Tomohiro Tsuji

GPU-accelerated molecular dynamics simulation for study of liquid crystalline flows

We have developed a GPU-based molecular dynamics simulation for the study of flows of fluids with anisotropic molecules such as liquid crystals. An application of the simulation to the study of macroscopic flow (backflow) generation by molecular reorientation in a nematic liquid crystal under the application of an electric field is presented. The computations of intermolecular force and torque are parallelized on the GPU using the cell-list method, and an efficient algorithm to update the cell lists was proposed. Some important issues in the implementation of computations that involve a large number of arithmetic operations and data on the GPU that has limited high-speed memory resources are addressed extensively. Despite the relatively low GPU occupancy in the calculation of intermolecular force and torque, the computation on a recent GPU is about 50 times faster than that on a single core of a recent CPU, thus simulations involving a large number of molecules using a personal computer are possible. The GPU-based simulation should allow an extensive investigation of the molecular-level mechanisms underlying various macroscopic flow phenomena in fluids with anisotropic molecules.

Molecular dynamics simulation of backflow generation in nematic liquid crystals

The mechanism of backflow generation in nematic liquid crystals under the application of an electric field is investigated by molecular dynamics simulation, and the roles of intermolecular interaction in the generation of bulk velocity are investigated. It is confirmed that the reorientation of molecules by the application of an electromagnetic field induces a transient “S-shaped” bulk velocity profile. The rotation and rearrangement of molecules during the reorientation process generate a local bulk velocity gradient.

Proposal of mechanics of liquid crystals and development of liquid crystalline microactuators

We propose a research field “mechanics of liquid crystals,” in which liquid crystals are studied from a mechanical viewpoint. The unsteady behaviors of a liquid crystal between parallel plates under an electric field are investigated. The imposition of the electric field on the liquid crystal induces flows, whose profile and magnitude strongly depend on the twist angle of the director at the plates. A visualization experiment confirms the generation of flows. The mechanism of such generation can be explained by considering that the rotation of molecules generated by the imposition of the electric field induces a local velocity gradient.

Three-dimensional molecular dynamics simulations of reorientation process and backflow generation in nematic liquid crystals under application of electric fields

The dynamic responses of nematic liquid crystals in a parallel-plate cell under the application of electric fields were investigated using three-dimensional molecular dynamics simulations, which should provide more precise dynamics as compared to those in two-dimensional molecular dynamics simulations as in our previous work [Sunarso et al., Appl. Phys. Lett. 93, 244106 (2008)]. The study is focused on the reorientation process and the generation of backflow, which should be important in the development of liquid crystalline actuators. It is shown that bulk reorientation is coupled with the generation of backflow owing to the conversion of electric-field-induced molecular rotation into bulk translational motion. The increase in electric torque due to the increase in electric field strength results in a faster change in the bulk orientation, thus accelerating the development of the flow field and increasing the magnitude of the generated velocity field. Different initial orientation angles result in simila...

Velocity profiles of electric-field-induced backflows in liquid crystals confined between parallel plates

For the purpose of developing liquid crystalline microactuators, we visualize backflows induced between two parallel plates for various parameters such as the twist angle, cell gap, applied voltage, and molecular configuration mode. We use 4-cyano-4′-pentyl biphenyl, a typical low-molar-mass nematic liquid crystal. By increasing the twist angle from 0° to 180°, the velocity component parallel to the anchoring direction of the lower plate changes from an S-shaped profile to a distorted S-shaped profile before finally becoming unidirectional. In contrast, the velocity component perpendicular to the anchoring direction evolves from a flat profile at 0° into an S-shaped profile at 180°. Because both an increase in the applied voltage and a decrease in the cell gap increase the electric field intensity, the backflow becomes large. The hybrid molecular configuration mode induces a larger backflow than that for the planar aligned mode. The backflow develops in two stages: an early stage with a microsecond time scale and a later stage with a millisecond time scale. The numerical predictions are in qualitative agreement with the measurements, but not quantitative agreement because our computation ignores the plate edge effect of surface tension.

