c-director relaxation around a vortex of strength +1 in free-standing smectic-C films

The relaxation of director fields in freely suspended smectic films is studied experimentally by means of polarizing microscopy, and analyzed by solving the torque balance equation under appropriate initial and boundary conditions. We consider in particular the role of anchoring conditions of the c-director at particles and defects in the film. The structure of regular relaxation patterns allows to determine the elastic anisotropy of smectic materials. The splay elastic constant can exceed the bend constant by a factor of two and more. A remarkable consequence of this anisotropy is the stick-slip-like relaxation around a central defect of topological strength s = + 1.     

Surface controlled nematic bistability

One discusses phenomenologically the statics and dynamics of surface anchoring of nematic liquid crystals. In statics, for symmetrical situations, one can induce, under the action of an external field, a surface breaking with a bifurcation between two opposite orientations of the director. In dynamics, one shows that the surface director relaxation is intrinsically fast, and accompanied by a transient surface shear flow. In thin cells with planar anchorings, filled by cyanobiphenyl materials which orient normal to the plates under a strong electric field, there are now two effects which couple the relaxation of the surface director: a static one, which favors parallel orientation and a uniform cell texture, and a transient dynamical coupling from the surface flow, which forces π-twisted texture. With suitable electric signals, one can control the surface bifurcations to give at will one of these two bistable textures. Results are presented which show that these mechanisms could lead to a new class of surface controlled, fast nematic bistable displays. Reproduced from ‘Liquid Crystals Today’ (Vol. 8, p. 1–7, 1998) by permission from Taylor and Francis.     

Influence of defect states on band gaps two-dimensional phononic crystal in the of4340 steel in an epoxy

The band structures of a new two-dimensional triangle-shaped array geometry of 4340 steel cylinders of square cross section in an epoxy resin were studied by the plane-wave expansion and supercell calculation method. The band gaps of this type of phononic crystals with different defects were calculated such as defect-free, 60 crystal linear defect states, 120 crystal linear defect states, and 180 crystal linear defect states. It was found that the band gap will emerge in different linear defects of the phononic crystals and the bandwidth of linear defect states is larger than that of the free-defect crystal by about 2.14 times within the filling fraction F = 0.1-0.85. In addition, the influence of the filling fraction on the relative width of the minimum band gap is discussed.     

缺陷态对4340钢-环氧树脂二维声子晶体带隙的影响

采用平面波展开法及平面波超元胞法研究缺陷态对4340钢-环氧树脂二维声子晶体带隙的影响. 以环氧树脂作为基底,把正方形结构排列的4340钢均匀的插入基底中,计算完整晶体、45°线缺陷态晶体、90°线缺陷态晶体、135°线缺陷态晶体、180°线缺陷态晶体的带隙. 结果表明,缺陷态的带隙宽度与完整晶体的带隙宽度相比最大可增大约31倍;对于不同的缺陷结构在F=0.1-0.9的范围内均有带隙出现且随着缺陷态角度的增大. 同时讨论带隙数目N,最低带隙相对宽度Δω/ω_g与填充率F之间的关系. 关键词： 缺陷态 带隙宽度 带隙数目     

The influence of external fields on the orientational relaxation of nematic liquid crystals

The relaxation of the director field n, the velocity field v, and the shear and normal components of the stress tensor is theoretically investigated by numerically solving the system of nonlinear hydrodynamic equations. These equations describe the director reorientation with allowance made for the field of velocities induced by the rotation of the director field. The relaxation time and the influence of the velocity field on the relaxation processes are analyzed for a number of hydrodynamic regimes, which arise in a liquid-crystal cell under the effect of external electric and magnetic fields.     

