Cholesteric Liquid Crystal Droplets Research Articles

Optically responsive dry cholesteric liquid crystal marbles

Dry liquid crystal marbles are structures that consist of cholesteric liquid crystal (CLC) droplets prepared by the mixture of chiral-doped thermotropic LCs encapsulated by cellulose nanocrystals (CNCs) that have been dried under ambient conditions. The characterizations revealed that CLC droplets were successfully encapsulated by self-standing CNC shells and responsive to the external gaseous stimulus. The dry LC marbles offer several advantages over previously reported LC-based gas sensors, such as fast response against minor external stimuli, and ease of handling, which make them particularly attractive for practical applications in sensing. We demonstrate the use of these marbles for detecting toluene vapor, a common industrial solvent and pollutant, which we also use to understand the response characteristics. The dry CLC marbles exhibit a significant response to toluene vapor with a detection limit below 500 ppm, attributed to the change of pitch size of the helical structure of CLC droplets induced by the toluene vapor. The CNC-capsulated CLC droplets were stable in emulsion for up to two weeks, and their dried form exhibited a sensitive response upon toluene exposure. The real-time experiments revealed that the LC marbles can be used multiple times without a significant loss of sensitivity, where 90 % of the maximum response was observed at 13.3 ± 4.7 s. These dry LC marbles can also be utilized in other areas, including drug delivery, optical devices, and biosensors.

Polymer dispersed cholesteric liquid crystals with combined photo- and mechanochromic response

The development of stimuli-sensitive systems is one of the newest and most promising directions in the field of modern materials science. The combination of various types of sensitivity in the same material opens the possibility of creating complex devices with a wide range of potential applications. This work deals with the design and preparation of new composite materials based on plasticized polyvinyl alcohol (PVA) films containing droplets of cholesteric liquid crystals (CLC). Mechanical stretching of elastic composite films results in a change in optical properties, namely selective light reflection (SLR), from CLC droplets. This in turn transforms the structure and optical properties of the entire polymer matrix. Such a process represents a certain mechano-optic response of the polymer matrix to the influence of mechanical deformation. The introduction of a chiral photochromic dopant into a composite film makes it possible to transform such a film into a photochromic material with the ability to control its optical properties using, for example, UV irradiation. Therefore, mechanical deformation of a polymer composite leads to a change in its optical properties (SLR) caused by a change chiral supramolecular structure of cholesteric mesophase. At the same time, UV irradiation of a polymer film induces E-Z isomerization process in the chiral photochromic dopant, altering its molecular shape and helical twisting power. This enables the using of this process for remote and contactless control of the composite optical properties. In addition, this feature allows one to implement the photopatterning and the recording of a latent image, which appears under the influence of a mechanical field.

Complex-Shape Solid-State Photonic Droplets Prepared via Phase Separation and Microfluidics.

Complex-shape solid-state cholesteric liquid crystal (CLCsolid) droplets were prepared via solvent removal, phase separation, and photopolymerization of uniformly sized reactive CLC (rCLC)/fluorocarbon oil (FCO)/dichloromethane (solvent) droplets produced via a microfluidic method. The interfacial energies between rCLC and FCO, rCLC and water, and FCO and water of a rCLC/FCO droplet in an aqueous solution were precisely controlled through the specified surfactants. The shape of the rCLC/FCO droplet was strongly dependent on the balances among these interfacial energies, enabling the preparation of complex-shape droplets through the controlled concentration of the used surfactants. The complex-shape rCLC/FCO droplets showed photonic patterns consisting of a central reflection from a convex surface, cross-communication from a convex surface between adjacent particles, a photonic reflection band from the outer upward-facing concave surface, and total internal reflection from the inner upward-facing surface. Complex-shape CLCsolid particles obtained after photopolymerization and extraction of a nonreactive chiral dopant and FCO showed photonic patterns similar to those before photopolymerization without much deterioration of the photonic structure. These complex patterns make CLCsolid and rCLC/FCO droplets promising anticounterfeiting materials.

