Backflow Effect Research Articles

Fast phase modulation using dual‐frequency nematic liquid crystals

Abstract— Experimental study and numerical simulations of optical phase modulators based on nematic liquid crystals characterized by frequency sign inversion of the dielectric anisotropy has been performed. The received results point out an extreme role of back‐flow effects and electro‐hydrodynamic instability, which make restrictions for creating high‐speed phase modulators with large switchable values of phase retardation. It is shown by numerical simulations that the hydrodynamic destabilization can be suppressed by decreasing the ratio of Leslie coefficients |α2/α4,5|.

Effect of backflow on the orientational and dissipation processes in Langmuir films

The numerical study of the system of hydrodynamic equations that include both director motion and fluid flow, for a number of dynamic regimes in the 4-n -pentyl- 4' -cyanobiphenyl multilayer film on the water surface has been carried out. Calculations show that the relaxation time over which the torques exerted per unit of liquid crystals volume puts the director n[over] to be normal to the air-water interface, is one order of magnitude less in the case of accounting for the backflow effect than without accounting for that effect. The role of the charged water surface potential on the orientational relaxation process in the 5CB multilayer film on the water surface also has been investigated.

Dissipation processes in Langmuir films under lateral compression

The numerical study of the system of hydrodynamic equations that include both director motion and fluid flow, for a number of dynamic regimes in the 4- n-pentyl-4′-cyanobiphenyl multilayer film on the water surface has been carried out. Calculations show that the relaxation time over which the torques exerted per unit LC's volume puts the director nˆ to be normal to the air–water interface, in one order of magnitude less in the case of accounting for the backflow effect than without accounting for that effect. The role of the charged water surface potential on the orientational relaxation process in the 5CB multilayer film on the water surface also has been investigated.

Tsunami-related deposits in temperate carbonate ramps, Sorbas Basin, southern Spain

Tsunami-related deposits occur in Upper Miocene (uppermost Tortonian–lowermost Messinian) temperate carbonates in the Sorbas Basin, SE Spain. These carbonates exhibit two distinct depositional models. At the northern margin, small, locally steepened ramps developed on an irregular palaeotopography. These ramps displayed bryozoan accumulations at the toe of submarine cliffs changing laterally to coralline algal rudstones to floatstones. A gentle homoclinal ramp extended along the southern margin of the basin. Bivalve (brachiopod/bryozoan)-rich carbonates formed in the mid-ramp, whereas coralline algal-rudstones spread over the outer-ramp, changing basinwards to packstones with planktonic foraminifers. During the tsunami event large amounts of sediment were eroded from the carbonate ramps and redeposited. Two types of tsunami deposits are intercalated in outer-ramp sediments at both margins of the basin. In the steep outer-ramps of the northern margin, some folded layers are eroded and overlain by convex upward, stratified megahummocks. In the southern ramp, an abnormal thick shell-debris bed (TSB) occurs. Distinctive sedimentary features of these tsunamites and the inferred inflow and backflow effects were controlled by different palaeotopographic profiles. At the northern margin, inflow tsunami wave(s) struck the steep ramps, causing folding of underlying beds and excavating a large, irregular erosive surface. Backflow surges filled the inflow scours with the removed sediment, producing the megahummocky sets. At the southern margin, incoming tsunami surge(s) crossed the gentle ramp and eroded the sediments, especially in the inner-ramp settings. Backwash transported part of the previously removed sediments basinwards, depositing them as a thick bioclastic bed on the outer-ramp. The triggering mechanism of the tsunamis was probably related to seismic events recorded in the adjacent Tabernas Basin as several coeval seismites.

Stress-Induced Electromigration Backflow Effect in Copper Interconnects

The electromigration threshold in copper interconnect is reported in this paper. The critical product (jL) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> is first determined for copper oxide interconnects with temperature ranging from 250degC to 350degC from package-level experiments. It is shown that the product does not significantly change in this temperature range. Then, (jL) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> was extracted for copper low-k dielectric (k=2.8) interconnects at 350degC. A larger value than that for oxide dielectric was found. Finally, a correlation between the n values from Black's model and with jL conditions was established for both dielectrics

エンジン吸気量計測における逆流補正信号処理方法

エンジン吸気量計測における逆流補正信号処理方法

Rapid switching in a dual-frequency hybrid aligned nematic liquid crystal cell

We report the optical characterization of a dual-frequency hybrid aligned nematic (HAN) liquid crystal cell driven between two states using a multiple-frequency sinusoidal pulse. The complex dynamic director structure formed during the fast switching process is resolved in unprecedented detail on a submillisecond time scale. The results reveal backflow effects and a total switching time that is substantially faster than that achievable with conventional HAN cells.

