Molecular dynamics simulation of backflow generation in nematic liquid crystals

Abstract

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. © 2008 American Institute of Physics. ͓DOI: 10.1063/1.3050111͔Liquid crystals exhibit many features that are interesting from technical and scientific viewpoints. The ability to control their molecular orientation by the application of an electric field has led to the development of liquid crystal displays ͑LCDs͒. Research in this field has driven the large-scale production of LCDs. As LCD technology has been established, the development of different applications of liquid crystals has attracted growing attention. Recently, the potential development of a liquid-crystal-based microactuator or manipulator has been proposed.1,2 The basic idea is to employ the flow induced by an electric field to control the motion of an object. The induced flow is known as backflow.Early studies on backflow dealt with its effects on the switching process. [3][4][5] In more recent studies, the potential application of backflow for controlling the motion of an object has been addressed.1,2 The backflow has been studied mainly in terms of the Ericksen-Leslie macroscopic continuum theory. [6][7][8] There are also different methods to study the effect of backflow such as the method used in Ref. 9, where the Lattice-Boltzmann simulation was used to solve the Beris-Edwards equations. 10 In Ref. 1, the continuum level mechanism of backflow has been proposed. However, to understand the detailed mechanism at the molecular level, further investigation is required. In the present work, we study the mechanism by molecular dynamics simulation. The roles of intermolecular interaction in the generation of bulk velocity are investigated.We consider the dynamics of a nematic liquid crystal confined between parallel plates. The computational domain and molecular model are shown in Fig. 1. The liquid crystal molecules are represented as ellipsoids. Without the electric field effect, the molecules interact through the Gay-Berne potential. ͑1͒Here, û i and û j are unit vectors representing the orientation of molecules i and j, respectively, and r ij is the unit vector of vector r ij ͑r ij = r ij / r ij ͒ that connects the centers of mass of molecules i and j. ͑û i , û i , r ij ͒ represents the contribution of molecular orientation on the intermolecular distance, while ͑û i , û i , r ij ͒ defines the potential well. The parameters of the potential ͑related to ͑û i , û i , r ij ͒ and ͑û i , û i , r ij ͒͒ are molecular length scale 0 , energy scale 0 , molecular aspect ratio r , energy ratio r , and the constants for the potential well and ....

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

“…The plot of bulk velocity at t * = 130 shows an S-shaped velocity profile. This confirms the generation of a bulk flow due to molecular reorientation, as demonstrated by a 2D molecular dynamics simulation [19] and simulation using macroscopic continuum approach [17] and observed in a visualization experiment [20].…”

“…For the development of microactuators and micromanipulators, the understanding of the molecular-level mechanism of backflow is essential. A study using a 2D molecular dynamics simulation [19] showed that the rotation and rearrangement of molecules under the application of an electric field introduces a local bulk velocity gradient, which plays an important role in the generation of macroscopic flow. The computed velocity profile in the 2D simulation is qualitatively in agreement with that observed in visualization experiment [20].…”

Anomalous Brownian motion of colloidal particle in a nematic environment: effect of the director fluctuations

“…Translational motion is coupled to the orientational dynamics of n(r, t ). In its turn, director reorientations induce torques and forces that cause the material to flow (the so-called backflow effect [33,41,42]) and thus modify v(r, t ). The director fluctuations establish an intrinsic memory at the scales τ d < τ < τ h , which is typically much longer than the hydrodynamic memory time of the isotropic fluid [32].…”

A Computational Predictive Approach for Controlling the Morphology of Functional Molecular Aggregates on Substrates

“…In this definition, S = 0 denotes a completely isotropic phase, whereas S = 1 a completely ordered phase . The long‐range molecular ordering in PTCDI‐C13 aggregates was analyzed by the intermolecular orientational correlation function, computed as where N is the total number of molecules, n i (r) is the number of molecules within a distance r , and r + δr from molecule i , θ , and φ are the intermolecular angular order parameters, defining the orientation between the π cores of individual PTCDI‐C13 molecules.…”