid crystal oblate droplets with conical boundary conditions (CBCs) under the action of electric field were studied by computer simulations and polarized optical microscopy. Droplets with CBCs were shown to prefer an axial-bipolar structure, which combines a pair of boojums and circular disclinations on a surface. In contrast to droplets with degenerate planar boundary conditions (PBCs), hybridization of the two structure types in droplets with CBCs leads to a two-minima energy profile, resulting in an abrupt structure transition and bistable behavior of the system. The nature of the low-energy barrier in droplets with CBCs makes it highly sensitive to external stimuli, such as electric or magnetic fields, temperature, and light. In particular, the value of the electric field of the structure reorientation in droplets with CBCs was found to be a few times smaller than the one for droplets with PBCs, and the droplet state remained stable after switching off the voltage.