Comparative Study of Director Responses in IPS and VA Modes Based Upon Physical Properties of Practical LC Materials

ABSTRACT The director response times of pure IPS and VA modes were compared by inserting respective material constant values of practical LC mixtures into the equations based upon the Leslie-Ericksen theory. The VA mode was confirmed to be superior to the IPS mode from this aspect due to the difference of the Frank elastic constants for bend and twist. Further superiority was shown quantitatively because of the contribution of the back-flow effect in the VA mode. These results were supported by referring to the molecular theory of the physical properties of LC substances.

Influence of the backflow effect on the orientational dynamics induced by light in nematics

We investigate the dynamical phenomena induced by a circularly polarized plane wave incident perpendicularly on a homeotropically aligned nematic layer. We study theoretically the influence of the velocity field (backflow effect) on the bifurcation scenario. Whereas backflow leads to substantial quantitative changes of secondary bifurcation thresholds, the overall bifurcation scenario remains unchanged. In the regime of uniform precession of the director with large reorientation, an unanticipated spatial oscillation of the flow field across the layer is found. Quantitative comparison with experimental large-aspect ratio systems is now possible.

Modeling a tethered polymer in Poiseuille flow

We investigate the behavior of a tethered polymer in Poiseuille flow using a multiscale algorithm. The polymer, treated using molecular dynamics, is coupled to a solvent modeled by the stochastic rotation algorithm, a particle-based Navier-Stokes integrator. The expected series of morphological transitions of the polymer: sphere to distorted sphere to trumpet to stem and flower to rod are recovered, and we discuss how the polymer extension depends on the flow velocity. Backflow effects cause an effective increase in viscosity, which appears to be primarily due to the fluctuations of the free end of the polymer.

Two-dimensional micromanipulation using liquid crystals

We developed a two-dimensional micromanipulation method using liquid crystals. The combination of the backflow effect in the twisted nematic cell and the electrostatic force for the naturally charged microparticles causes the net movement. By adjusting the polarity and the duty ratio of the rectangular voltage, the particles can be driven two-dimensionally and bidirectionally. The velocity of the particles depended on the duty ratio, amplitude, and frequency of the applied voltage.

Molecular dynamics simulation study on the transient response of solvation structure during the translational diffusion of solute

The transient response function of the density profile of the solvent around a solute during the translational diffusion of the solute is formulated based on the generalized Langevin formalism. The resultant theory is applied to both neat Lennard-Jones fluids and cations in liquid water, and the response functions are obtained from the analysis of the molecular dynamics simulations. In the case of the self-diffusion of Lennard-Jones fluids, the responses of the solvation structures are in harmony with conventional pictures based on the mode-coupling theory, that is, the binary collision in the low-density fluids, the backflow effect from medium to high density fluids, and the backscatter effect in the liquids near the triple point. In the case of cations in water, the qualitative behavior is strongly dependent on the size of cations. The pictures similar to simple dense liquids are obtained for the large ion and the neutral molecule, while the solvent waters within the first solvation shell of small ions show an oscillatory response in the short-time region. In particular, the oscillation is remarkably underdumped for lithium ion. The origin of the oscillation is discussed in relation to the theoretical treatment of the translational diffusion of ions in water.

Bistable chiral tilted-homeotropic nematic liquid crystal cells

A bistable chiral tilted-homeotropic nematic cell which uses dual-frequency liquid crystal is demonstrated. This cell can be switched between the tilted homeotropic state and the twisted state. The switching mechanisms are achieved by the backflow effect together with the anisotropic properties of the dual-frequency liquid crystal material. The experimental results of this bistable cell are described explicitly.

Hydrodynamic and Brownian Fluctuations in Sedimenting Suspensions

We use a mesoscopic computer simulation method to study the interplay between hydrodynamic and Brownian fluctuations during steady-state sedimentation of hard sphere particles for Peclet numbers (Pe) ranging from 0.1-15. Even when the hydrodynamic interactions are an order of magnitude weaker than Brownian forces, they still induce backflow effects that dominate the reduction of the average sedimentation velocity with increasing particle packing fraction. Velocity fluctuations, on the other hand, begin to show nonequilibrium hydrodynamic character for Pe>1.

Lattice Boltzmann algorithm for three-dimensional liquid-crystal hydrodynamics.

