Yusril Yusuf

Prewavy Pattern: A Director-Modulation Structure in Nematic Liquid Crystals

Abstract A periodic structure (the prewavy pattern) found below the onset of electroconvection in a homeotropically aligned nematic liquid crystal is reported. It is characterized by a periodic modulation of the director in the xy-plane, which can be distinguished from electroconvection patterns with modulation in the xz-plane. The prewavy instability is investigated in detail; a phase diagram in the frequency-voltage plane, the voltage dependence of the azimuthal rotation angle, the voltage and frequency dependence of the wavelength of the prewavy pattern, and the director field in the prewavy pattern are provided. This prewavy pattern always evolves into the so-called chevrons above onset of convection. The wavelength of the chevrons and the orientation of their alternating zig and zag rolls depend on the director structure of the prewavy pattern.

Multifunctional liquid crystal elastomers: Large electromechanical and electro-optical effects

A multifunctional main chain liquid crystal elastomer (MCLCE) with large mechanical and optical effects in applied electric fields is investigated, when MCLCE is swollen in a low molecular weight liquid crystal, 4-n-pentyl-4-cyanobiphenyl, a nematic solvent. The size change by the field effects is linearly proportional to the transmittancy change. This suggests the possibility of broad application as a field-induced-optical actuator device.

Trifunctionally Cross-Linked Liquid Single Crystal Elastomers: Swelling Dynamics and Electromechanical Effects

Here, we study the influence of cross-linking density, X, on the swelling, thermal and electric field response of trifunctionally cross-linked liquid single crystal elastomers (V3 LSCEs) swollen with low molecular weight liquid crystals, 4-n-pentyl-4-cyanobiphenyl (5CB). The cross-linker concentrations are X3V3=3.3 mol %, X5V3=5 mol %, and X7V3=7 mol %. LSCE networks are characterized by frozen-in orientation order, P. Xc is a constitutional critical point for LSCE networks. When X Xc (P>0), supercritical behavior prevails over a range of temperatures, ΔTs. Taking ΔTs∝(X-Xc)1/2, we find a critical cross-linking density, Xc≈3%, for V3 LSCEs. 5CB diffuses ⊥n into V3 similar to isotropic gels with typical times, τ⊥n in minutes, that decreases with increasing (X-Xc). Swelling with reorientation effects is initiated by 5CB propagating into V3 followed by relaxation of the front profile with τ⊥(2) (twist), and τ|| (13) (splay-bend) that both increase nearly parabolically with increasing (X-Xc). Front speeds are 50% faster in 5V3 than in 7V3 but no fronts were observed in 3V3. Compared to dry V3 LSCE volumes, Vd, the swollen volumes, Vs, increased as X→Xc: Vs/Vd~(X-Xc)-0.27 reducing the frozen-in orientational order at X to \tildeX=X(Vd/Vs). The maximum electromechanical effect found in swollen V3 was δMAX~(\tildeX-\tildeXc) with \tildeXc~0.25%. The size of the effect is much smaller than observed in 8A2 which has a larger P. For 7V3 the effect is about 4 times smaller and for 5V3, it is an order of magnitude smaller.

Anomalous diffusion and Lévy distribution of particle velocity in soft-mode turbulence in electroconvection

We study the complex behavior in soft-mode turbulence (SMT), a recently discovered type of spatiotemporal chaos observed in electrohydrodynamic instability (EHD) of a nematic liquid crystal with homeotropic alignment. A particle, small compared to the characteristic length of the macroscopic flow, injected in SMT travels with random velocity such as an active Brownian motion. Tracking the particle trajectory induces changes in the flow velocity as a function of space and time (the Lagrange picture). The mean amplitude of the velocity is linearly proportional to the control parameter e, normalized voltage in EHD. The probability density distribution of the particle velocity changes from Levy for small e to Gaussian distribution for large e through intermediate distributions. The different regimes of anomalous diffusions are also observed. The stochastic properties of SMT are discussed based on these results.