Effect of magnetic fields on out-of-plane orientations of nematic liquid crystalline polymers under simple shear flow

The effect of magnetic fields on out-of-plane orientations of liquid crystalline polymers (LCPs) under simple shear flows is numerically analyzed using the Doi–Hess equation. The evolution equation for the probability distribution function of the LCP molecules is directly solved without any approximation closure. The initial director is parallel to the vorticity direction. Two cases of the magnetic fields are considered (1) the magnetic field parallel to the flow direction, and (2) the magnetic field parallel to the velocity gradient direction. For both cases a log-rolling orientation state is detected at low shear rates. However, the director is quickly aligned along the direction of magnetic fields because of the deformation of molecules. The field affects on the scalar order parameter rather than the major orientation direction for the magnetic field parallel to the flow direction. On the other hand regarding the magnetic field along the vorticity gradient direction, the effect of the magnetic field is more remarkable on the major orientation in comparison with the effect on the scalar order parameter. Also it is be found that the order parameter is increased obviously with increasing the magnetic fields. It is an efficient way to improve the performance of LCP materials.

Fundamental study on the application of liquid crystals to actuator devices

In a fundamental study to develop liquid crystal microactuators, we prepared a sandwich cell with a movable upper plate and used backflow induced by applying repetitively a rectangular wave voltage to drive the upper plate in its plane. We used 4-cyano-4′-pentyl biphenyl, a low-molar-mass nematic liquid crystal. The speed of the plate depends significantly on the frequency of the applied voltage. With specific settings of applied voltage, duty ratio, plate gap, and upper plate mass, the speed increases with increasing frequency, attaining a maximum value of 120u2009μm/s at 175u2009Hz. Further increases in frequency, however, produce a gradual decrease in plate speed because the molecules of the liquid crystal respond too slowly to the change in voltage at the higher frequencies. In addition, to expand the field of application of liquid crystal actuators, we performed an experiment to control the direction of movement of the upper plate by patterning the electrodes and the alignment layer to govern the orientation...

Back‐Flow of Nematic Liquid Crystals and Its Application to Liquid Crystalline Microactuators

With the aim of the development of liquid crystalline microactuators, the numerical and experimental studies of the back‐flow effect of nematic liquid crystals have been performed. The Leslie‐Ericksen continuum theory is used to simulate the back‐flow of a nematic liquid crystal, 4‐n‐pentyl‐4‐cyanobiphenyl (5CB), between parallel plates. The lower plate is fixed, and the upper plate can move freely. From the results, it is found that the translational movement of the upper plate occurs owing to the shear stress arisen from the back‐flow, when an electric field is imposed between the upper and lower plates. By applying the pulsed voltage, continuous movement of the upper plate is able to be achieved. The effects of the applied voltage, frequency of the pulse, gap width, and the surface molecular anchoring condition on the driving efficiency of the actuators are thoroughly investigated to obtain the optimal condition for the actuators. For example, the averaged driven speed of the upper plate increases with...

Liquid Crystal Flow Induced by Annihilation of a Pair of Defects (1st Report, Relationship between Orientation State and Computation Accuracy)

As the first report of liquid crystal flow induced by annihilation of a pair of defects, we have studied the relationship between orientation state of molecules and computation accuracy using the Doi theory, in which the orientation state is described with the orientation distribution function. Computation of the function is a huge task, so that we have approximated the function with a series of spherical harmonic functions in order to reduce the task. The computation accuracy is assumed to depend strongly on both the nematic potential intensity and shear rate. It is found from the numerical results that many terms of spherical harmonic functions are required for the high potential intensity and shear rate because the function becomes steep. In addition, we propose diagrams from which we can easily obtain the necessary number of terms as a function of the potential intensity and shear rate.

Numerical and Experimental Studies of Liquid Crystalline Flow Induced by Annihilation of Paired Defects

Numerical and experimental studies of liquid crystalline flow induced by the annihilation of paired defects have been achieved by using the Doi theory coupled with the Marrucci-Greco potential. The molecular orientation distribution function is approximated by a series of spherical harmonic functions, to reduce the computational task. It is empirically known that the defects with different types of molecular orientation configuration attract each other, and finally annihilate. During this annihilation process, the liquid crystalline flow will be induced through the rotation of the molecular orientation direction. It is found from the numerical results that vortex-like liquid crystalline flow is induced around the defects come closer. The numerical predictions are induced liquid crystalline flow is confirmed by the experiment.Copyright