Optical detection of bovine serum albumin using charged cholesteric liquid crystal droplets functionalized with surfactant

Monodispersed micrometer-sized Cholesteric liquid crystal (CLC) droplets were prepared by using the flow-focusing microfluidic device, which was used for the detection of protein bovine serum albumin (BSA). The PVA/SC12S stabilized micro-sized CLC droplets (CLCPVA/SC12S) exhibited flashing spots with red reflection. These CLC droplets were effective in the detection of BSA in an aqueous solution through a configurational change from red reflection flashing spots (homeotropic configuration) to a central spot of red reflection (planar configuration) with a detection limit of 0.15 µM. We found that the electrostatic interaction caused by the adsorption of negatively charged BSA below isoelectric point on the positively charged CLC droplets can cause the arrangement of the CLC molecules in droplets to change from flashing spot to central spot reflection. Our results also highlight that the limit of detection and response times of CLCPVA/SC12S droplets are tunable as a function of the concentration and pH of protein. These CLCPVA/SC12S droplets are highly sensitive and are inexpensive and simple to make. With this novel approach, we can detect protein by observing optical reflection patterns with/without crossed polarizers in CLCPVA/SC12S droplets, which, on the contrary, are required in the case of the nematic biosensors.

Understanding directed assembly of concentrated nanoparticles at energetically heterogeneous interfaces of cholesteric liquid crystal droplets

Colloidal self-assembly has gained significant interest in scientific and technological advances. We investigated the self-assembly of the colloids at fluidic interfaces that mediate elastic interactions. Whereas past studies have reported the assembly of micrometer- or molecular-sized species at aqueous interfaces of liquid crystals (LCs), herein we study the assembly of intermediate-sized nanoparticles. Specifically, surface-modified silica nanoparticles (50 to 500 nm) were adsorbed at the liquid crystal-water interfaces and their positioning was investigated using electron microscopy after polymerization. The study revealed that the electric double layer forces and the elastic forces caused by LC strain are dominant in the assembly of nanoparticles and their contributions can be tuned to direct the self-assembly guided by the sub-interface symmetry of confined cholesteric LCs. At high ionic strengths, we observed a strong localization of nanoparticles at the defects, whereas intermediate strengths resulted in their partial enrichment into cholesteric fingerprint patterns with an interaction energy of ≈3 kBT. This result is comparable with the calculations based on the strength of the binary interactions of the nanoparticles. The findings also support the role of ion partitioning at the LC-aqueous interfaces on the formation of the assemblies. The results can be utilized for applications in sensors, microelectronics, and photonics.

Asymmetric Pairing of Cholesteric Liquid Crystal Droplets for Programmable Photonic Cross-Communication.

The photonic cross-communication between photonic droplets has provided complex color patterns through multiple reflections, potentially serving as novel optical codes. However, the cross-communication is mostly restricted to symmetric pairs of identical droplets. Here, a design rule is reported for the asymmetric pairing of two distinct droplets to provide bright color patterns through strong cross-communication and enrich a variety of optical codes. Cholesteric liquid crystal (CLC) droplets with different stopband positions and sizes are paired. The brightness of corresponding color patterns is maximized when the pairs are selected to effectively guide light along the double reflection path by stopbands of two droplets. The experimental results are in good agreement with a geometric model where the blueshift of stopbands is better described by the angles of refraction rather than reflection. The model predicts the effectiveness of pairing quantitatively, which serves as a design rule for programming the asymmetric photonic cross-communication. Moreover, three distinct droplets can be paired in triangular arrays, where all three cross-communication paths yield bright color patterns when three droplets are selected to simultaneously satisfy the rule. It is believed that asymmetric pairing of distinct CLC droplets opens new opportunities for programmable optical encoding in security and anti-counterfeiting applications.

Lattice Boltzmann Modeling of Cholesteric Liquid Crystal Droplets Under an Oscillatory Electric Field

We numerically study the dynamics of quasi-two dimensional cholesteric liquid crystal droplets in the presence of a time-dependent electric field, rotating at constant angular velocity. A surfactant sitting at droplet interface is also introduced to prevent droplet coalescence. The dynamics is modeled following a hybrid numerical approach, where a standard lattice Boltzmann technique solves the Navier-Stokes equation and a finite difference scheme integrates the evolution equations of liquid crystal and surfactant. Our results show that, once the field is turned on, the liquid crystal rotates coherently triggering a concurrent orbital motion of both droplets around each other, an effect due to the momentum transfer to the surrounding fluid. In addition the topological defects, resulting from the conflict orientation of the liquid crystal within the drops, exhibit a chaotic-like motion in cholesterics with a high pitch, in contrast with a regular one occurring along circular trajectories observed in nematics drops. Such behavior is found to depend on magnitude and frequency of the applied field as well as on the anchoring of the liquid crystal at the droplet interface. These findings are quantitatively evaluated by measuring the angular velocity of fluid and drops for various frequencies of the applied field.

