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We performed numerical simulation to investigate the effects of wall slip on flow behaviors of Newtonian and non-Newtonian fluids in macro and micro contraction channels. The results show that the wall slip introduces different vortex growth for the flow in micro channel as compared to that in macro channel, which are qualitatively in agreement with experimental results. The effects of slip on bulk flow behaviors depend on rheological property of the fluid. For Newtonian fluid, the wall slip always reduces the vortex length, while for non-Newtonian fluid, the strength of the slip determines whether the vortex length is reduced or increased. Analyses on the velocity and stress fields confirm the channel size dependent phenomena, such as the reduction of wall shear stress with the decrease in channel size. With the increase in average shear rate, the Newtonian fluid shows the reduction of wall shear stress that increases in the same trend with slip velocity-wall shear stress function, while for non-Newtonian fluid, the effect of the slip is suppressed by shear thinning effect and, therefore, the reduction of wall shear stress is less sensitive to the change in average shear rate and slip velocity-wall shear stress function.

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 ....

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.