Conventional optical microscopes visualize static inhomogeneities such as orientation, concentration, and density within a material. On the other hand, our newly developed fluctuation microscope is a new microscope that directly visualizes the distribution of dynamics within a material as "dynamic inhomogeneity" as a two-dimensional image. We mixed liquid crystal molecule E44 and mesogen molecule with acrylate groups A6OCB and UV-polymerized them in a liquid crystal alignment cell to investigate the dynamics in the material near the liquid-glass transition point of the side-chain polymer swelling system, and near the sol-gel transition point by adding a very small amount of cross-linking agent.
As a result, we succeeded in directly observing the characteristics of spontaneous dynamic heterogeneity formation. To investigate the correlation between the dynamic inhomogeneity generation and macroscopic viscoelasticity, simultaneous multi-frequency dynamic Young's modulus measurements were also performed. We found that when the temperature is lowered from the high-temperature nematic phase state, the remarkable dynamic heterogeneity appears in intermediate temperature range.
In the glass or gel states, materials have perfect isotropic symmetry and transparency. However, even if physical quantities such as density, concentration, and orientation are uniform, spatial heterogeneity in mobility may appear, which is called “dynamic heterogeneity”. we have invented a new fluctuation microscope that directly visualizes the dynamic inhomogeneity. In this report, as a model system for observing dynamic inhomogeneity, we observed swollen liquid crystalline nematic gels and polymers. Orientational order is encapsulated in polymer chains. Both dynamics of the nematic director and conformation of polymer chains are dynamically coupled with each other. We have recorded this polymerization process using a fluctuation microscope from immediately after UV irradiation, and directly confirmed that the dynamics of the orientation fluctuation gradually decelerates with the passage of polymerization process. We also success to confirm that the dynamic inhomogeneity appears in space, based on the time evolution analysis of the image of special distribution of the amplitude and the relaxation time of the director fluctuation observed by the fluctuation microscopy.
We recently invented a new fluctuation microscopy principle and constructed a prototype system for observing dynamic heterogeneity of soft matter. We proved that a spatial resolution of 1 μm could be achieved with this microscope. We also prepared a nematic liquid crystal cell with a mixture of azo dye and liquid crystal. When this cell was irradiated with the blue letter "JUN" using a pattern irradiation device, the relaxation time of the irradiated area slowed down, and the relaxation time change appeared as a character pattern on the fluctuation microscope. The post-irradiation changes of the character patterns were recorded as moving images of the dynamic heterogeneity. The speed at which the pattern appears is faster than the current frame rate of video recording (~20ms).
Glass, gel, or soft matter material near the phase transition point has high symmetry and various physical quantities are spatially uniform. Such uniformity is isotropic and highly transparent and has practically important and useful physical properties. On the contrary, high symmetry makes observation by structural analysis equipment such as microscopes and X-rays ineffective and is a major obstacle to material design and synthesis based on physical property measurement. Therefore, we recently found a new principle of a fluctuation microscope for observing dynamic inhomogeneities in soft matter and completed a prototype. In gel, glass and mixed or complex systems containing different species of molecules, cooperative dynamic modes appear hierarchically based on local molecular motion, even if they are liquid state without static material structure. appear. This fluctuation microscope directly visualizes the dynamic spatial inhomogeneity of such fluctuations in modes of collective motions.
We have investigated the new types of the continuous isotropic(I)-nematic(N) phase transition originated from the shape transformation of micelles from sphere to rod in extremely dilute concentration region(3~9wt%). In this paper, we measure the response time of the flow birefringence dependent on the AC flow field and discuss the origin and dynamics of the flow induced birefringence. The flow birefringence is 3-order slower than the relaxation time of the order parameter fluctuation in the isotropic phase near I-N transition point.
