Everett Wang

Electric-field-induced patterns and their temperature dependence in a bent-core liquid crystal.

Two kinds of electroconvection patterns in an ether-bridged bent-core nematic liquid crystal material (BCN), which appear in different frequency ranges, are examined and compared. One is a longitudinal pattern with the stripes parallel to the orientation of the BCN and with a periodicity of approximately the cell thickness, occurring in the high-frequency range of several hundreds Hz; the other one is oblique stripes, which results in a zigzag pattern, and appears in the low-frequency range of several tens Hz. In addition, within an intermediate-frequency range, transformations from oblique to longitudinal and then to normal stripes occur at increased ac voltages. In particular, we investigated the temperature behavior of longitudinal and oblique stripes: When the temperature T increases and approaches the clearing temperature Tc, the contrast of the domains is enhanced and the frequency range of existence becomes wider, while the onset voltages increase only moderately instead of diverging, thus suggesting an isotropic mechanism of pattern formation.

Symbolic derivation of nonlinear benchmark bicycle dynamics with holonomic and nonholonomic constraints

We present a symbolic method for modeling nonlinear multibody underactuated systems with holonomic and nonholonomic constraints. Using MAPLE software, we are able to solve the quartic holonomic constraint analytically. We then use the constraints and extra Lagrange-Euler equations to systematically eliminate all the auxiliary coordinates and Lagrange multipliers, thereby obtaining a minimum set of unconstrained nonlinear analytic ordinary differential equations corresponding to the degrees of freedom of the system. The method is applied to a benchmark bicycle, in which all the six ground contact constraint equations are eliminated, leaving analytic coupled ordinary differential equations corresponding to the bicycle rear body roll, steer angle, and rear wheel rotation degrees of freedom without any approximation. This reduced analytic model offers insights in understanding complex nonlinear bicycle dynamic behaviors and enables the development of an efficient model suitable for real time control outside of the linear regime.

Observation of the photorefractive effects in bent-core liquid crystals

We present a new observation of photorefractive (PR) effects in bent-core nematic (BCN) liquid crystal (LC) materials, where two kinds of optical-induced gratings are demonstrated and compared in pure and surface-doped BCN systems. The experimental results showed that these two kinds of gratings exhibit distinctive different polarization-dependent and angular-dependent behaviors, respectively. Furthermore, we supplied the pure and surface-doped rodlike LC systems for comparison, which revealed that V shape molecular structure of BCN can produce charge carrier more efficiently than rodlike molecular structure does. Thus BCN materials can offer an exciting potential for optical information processing.

A fast light-induced grating in bent-core nematic liquid crystals with in-plane switching

We observed a fast light-induced grating in bent-core nematic liquid crystal (LC) using an in-plane switching cell. Combining weak writing beams and a dc field, this grating can be easily built up and decay within 10 ms, which is two orders of magnitude faster than that in conventional device composed of doped rodlike LC and planar cell. Besides, this grating is produced by the periodic twist deformation of LC, whereas the conventional grating is produced by the periodic bend deformation, therefore, this grating can be used to explore the twist elastic properties of LC systems.

Development of Efficient Nonlinear Benchmark Bicycle Dynamics for Control Applications

We present a symbolic method for modeling a nonlinear multibody bicycle with holonomic and nonholonomic constraints. The method, developed for robotic multibody dynamics, is applied to a benchmark bicycle, in which all six ground contact constraint equations are eliminated, leaving analytic coupled ordinary differential equations corresponding to the bicycle rear body roll, steer angle, and rear wheel rotation degrees of freedom without any approximation. We have shown that the nonlinear dynamics of the bicycle satisfies an underactuated manipulator equation and demonstrated an analytic method to solve the vehicle pitch angle from a quartic equation. This reduced analytic model offers insights in understanding complex nonlinear bicycle dynamic behaviors and enables the development of an efficient model suitable for real-time control outside of the linear regime.

Voltage selectable dual-mode optically-induced grating in ZnTPP doped chiral nematic liquid crystals

Optically-induced grating in ZnTPP doped chiral nematic liquid crystals (CLC) is demonstrated by dual-mode operations, where the dynamic or storage functionality is selected by the amplitude of dc voltage. While the storage can be erased and rewritten by a high dc pulse, it can persist without an electric field, which is the result when the CLC undergoes a switching between a planar and a focal conic state. This tunable dual-functionality grating combines the advantages of bistable storage of CLC and high photosensitivity of dopant, and thus can be potentially exploited in photonic devices to implement different optical information processing in situ.

Polarity-dependent bistable optical grating in chiral bent-core nematic liquid crystals

We examined the flexodomains (FDs) in chiral bent-core nematics (BCNs), and demonstrated the morphology changed from parallel stripes in pure BCN to oblique ones in chiral BCNs. While the magnitude of obliqueness angle strongly depended on the concentration of chiral dopant, its sign was determined by the polarity of the driving voltage, thus FDs appeared alternately as symmetrical oblique stripes in the positive and negative half a.c. voltage cycles, respectively. Also the HTP value of chiral dopant in BCNs can be determined based on this phenomenon. The polarity-dependent behavior of FDs can be potentially exploited in photonic devices with a bistable function.