Jang-Kun Song

Electro-optical switching of graphene oxide liquid crystals with an extremely large Kerr coefficient

The sensitive response of the nematic graphene oxide (GO) phase to external stimuli makes this phase attractive for extending the applicability of GO and reduced GO to solution processes and electro-optic devices. However, contrary to expectations, the alignment of nematic GO has been difficult to control through the application of electric fields or surface treatments. Here, we show that when interflake interactions are sufficiently weak, both the degree of microscopic ordering and the direction of macroscopic alignment of GO liquid crystals (LCs) can be readily controlled by applying low electric fields. We also show that the large polarizability anisotropy of GO and Onsager excluded-volume effect cooperatively give rise to Kerr coefficients that are about three orders of magnitude larger than the maximum value obtained so far in molecular LCs. The extremely large Kerr coefficient allowed us to fabricate electro-optic devices with macroscopic electrodes, as well as well-aligned, defect-free GO over wide areas.

Hierarchical elasticity of bimesogenic liquid crystals with twist-bend nematic phase

In 2001, Dozov predicted that twist-bend nematic phase can be spontaneously formed when K33   2, and this phase has recently been discovered in bimesogens. To verify Dozovs hypothesis, we have measured precisely the temperature dependence of the elastic constants of CB7CB in the entire temperature range of nematic phase and in twist-bend nematic phase close to the transition temperature by combing the Freedericksz threshold methods for a twist nematic and an in-plane switching cells. Anomalous changes in K22 and K33 are observed across the phase transition. The elasticity estimated via extrapolation of the data in the high temperature region of the nematic phase seems to fully satisfy Dozovs hypothesis although the elasticity data in the vicinity of the phase transition exhibit opposite trends. This can be explained by the general nature of a hierarchical system where the macroscopic elasticity is governed mostly by the distortion of a higher level structure.

Optimization of particle size for high birefringence and fast switching time in electro-optical switching of graphene oxide dispersions

In order to use graphene oxide (GO) dispersions for electro-optical applications, both a high GO concentration and a high electrical sensitivity are essential; however, these have not been achieved to date. Here, we report that by optimizing the mean size of GO particles to approximately 0.5 μm, one can obtain a high GO concentration of up to 2 wt% and high electrical sensitivity simultaneously. By reducing the mean GO-particle size, the interparticle interaction and the rotational viscosity can be significantly reduced, and a high-concentration isotropic phase can be obtained. As a result, the maximum birefringence increases and the dynamic response becomes faster. However, further decrease of the mean size below 0.1 μm causes a decrease in the anisotropy of electrical polarizability, resulting in the reduction of the electrical sensitivity of GO dispersions.

Alignment of Liquid Crystals Using a Molecular Layer with Patterned Molecular Density

The surface of self-constructed molecular density modulation (SDM) exhibits a wide range of liquid crystal alignment capabilities including planar, tilted, and homeotropic alignments, disclination-free uniform and heterogeneous alignments, and even spatially varying alignments through the single non-contact process. Alignment defects are eliminated by temporary lowering the frictional energy barrier via the open-boundary elastic stabilization (OES) treatment.

Self-constructed stable liquid crystal alignment in a monomer-liquid crystal mixture system

Here, we demonstrate excellent liquid crystal (LC) vertical alignment without using an alignment layer printing process by introducing octadecyltrichlorosilane (OTS) into the LC mixture. Further, we investigated the alignment mechanism by analysing the surfaces of the substrates. The optimum concentration of OTS was found to be about 0.03 wt%, which is 1/100 of that in the previously reported polyhedral oligomeric silsesquioxane (POSS)–LC system. Moreover, the OTS–LC system exhibited a more stable LC alignment compared with the POSS–LC system. These differences may arise from the different strengths of surface–dopant interactions; that is, the covalent bond in the OTS–LC system and the van der Waals interactions in the POSS–LC system. We also demonstrated that the method can be used in a capillary tube, which may serve as a new method facilitating the application of LCs with curved surfaces.

Manipulation of structural color reflection in graphene oxide dispersions using electric fields.