We have invented the new principle to produce the slippery interfaces on the glass plates. Slippery interfaces are created by the localized disorder effect near the substrates. We designed and realized several proto types of the model for self-organized slippery interfaces. In the conventional LC display, switching dynamics is only defined by the motion of bulk director, because the director on the glass substrate is completely fixed due to the strong anchoring condition. By introducing the slippery interface, the motion of the surface director play key role for the switching dynamics as a new hydrodynamic variable. The reduction of the driving voltage and enhancement of mode efficiency can be achieved by the rotation of surface director. Furthermore, even the response time can be accelerated by the lubrication for the motion of surface director.
We characterize the slippery interface by measuring the dynamics of surface director ns after switching on/off or under the rotating the external magnetic field. The anchoring energy and the viscosity of the surface director can be evaluated directly from the dynamics of ns. On the other hand, the gliding phenomena, which is the movement of so-called easy axis nh, is investigated from the dynamics of ns after switching off the magnetic field. The gliding feature can be represented by the viscosity of the movement of the easy axis. It should be important to distinguish the dynamics of ns and nh independently. This work was supported by JST-CREST (JPMJCR1424).
"Ferroelectric smectic (SmC) phase" Anchoring effects on the polymer films in the liquid crystal (LC) display devices plays key role to create the restoring force to the black state for any types of display modes, such as IPS, STN, VA and OCB etc. On the contrary, we have designed the slippery interfaces in the homeotropic ferroelectric (SmC*) liquid crystals for the DH-FLC mode as walls wetted on the electrodes in the in-plane switching cell, and success to reduce the driving voltage keeping the ultra-fast response.
"Principle & Design" We have invented the new principle to produce the slippery interfaces on the glass plates or even nano-interfaces embedded in the materials such as PSChBP. Slippery interfaces are created by the disorder effect induced by the several types of frustration. For example, the impurities with surface affinity weaken or melt the liquid crystalline (LC) order near the interfaces, then boundary region of LC spontaneously play roles on the slippery interfaces. Therefore, the anchoring effect disappears, and molecular motion is lubricated by the slippery interfaces. Especially, change of the anchoring to the slippery condition can be controlled by UV illumination on localized azo dye surfaces.
"Evaluation" We measure the dynamics of surface director rotation under rotating magnetic field, and analyze the response of the surface director by changing the strength of the magnetic field and rotation speed of the cell. Therefore, we correctly evaluate the change of the anchoring condition which is related to the anchoring energy and viscosity of surface director. Anchoring phenomena is strongly dependent on the model of the slippery interface, the temperature and even the LC materials on the interface.
Graphene, a monoatomically thick film made by carbon atoms arranged in honeycomb lattice, for its exceptional electrical, thermal and mechanical properties is one of the most attractive materials to be incorporated in electronic devices and in composites. New interest has been recently arisen from water suspensions of flakes of graphene oxide (GO), obtained from chemical exfoliation of graphite, since they form liquid crystal (LC) phases, for the easiness of handling graphene, otherwise forming aggregates, and their high yield. Interestingly, GO LC suspensions are responsive to electric fields with an extremely high Kerr coefficient resulting in an induced birefringence at macroscopic scale, achieved with very low electric fields. The LC phase formation and its responsiveness to electric fields are dependent on suspension characteristics such as flake average dimension, aqueous matrix and flake properties. In particular, bare graphene flakes have larger response to electric fields, due to their higher polarizability, than GO flakes. As it will be described, this results in improved electro-optic performance of reduced-graphene compared to GO LC with remarkably higher optical transmission for the same field strength thanks to a more efficient flake reorientation enabling a larger optical modulation.
We designed the fast switching & low driving voltage principle with nano slippery interfaces in nematic gels. Confinement effect by the gel network can accelerate the response time of nematic due the localization of director motion as similar way to the cholesteric blue phase. However, the anchoring of the director on the gel network induces the driving voltage. Slippery effect on the nano interface of the gel network can reduce the driving voltage due to the lubrication of the director motion. Since azo dye acrylates are co-polymerized with nematic gels (Azo-NGel), we can demonstrate the UV-switchable nano-slippery interface on the nematic gels.