Electrospun Nanofibers Based on Polyvinyl Alcohol/Chitosan and its Stability in KOH Solution

Fabrication of nanofibers based on polyvinyl alcohol and chitosan (PVA/chitosan) has been carried out by means of electrospinning with focusing on beads formation and diameter variation due to effects of weight ratio variations of PVA/chitosan. For electrospinning solution, the PVA/chitosan with artios of 10:1, 10:2 and 10:3 w/w were dissolved in 1% acetic acid. In addition, stability testing of PVA/chitosan nanofibers in an electrolyte solution was performed by soaking the nanofiber mat in KOH solution. As results, the addition of chitosan concentration increased the pH, conductivity, and viscosity of the solution. Beads were potentially generated for low chitosan concentration. Furthermore, the effect of viscosity and conductivity on diameter of electrospun nanofibers is very significant and need to be optimized. The average diameter of electrospun nanofibers were found to be 160 ± 18 nm, 120 ± 12 nm, and 148 ± 26 nm, for weight ratios of PVA and chitosan 10:1, 10:2 and 10:3, respectively. In addition, no significant change in morphology after being soaked the electrospun PVA/chitosan nanofibers in KOH 40 wt% solution for 24 hours. It indicates that the electrospun PVA/chitosan nanofibers is stable in electrolyte solution which shows a high potential for electrolyte membrane in fuel cells application.

Shape Anisotropy and Optical Birefringence Measurements of Dry and Swollen Liquid Single Crystal Elastomers

We study the relation between the shape anisotropy changes, Δα, and the optical birefringence, Δ n , of dry and swollen rigid bi-functionally (8.0 mol %, 8A2) and flexible tri-functionally [5 mol %...

Nonlinear Transport and Anomalous Brownian Motion in Soft-Mode Turbulence

Soft-mode turbulence (SMT) is a recently discovered type of spatiotemporal chaos observed in the electrohydrodynamic instability (EHD) of a nematic liquid crystal with homeotropic alignment. Its novelty is that it occurs as therst supercritical bifurcation from a stable stationary state in the weakly nonlinear regime of EHD. A particle, small compared to the characteristic length of the macroscopicow, injected in SMT travels with random velocity similar to a particle in Brownian motion. Wend that the particle trajectory exhibits anomalous Brownian motion such as pumpedight and that the macroscopic diusion constant due to weak turbulence is about 10 3 times larger than the diusion constant associated with the rest state. The stochastic properties of SMT are discussed.

Electric Field Mediated Growth Habits in B7

The ground state of the novel liquid crystal phase formed by achiral banana-shaped nitro-molecules and known as B7 is not understood. While spirals and myelins spontaneously appear on cooling from the isotropic liquid, neither mature to form B7 monocrystals. We tried suppressing the flow associated with these growth transients by applying large electric fields to thin samples (1-4 w m thick). While a field untwists some spirals and coarsens the less mobile myelin textures, it stimulates even more flow and a B7 texture with variable but exceedingly small length scales.

Sensitivity Improvement of Ammonia Gas Sensor Based on Poly(3,4-ethylenedioxythiophene):Poly(styrenesulfonate) by Employing Doping of Bromocresol Green

The aim of this research is to improve the sensitivity of ammonia gas sensor (hereafter referred to as sensor) based on poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) by employing the doping dye of bromocresol green (BCG). The doping process was carried out by mixing the BCG and the PEDOT:PSS in a solution with an optimum ratio of 1 : 1 in volume. The sensor was fabricated by using spin-coating technique followed by annealing process. For comparison, the BCG thin film and the PEDOT:PSS thin film were also deposited with the same method on glass substrates. For optical characterization, a red-light laser diode with a 650 nm wavelength was used as light source. Under illumination with the laser diode, the bare glass substrate and BCG film showed no absorption. The sensor exhibited linear response to ammonia gas for the range of 200 ppm to 800 ppm. It increased the sensitivity of sensor based on PEDOT:PSS with BCG doping being about twofold higher compared to that of without BCG doping. Furthermore, the response time and the recovery time of the sensor were found very fast. It suggests that the optical sensor based on BCG-doped PEDOT:PSS is promising for application as ammonia gas sensor.

Temperature Dependence of Electromechanical Effects in a Swollen Polydomain Liquid Crystalline Elastomer

We report electromechanical effects in a swollen polydomain liquid crystalline elastomer (POLY-LCE). Owing to the director reorientation, we could directly observe its deformation in the unconstrained elastomer gel state. Using optical image analysis, the deformation amplitude, threshold voltage, and temporal characteristics of the POLY-LCE were quantitatively measured with varying voltage and temperature. From the experimental results we discuss the deformation mechanism and property of the POLY-LCE.