Laser Processing of Liquid Crystal Droplets with Diverse Internal Structures

To control the spatial placement and organize micro/nanodroplets (NDs) has fundamental importance both in science and engineering. Cholesteric liquid crystal (CLC) droplets with topological diversity can offer many self-assembly modalities to arrange guest NDs in their spherical confinement; however, limited progress has been achieved due to difficulties of loading NDs into stabilized host droplets. Here, a laser injection technique is introduced, through which a controlled number of NDs were injected from a pre-selected location onto the surface of the host droplet. The sequentially injected NDs spontaneously drifted toward areas with topological defects and self-assembled along its geometry or local director field into a predefined shape. Within CLC droplets with different topological structures, guest NDs self-assembled near areas with defect points as twisting radial chains and quill-like assembly structures, and along defect lines as discrete beads and helical threads, respectively. The injection speed of the NDs, controlled by laser power, was found to play a key role in the assembly geometry of NDs as well as the internal structure of the CLC droplet processed. This study expands our abilities to precisely organize NDs in a spherical confinement and such droplet-based microsystems have potential applications for sensors, photonic devices, pharmaceuticals, and biotechnology.

Strain-induced recognition of molecular and chirality in cholesteric liquid crystal droplets for distance and curvature sensing.

Chirality is universal in nature and in biological systems, and the chirality of cholesteric liquid crystals (Ch-LC) is both controllable and quantifiable. Herein, a strategy for precise chirality recognition in a nematic LC host within soft microscale confined droplets is reported. This approach facilitates applications in distance and curvature sensing as well as on-site characterization of the overall uniformity and bending movements of a flexible device. Due to interfacial parallel anchoring, monodisperse Ch-LC spherical microdroplets show radial spherical structure (RSS) rings with a central radical point-defect hedgehog core. Strain-induced droplet deformation destabilizes the RSS configuration and induces the recognition of chirality, creating "core-shell" structures with distinguishable sizes and colors. In practice, an optical sensor is achieved due to the rich palette of optically active structures that can be utilized for gap distance measuring and the monitoring of curvature bending. The properties reported here and the constructed device have great potential for applications in soft robotics, wearable sensors, and advanced optoelectronic devices.

Optically driven liquid crystal droplet rotator

In this study, the rotation of liquid crystal droplets induced by elliptically polarized laser light was investigated using optical tweezers. The rotation mechanism was analyzed based on the arrangement of liquid crystal molecules within the droplets. The change in the rotation behavior of nematic liquid crystal (NLC) droplets was evaluated by varying the droplet size. The experimental results were analyzed based on the waveplate effect and light-scattering process. The rotation behavior of cholesteric liquid crystal droplets was examined by varying the droplet size and helical pitch, which was controlled by the chiral dopant concentration. The results are discussed in terms of the selective reflection of the incident beam by the helical structure. The dependence of the rotation frequency on the ellipticity of the incident beam was also studied. The main contribution to the rotation gradually changes from light transmission to reflection with increasing chirality of the droplet. An NLC rotator system was constructed using holographic optical tweezers. Such an optically controllable rotator is a typical micro-optomechanical device. Complex flow fields, including multiple vortex and localized shear fields, were realized at the micron scale.

Temperature gradient sensing mechanism using liquid crystal droplets with 0.1-mK-level detection accuracy and high spatial resolution

We proposed the detection mechanism of the micro-levels of temperature gradient in a micro-electromechanical system using the unidirectional rotation of cholesteric-liquid crystal (Ch-LC) droplets. Ch-LC droplets in the presence of an isotropic phase subjected to a heat flux rotate with a speed proportional to the magnitude of the temperature gradient. We further quantified the temperature gradient-to-torque conversion efficiency to apply the thermomechanical cross-correlation to the detection of temperature gradient. Then, we observed the rotational behavior of Ch-LC droplets after introducing them onto model devices containing patterned Au thin-film electrodes. Direct electric current applied to these Au electrodes results in unidirectional rotation of the Ch-LC droplets in response to heat flux generated from the Au electrodes. By evaluating the possible temperature gradient detection resolution using Ch-LC droplet rotation, we show that Ch-LC droplets can achieve both high spatial resolution (~ 10 µm) and high detection accuracy (~ 0.1 mK/µm).