Dynamic Light Scattering (DLS) provides us the information of the dynamics of director rotation. Dispersion relation of the pure 7CB-T15 mixture completely satisfies the prediction of hydrodynamic modes in the nematic. On the contrary, the relaxation frequency of the Azo-NGel keeps fast response in the low wave-number region due to the confinement effect.
Electro-Optic response (EO) of the Azo-Ngel without UV illumination shows the serious increase of the driving voltage compared to the pure 7CB-T15 mixture. When the Azo-Ngel is illuminated by UV-light Slippery interfaces can be created by the trans-cis isomerization of the Azo dye of the side-chain on the nano network of nematic gels. EO under UV illumination shows the drastic reduction of the driving voltage keeping the fast response time. We have also confirmed that the dispersion relation of the UV illuminated Azo-Ngel is almost the same as that of Azo-Ngel without UV illumination by the DLS measurement. Thus, the confinement effect for the nematic gels is still effective after the slippery interface is created by the UV illumination.
We success to demonstrates that the acceleration of the response time is compatible with reduction of the driving voltage by the localization and lubrication of director motions due to the design of the nano-slippery interfaces in the nematic gels.
Anchoring effects on the polymer films in the liquid crystal (LC) display devices plays key role to create the restoring force to the black state. However, the chiral materials with spontaneous helix, such as deformed helix mode in SmC* (DH-FLC) or the polymer stabilized blue phase (PSChBP), can recover black state by rewinding motion of the helix itself. We have invented the principle and design of slippery interfaces, which has zero anchoring force for attached LC molecules on the interfaces, and confirmed the drastic reduction of driving voltage in DH-FLC mode of SmC* (<1 order) keeping the fast switching response (tau~50 micro sec). We have reported the lateral slippery interfaces consist of the phase separated liquid phases created by tran-cis isomerization of doped azo dye. It is not enough to the complete transmission of the light(I/I0~1) by applying the typical driving voltage (~1.0V/micro m) for current IPS panels. It is also problem that slippery interface become effective only just below the I-SmC phase transition temperature (TIC-T<20°). Here, we report new type of the vertical slippery interface realized by the spin coated swollen azo-LC gel films on the glass substrates. Under UV irradiation, trans-cis isomerization of the azo-dye co-polymerized in the azo-LC gel film, induces the vertical slippery interfaces by the disordering effect. Since the co-polymerized azo-dye cannot be dissolved into LC, the disordering effect is completely localized in the interface between swollen azo-LC gel and bulk SmC* material. Then the slippery interfaces can be stabilized over wide temperature range. We greatly improve the reduction of the driving voltage, I/Io=1, 1.0V/micro m for rather slow change of the driving voltage (tau~1msec 2.5msec pulse), I/I0=0.6, 1.5V/micro m for fast change (tau~50 micro sec, 250 micro sec pulse) by lubrication of intra and inter helix C-director rotation motions.
The efficiency of the conduction of photocurrent in discotic liquid crystals is known to depend on the quality of the
columnar organization. Solvents have shown to be able to influence the formation of wire structures on substrates
promoting very long and ordered wired formations or bulkier structures depending on the affinity of the solvent with
parts of the molecular structure of discotics. Here we present a study on the effect of solvents when the liquid crystal is
confined between two substrates with the columns running perpendicular to them, geometry used in solar cells. We
focused on toluene and dodecane, solvents that have shown to promote on substrates the formation of aligned and long
nanowires and bulk large and isolated fibers, respectively. The phase transition behavior indicates that toluene does not
interfere with the columnar formation while dodecane strongly influence increasing the disorder in the structure.
Publisher’s Note: This paper, originally published on 5 October 2015, was withdrawn per author request, if you have any questions please contact SPIE Digital Library Customer Service for assistance.