Aqueous graphene oxide (GO) dispersions with a photonic crystal structure are carefully prepared to produce structural color reflection. We fabricate a simple reflective GO cell with a unique electrode design and demonstrate that the resulting structural color reflection is electrically erasable and rewritable. GO concentration and the direction of the electric field are vital factors in the development of the device. The resulting device works well, although it exhibits a rather slow response time; in particular, the spontaneous recovery time from dark to bright color reflection requires tens of minutes. Through the application of a horizontal electric field parallel to the substrate, the recovery time can be improved, resulting in a recovery period of 3 min. Although many unsolved issues remain, the findings in GO dispersion may provide a new possibility for color filter-less bi-stable color displays with low power consumption.

Dielectrophoretic manipulation of the mixture of isotropic and nematic liquid

In various applications involving liquid crystals, the manipulation of the nanoscale molecular assembly and microscale director alignment is highly useful. Here we show that a nematic–isotropic mixture, a unique bi-liquid system, has potential for the fabrication of microstructures having an ordered phase within a disordered phase, or vice versa. The volume expansion and shrinkage, migration, splitting, mergence and elongation of one phase within the other are easily accomplished via thermal treatment and dielectrophoretic manipulation. This is particularly achievable when one phase is suspended in the middle. In that case, a highly biased ordered-phase preference of surfaces, that is, the nematic-philic nature of a polyimide layer and the nematic-phobic nature of a self-assembled monolayer of chlorosilane derivatives, is used. Further, by combining this approach with photopolymerization, the patterned microstructure is solidified as a patterned polymer film having both isotropic and anisotropic molecular arrangements simultaneously, or as a template with a morphological variation.

Flow-induced ordering of particles and flow velocity profile transition in a tube flow of graphene oxide dispersions

We measured the S- and P-order parameters of flow-induced ordered graphene oxide (GO) particles and the flow velocity profiles for a flowing aqueous GO dispersion in a tube, by using an optical method. The order parameters clearly exhibit increasing concentric biaxial ordering as the flow velocity increases, with the exception of a disordered centre. Newtonian to non-Newtonian transition in the flow velocity profile is found, changing from a parabolic shape to a fuller shape at very low Reynolds numbers less than 10. This is attributed to the shear thinning effect (i.e., an ordering-induced reduction in viscosity). In the Newtonian flow, a uniaxial ordering was dominant; whereas a biaxial ordering sharply increased in the non-Newtonian flow, indicating that both the ordering of GO particles and the interparticle interactions influence the flow profile transition.

Ionic impurity control by a photopolymerisation process of reactive mesogen

Extremely low ion density, 10−8−10−11 molar fraction, which inevitably exists due to residual ion impurities even in a purified liquid crystal (LC) compound, can significantly influence the electro-optic response in LC devices. We found that the density of ionic molecules increased with the addition of various dopants including triphenylphosphine oxide (TPPO), molecules with the functional group of aldehyde, epoxide and so on into a nematic LC cell by observing the electrical response of the LC cells and that the irradiation of ultraviolet (UV) light accelerated the generation of ionic molecules, indicating degradation of organic materials. However, the addition of reactive mesogen (RM) compounds to the LC mixture significantly decreased the effective density of ions during and after the photopolymerisation process. The cured RM networks effectively captured the ion impurities during the photopolymerising process and their ion capturing ability was sustained even after completing the photopolymerisation process. This observation may provide a simple and useful way to control the effective ion density in a liquid medium down to extremely low levels.

Antiferroelectric dielectric relaxation processes and the interlayer interaction in antiferroelectric liquid crystals

Antiphase relaxation process of antiferroelectric phase is investigated in binary mixtures of antiferroelectric and ferroelectric liquid crystalline compounds. It is found that the frequency of the antiphase mode ranges from 2to4MHz, being one of the fastest collective processes reported so far. This can be exploited in a photonic switch with a submicrosecond response time. However, the frequency of the antiphase mode decreases by the mixing ratio of the ferroelectric compound. The decrease in the frequency is shown to be due to a reduction in the elastic constant in between the adjacent layers.