We describe a lattice Boltzmann algorithm to simulate liquid-crystal hydrodynamics in three dimensions. The equations of motion are written in terms of a tensor order parameter. This allows both the isotropic and the nematic phases to be considered. Backflow effects and the hydrodynamics of topological defects are naturally included in the simulations, as are viscoelastic effects such as shear-thinning and shear-banding. We describe the implementation of velocity boundary conditions and show that the algorithm can be used to describe optical bounce in twisted nematic devices and secondary flow in sheared nematics with an imposed twist.

Anisotropic shear viscosity in nematic liquid crystals: new optical measurement method

We propose a new optical method and the experimental set-up for measuring the anisotropic shear viscosities of nematic liquid crystals (LCs). LC shear viscosities can be optimized to improve liquid crystal display (LCD) response times, e.g. in vertical aligned nematic (VAN) or bistable nematic displays (BND). In this case a strong back-flow effect essentially determines the LCD dynamic characteristics. A number of shear viscosity coefficients defines the LCD response time. The proposed method is based on the special type of a shear flow, namely, the decay flow, in the LC cell with suitably treated substrates instead of magnetic or electric field application. A linear regime of a quasi-stationary director motion induced by a pressure difference and a proper configuration of a LC cell produces decay flow conditions in the LC cell. We determine three principal shear viscosity coefficients by measuring relative time variations of the intensity of the light passed through LC cells. The shear viscosity coefficient measurements provide a new opportunity for the development of new LC mixtures with fast response times in VAN, BND and other important LCD types.

Variable optical attenuator based on polymer stabilized twisted nematic liquid crystal

A variable optical attenuator (VOA) based on polymer stabilized twisted nematic (PSTN) liquid crystal (LC) is demonstrated. Comparing with the normal twisted nematic LC-based VOA, PSTN exhibits a much faster response time. Moreover, the polymer networks effectively eliminate the backflow effect which exists in the normal TNLC cell. The attenuation mechanism of the PSTN LC was studied. Both polarization rotation and light scattering effects are found to contribute to the optical attenuation. The ratio between these two mechanisms can be adjusted by changing the polymer concentration and polymer network domain size.

Waves at the nematic-isotropic interface: the role of surface tension anisotropy, curvature elasticity, and backflow effects.

Recently, a theoretical description of waves at the nematic-isotropic interface has been proposed using a generalized dynamical Landau-Ginzburg-de Gennes theory [V. Popa-Nita and T. J. Sluckin, Phys. Rev. E 66, 041703 (2002)]. This calculation assumed an isotropic surface tension, i.e., independent of the director orientation at the interface and neglected all coupling between the director and the hydrodynamic flow. As a consequence, the director was assumed to keep a fixed orientation and do not couple with the oscillations of the interface. These assumptions are rather crude in real nematics where surface tension anisotropy may be as large as 20% and where hydrodynamic coupling with the director is known to be important. In this paper we propose to take into account these two effects: as a result, interface oscillations couple with the director field via hydrodynamic flows and backflow effects. We analyze how these phenomena change the dispersion relation. Finally, we review experiments on the nematic-isotropic interface and discuss how to measure experimentally the dispersion relation.

Spin back-flow effect in spin-polarized transport

The influence of electron correlations on spin-polarized transport is examinedwithin a phenomenological approach based on the Landau theory of Fermi liquids.The inclusion of a spin-dependent interaction in the drift terms of the Boltzmanntransport equation satisfied by the spin-dependent distribution function nσ leadsto an ‘apparent’ charge current that is different from the spin σcurrent associated with the quasiparticle momentum flow. This effect originates inthe back-flow current of opposite spins and its magnitude is proportional to theLandau phenomenological parameter that describes the angular average of theopposite-spin interaction and with the relative drift velocity of the two spinspecies.

Analytical Prediction of the Chip Back-Flow Angle in Machining With Restricted Contact Grooved Tools

This paper develops a new analytical model to predict the chip back-flow angle in machining with restricted contact grooved tools. The model is derived from a recently established universal slip-line model for machining with restricted contact cutaway tools. A comprehensive definition of the chip back-flow angle is presented first, and based on this, a quantitative analysis of the chip back-flow effect is established for a given set of cutting conditions, tool geometry, and variable tool-chip interfacial stress state. The model also predicts the cutting forces, the chip thickness, and the chip up-curl radius. A full experimental validation of the analytical predictive model involving the use of high speed filming technique is then presented for the chip back-flow angle. This validation provides a range of feasible/prevalent tool-chip interfacial frictional conditions for the given set of input conditions.