The self-organization of discotic liquid crystal molecules in columns has enormous interest for soft nanoelectronic applications. A great advantage of discotic liquid crystal is that defects can be self-annealed in contrast to typical organic materials. Through the overlap of molecular orbitals, the aromatic cores assemble into long range ordered one-dimensional structures. Very thin structured films can be obtained by spin-coating from solution and the resulting morphologies are strongly dependent on the interaction between discotics and solvent molecules. Toluene produces films formed by very long nanowires, spontaneously aligned along a common direction and over fairly large areas. These nanostructured films are a result of the interplay between liquid crystal self-organization and solvent driven assembly. The ordered nanowire structures exhibit improvement in the electrical properties compared to misaligned structures and even to pristine HAT5, deposited without the aid of solvent. In this study we show that the toluene-based deposition of discotic liquid crystals is advantageous because it allows a uniform coverage of the substrate, unlike pristine HAT5 but also thanks to the type of induced structures exhibiting one order of magnitude higher conductivity, in the aligned nanowire films, compared to bare HAT5 ones.
Drops or shells of a planar-aligned short-pitch cholesteric liquid crystal exhibit unique optical properties due to the combination of Bragg reflection in the cholesteric helix and a radial orientation of the helix axis. If such a droplet is illuminated from above, light is reflected into a continuous set of cones, the opening angles of which depend on where on the droplet the light hits its surface. For the wavelength that fulfills the Bragg condition the reflection is dramatically enhanced, yielding the light cones colored. A photonic cross communication scheme arises for certain angles, reflecting light back to the observer from a different droplet than the one originally illuminated. This gives rise to an intricate pattern of colored and circularly polarized spots. A number of interesting applications may be developed based on this pattern, e.g. in identification and authentication devices. We have carried out a detailed spectrophotometric analysis of the patterns, localized to individual spot maxima. A quantitative comparison between the measured spectra and the reflection wavelength expected from a model for the pattern generation allows us to conclude that the droplets are in fact not spherical but slightly ellipsoidal.
Lyotropic Liquid Crystals (LCs) are attractive materials as host systems for nanoparticles, in particular for carbon nanotubes (CNTs), due to the LC templating and dispersing action. Since carbon nanotubes have many remarkable properties their presence could also influence the aligning hosts and such effects need to be taken into account in CNTLC composites. CNTs can be dispersed efficiently in surfactant-based lyotropic hosts that can be removed after their templating action, being water based. However, residual surfactant has a detrimental effect on the nanotube properties and it becomes important to find ways to minimize its amount in CNT composites. In the present work we use, for CNT alignment, a lyotropic nematic LC host with a very low surfactant concentration, based on charge combination of cationic and anionic surfactant molecules. Small variations in the molar ratio of the two surfactants, still at a fixed total surfactant amount, yield a very different LC behavior. CNTs could be successfully dispersed in the host forming an overall low-surfactant composite. Interestingly, the presence of nanotubes strongly influences the behavior of the host, bringing a stabilization of the LC phase.
Biaxial nematic liquid crystals have attracted much attention from both fundamental and application points of view,
because the fast response based on the rotation of the minor director is expected. So far, different molecular designs have
been proposed for the emergence of the biaxial nematic phase. Among that, we have been interested in applying "preorganization"
concept on generating the biaxiality. Dimeric liquid crystal compounds have been prepared in line with
this concept in which two mesogenic parts are linked by the biphenyl connecting group. The pre-organized dimmer
shows an anomalous textural change, for vertically-aligned and free-standing film samples, at the smectic C (SmC)-
nematic (N) phase transition, in which the Schlieren texture of the SmC changes into the other Schlieren texture of the N
phase. There are two possible explanations for this textural change, i.e., the occurrence of the director change at the
SmC-N phase transition or the emergence of biaxiality in the N phase. The electric-field-induced birefringence has also
been measured in detail for investigating the biaxial nature of the sample.
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