Compartmentalized Janus droplets of photoresponsive cholesteric liquid crystals and poly(dimethylsiloxane)-based oligomers

A new kind of Janus droplet containing photoresponsive cholesteric liquid crystals (CLCs) was fabricated for the first time and their formation, compartment structure, mesophase texture and function were thoroughly investigated. In the droplets, the CLC compartments included a typical nematic LC (4'-pentyl-4-biphenylcarbonitrile) doped with an azobenzene-containing chiral dopant, and the other compartments were formed of a poly(dimethylsiloxane)-based oligomer. Janus droplets were fabricated through microphase separation of the incompatible components in chloroform solution dispersed in an aqueous medium, induced by slow evaporation of chloroform. The mesophase structures of the CLC phase in Janus droplets, both suspended in aqueous medium and spreading on substrates, were controlled by the bulk elastic free energy of the CLC phase, surface anchoring and confining geometries. The helix pitch of the cholesteric phase in the droplets was determined by the doping concentration of the chiral dopant. For the suspended Janus droplets with the helix pitch obviously smaller than the droplet sizes, the CLC compartments mainly possessed a bipolar structure instead of the Frank-Pryce structure typically observed on CLC droplets. After the Janus droplets spread on the substrates, the CLC compartments changed to crescent shapes due to the different wettability characteristics of the two compartments, and the formed stable and metastable CLC configurations were distinctively different from those in the suspensions. Interestingly, when the Janus droplets spreading on substrates were irradiated with a laser beam (λ = 488 nm) of low intensity, the directors in the CLC compartments rearranged to form fingerprint structures with minimum total energy.

Researches and applications of cholesteric liquid crystal droplets

Researches and applications of cholesteric liquid crystal droplets

Mechanochromic Responses of Cholesteric Liquid Crystal Droplets with Nanoscale Periodic Helical Structures Showing Reversible and Tunable Structural Color

Cholesteric liquid crystal (CLC) droplets with nanoscale periodic helical structures are omnidirectional under planar anchoring, and light is selectively reflected from the droplet center at all directions. However, when CLC droplets are too small, random light scattering dominates. In this regard, we present a facile method to increase the selective Bragg reflection of CLC droplets and demonstrate the unique mechanochromic responses of CLC-droplet-dispersed polymer films. The films are originally opaque and white, when the films are bidirectionally stretched, spherical CLC droplets are squeezed into oblate shape and the point defect at the center of each CLC droplet transitions to a ring defect, greatly increasing the selective Bragg reflection region and resulting in the appearance of structural color. By tuning the helical pitches via the chiral dopant concentration from 1.7 to 3.0 wt %, the desired structural color is achieved. Smart sensors show color changes in response to hydration and dehydration are designed, paving the way for the developments of CLC-based functional materials and smart devices.

Optical Fiber Sensor for Determination of Methanol Ratio in Methanol-Doped Ethanol Based on Two Cholesteric Liquid Crystal Droplets Embedded in Chitosan

In this paper, an optical fiber sensor based on cholesteric liquid crystal (CLC) is designed to detect the proportion and concentration of methanol and ethanol in alcohol solution. Due to the different polarity of methanol and ethanol molecules, the pitch of CLC will swell in different degrees. Therefore, the reflected light wavelength of CLCs will shift differently when the sensor is put into the mixed solution of two alcohols with different proportions and concentrations. The function of chitosan polymer network is to fix and protect CLC droplets, it also allows small molecules to pass through and interact with CLC. The sensitivities of the two CLCs are 0.296 nm/v% and 0.467 nm/v% to methanol, and 0.764 nm/v% and 1.133 nm/v% to ethanol, respectively. The detection range of the sensor for alcohol solution is 0.5v/v% -30v/v%. The linear relationship between wavelength shift and proportion and total concentration was obtained by least square method, and the related characteristics of the sensor were explored.

