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

Abstract

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 similar dynamics, although the magnitude of the generated velocity decreases with increasing initial orientation angle. The development of velocity profiles confirms the results of the experiment and the simulation using a macroscopic continuum approach. Simulations under various molecular aspect ratios show that with an increase in the aspect ratio, the reorientation process becomes slower due to the increase in moment of inertia and elastic torque, whereas the magnitudes of the velocity show the trade-off between the speed of the orientation change and the effectiveness of the molecular motion conversion. Furthermore, the simulation results show the spatial variation in the reorientation process as the result of interplay between electric torque, elastic torque, and backflow.

Bidirectional molecular reorientation induced by localized surface plasmon

“…The simulation are run in similar way as for the reorientation of LC molecules upon action of a static electric eld. 19,20 However, in our case we deal with a complicated spatial distribution of electric eld oscillating with light frequency. For calculation of quasistatic spatial distribution of electric eld around NP we have used the discrete-dipole approximation (DDA) method that was described in details in ref.…”

Shear thinning and thickening in spherical nanoparticle dispersions

“…We use the rotation matrix algorithm to enforce the rigid body motion. 40,41 While the SHAKE or RATTLE 42,43 algorithms use constraints to enforce the rigid body motion, it is not the case for the rotation matrix algorithm [44][45][46] . One can thus derive a reversible integration algorithm which in turn permits to do long simulations without any unphysical velocity scalings.…”

Development of novel microactuators driven by lquid crystalline materials

“…When an electric field is imposed on the LC material as shown in Fig.1(b), the LC molecules change their orientation direction to align parallel to the field, except for the molecules near the plate surfaces at which the effect of the surface treatment on the orientation is strong. In the molecular reorientation process, a bulk flow, called the backflow, is induced in the LC material [6][7][8] and the backflow causes a shear stress on the plates.…”