Shigeomi Chono

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.

Spatial development of director orientation of tumbling nematic liquid crystals in pressure-driven channel flow

The Leslie-Ericksen equations in the tumbling regime were solved for developing flow between parallel plates to study the two-dimensional spatial development of director orientation. Calculations were carried out using the physical properties of 8CB. Texture development is governed by the number of director rotations required to reach the equilibrium distribution for fully-developed flow, with complex textures developing at large values of both the tumbling parameter (e) and the Ericksen number (Er). The entry length for structure for a given value of e is a non-monotonic function of Er, and entry lengths of >40 can occur for the parameter range studied here when Er is near a value corresponding to a discontinuous jump in the number of rotations required to reach equilibrium.

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.

Numerical Simulation of Nematic Liquid Crystalline Flows Around a Circular Cylinder

Abstract Numerical solutions to the full set of partial differential equations for the Leslie-Ericksen theory are obtained for steady flows of non-tumbling nematic liquid crystals around a circular cylinder with an infinite axial length. The streamline pattern for fixed director orientation, which has been presented by Heuer et al., differs from our results, which are obtained without approximations. An upstream displacement of the streamlines is observed for small Ericksen numbers. This streamline displacement is shifted to the downstream region with increasing the Ericksen number because the effect of fluid inertia becomes large. The distinctive director orientation profile is predicted in the downstream region of a circular cylinder when the Ericksen number is large. Flow kinematics such as streamline pattern and director orientation profile change greatly between the Ericksen number of 10 and 50 for a planar anchoring configuration. For a homeotropic anchoring configuration, on the other hand, the eff...

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 120 μm/s at 175 Hz. 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...

Special Issue on Flow Phenomena of Complex Fluids

複雑流体とは,構成分子よりも遙かに大きなサイズの高次構造が内部に存在し,外場を含む周囲環境や流体自 体の流動状態によってその構造が変化する流体です.具体的には高分子流体,サスペンション,エマルション, 界面活性剤溶液,フォーム,液晶,MR/ER 流体,生体液など,身近なところで数多く存在しております.複雑流 体は一般に,ニュートンの粘性則には従わずまた弾性を有する場合もあるので,非ニュートン流体や粘弾性流体 と同義に扱われることもありますが,実際はこれらを包含する流体です.外場に反応したり,内部構造が流動状 態に反映されたりもするので,応用面から機能性流体と呼称されることもあります.近年,複雑流体の工学的重 要性が認識されてきましたが,その挙動は文字通り複雑ですので,これらを適切に活用するには科学的根拠に裏 付けされた詳細で,且つ広範な流動データの構築が求められております. 複雑流体に対する機運の高まりから,2001年から年次大会において,オーガナイズドセッション「複雑流体の 流動現象」が企画され,毎年数10件の研究発表がなされてきました.しかし当時,短時間での発表と質疑応答で は十分な理解が進まないことから,テーマを絞って徹底的に議論する場を切望する声が増えていました.そこで 2006年 4月,流体工学部門の中に「複雑流体研究会」を立ち上げ,招待講演会と大学院生や若手研究者による研 究発表会をそれぞれ年1回開催してまいりました.会員の所属は広域に渡っているにもかかわらず参加者も多く, 活発な活動がなされてきました.2010年 4月には日本機械学会 論文集 基準キーワード集に「複雑流体」が登録 されるなど,この研究会の活動と研究分野の重要性が再評価されてきたと理解しております. 2015 年度,本研究会は設立 10 年目を迎えます.この記念すべき年を迎えるにあたり,これまでの活動内容を