Polymer Dispersed Cholesteric Liquid Crystals With a Toroidal Director Configuration under an Electric Field.

The electro-optical properties of polymer dispersed liquid crystal (PDLC) films are highly dependent on the features of the contained liquid crystal (LC) droplets. Cholesteric LC droplets with homeotropic boundaries can form several topologically different orientational structures, including ones with single and more point defects, layer-like, and axisymmetric twisted toroidal structures. These structures are very sensitive to an applied electric field. In this work, we have demonstrated experimentally and by computer simulations that twisted toroidal droplets reveal strong structural response to the electric field. In turn, this leads to vivid changes in the optical texture in crossed polarizers. The response of droplets of different sizes were found to be equivalent in terms of dimensionless parameters. In addition, the explanation of this phenomenon showed a comparison of theoretical and experimental structural response curves aids to determine the shape of the droplet. Finally, we demonstrated that the addition of a dichroic dye allows such films to be used as optical filters with adjustable color even without polarizers.

3D-Printed Biomimetic Systems with Synergetic Color and Shape Responses Based on Oblate Cholesteric Liquid Crystal Droplets.

Living organisms in nature have amazing control over their color, shape, and morphology in response to environmental stimuli for camouflage, communication, or reproduction. Inspired by the camouflage of the octopus via the elongation or contraction of its pigment cells, oblate cholesteric liquid crystal droplets are dispersed in a polymer matrix, serving as the role of pigment cells and showing structural color due to selective Bragg reflection by their periodic helical structure. The color of 3D-printed biomimetic systems can be tuned by changing the helical pitch via the chiral dopant concentration or temperature. When the oblate liquid crystal droplets are heated up to isotropic, the opaque and colored biomimetic systems become transparent and colorless. Meanwhile, the isotropic liquid crystal droplets tend to become spherical, causing volume contraction along the film plane and volume dilation in the perpendicular direction. The internal strain combined with the gradient distribution of the oblate isotropic liquid crystal droplets result in corresponding shape transformations. The camouflage of a biomimetic octopus and the blossom of a biomimetic flower, both of which show synergetic color and shape responses, are demonstrated to inspire the design of functional materials and intelligent devices.

Detection of bile acids using optical biosensors based on cholesteric liquid crystal droplets

We report a novel highly sensitive bile acids (cholic acid and deoxycholic acid) biosensor based on PVA/SC12S-stabilized cholesteric liquid crystal droplets.

Frustrated structures and pattern formation after thermal quenches in cholesteric liquid crystal droplets

Structural evolution simulations with stochastic nature of cholesteric liquid crystal droplets in homeotropic and planar degenerated anchoring under a thermal quenching procedure from random initial states to organized supramolecular structures.

Optical Multisensor Array with Functionalized Photonic Droplets by an Interpenetrating Polymer Network for Human Blood Analysis.

Photonic solid-state cholesteric liquid crystal (CLCsolid) droplets intertwined with a poly(acrylic acid) (PAA) network that has an interpenetrating polymer network (IPN) structure (referred to as photonic IPN CLCsolid-PAA droplets) were used as individual sensors in the dots of a PAA-patterned array film after functionalization via immobilization of the receptors and a metal-ion treatment. The photonic IPN CLCsolid-PAA droplets in the PAA-patterned array film were pH-responsive and showed an observable change in the reflected central color. This "smart" property, coupled with the photonic color response, makes these devices ideal photonic sensors. The immobilization of urease and phenylboronic acid on the PAA network allowed for the application of several 10 μm photonic IPN CLCsolid-PAA droplets to the optical photonic biosensors through facilitated volumetric changes in the PAA network in response to urea and glucose analytes, with high selectivity for major components in human serum, acceptable sensitivity for use with human serum, and extreme stability due to a solid-state structure. The blueshift of the reflected color of the KOH-treated photonic IPN CLCsolid-PAA droplets could be used for divalent metal-ion detection. The compartmentalized photonic IPN CLCsolid-PAA droplets in the patterned array film could be used for multiple detection applications, as evidenced by the ability to conduct pH, divalent metal ion, urea, and glucose detections in one patterned array film. This new platform opens the door for many interesting applications with numerous combinations of responsive hydrogel matrices and receptors.