Effect of Saddle-Splay Elasticity on Stability of Disclination Rings in Nematic Liquid Crystals

In this paper, the stability of disclination ring in nematie liquid crystals is studied. In the presence of saddle-splay elasticity （characterized by k24） the disclination ring has a universal equilibrium radius. Depending on the values of the saddle-splay constant k24, the universal equilibrium radius is altered. When k24 〉 0.92k （m = 1/2） and k24 〉 0.88k （m =-1/2）, the disclination will be a point rather than a ring, where k is the Frank elastic constant in the one-constant approximation.     

Influence of defect states on band gaps in the two-dimensional phononic crystal of 4340 steel in an epoxy

The band structures of a new two-dimensional triangle-shaped array geometry of 4340 steel cylinders of square cross section into epoxy resin were studied by the plane-wave expansion and supercell calculation method. The band gaps of this type of phononic crystals with different defects were calculated such as defect-free, 60° crystal linear defect states, 120° crystal linear defect states, and 180° crystal linear defect states. It was found that the band gap will emerge in different linear defects of the phononic crystals and the bandwidth of linear defect states is larger than that of the free-defect crystal by about 2.14 times within the filling fraction F=0.1-0.85. In addition, the influence of filling fraction on the relative width of the minimum band gap is discussed.     

Anomalous viscoelastic response of nematic elastomers

We report a combined theoretical and experimental study of linear viscoelastic response in oriented monodomain nematic elastomers. The model predicts a dramatic decrease in the dynamic modulus in certain deformation geometries in an elastic medium with an independently mobile internal degree of freedom, the nematic director with its own relaxation dynamics. Dynamic mechanical measurements on monodomain nematic elastomers confirm our predictions of dependence on shear geometry and on nematic order, and also show a very substantial mechanical loss clearly associated with director relaxation.     

The influence of temperature fields on the orientational relaxation in hybrid liquid-crystal cells

The relaxation of the director field $ \hat n The relaxation of the director field , the velocity field v, and the shear and normal components of the stress tensor in a hybrid liquid-crystal cell with differently heated bounding surfaces is investigated theoretically. The system of nonlinear hydrodynamic equations that describe the director reorientation with allowance made for the field of velocities v induced by the director reorientation, on the one hand, and by the temperature gradient, on the other hand, is solved numerically. The relaxation time and the influence of the velocity field on the relaxation processes are analyzed for a number of hydrodynamic regimes arising in the hybrid liquid-crystal cell under the effect of the temperature field. Original Russian Text ? A.V. Zakharov, A.A. Vakulenko, 2008, published in Fizika Tverdogo Tela, 2008, Vol. 50, No. 3, pp. 557–564.     

Defect modes and magnetooptical activity of a one-dimensional magnetophotonic crystal

The reflection spectrum of a periodic transverse-magnetized structure containing a finite number of ferromagnet-dielectric periods has been obtained. The influence of a substitutional defect on the reflection spectrum is analyzed for normal incidence of light on the structure. It is shown that the intensity and position of the defect miniband in the bad gap of the spectrum depend on the location of the defect layer in the structure and on its type (magnetic or nonmagnetic). It is found that magnetooptical activity of the defect structure is higher than in a defect-free structure.     

Dynamical slowdown of polymers in laminar and random flows

The influence of an external flow on the relaxation dynamics of a single polymer is investigated theoretically and numerically. We show that a pronounced dynamical slowdown occurs in the vicinity of the coil-stretch transition, especially when the dependence on polymer conformation of the drag is accounted for. For the elongational flow, relaxation times are exceedingly larger than the Zimm relaxation time, resulting in the observation of conformation hysteresis. For random smooth flows, hysteresis is not present. Yet, relaxation dynamics is significantly slowed down because of the large variety of accessible polymer configurations. The implications of these results for the modeling of dilute polymer solutions in turbulent flows are addressed.     

Molecular dynamics of n-dodecylammonium chloride in aqueous solutions investigated by 2H NMR and 1H NMR relaxometry

Molecular dynamics in n-dodecylammonium chloride/water solutions for concentrations of 34 and 45 wt% was studied by 2H NMR and by 1H NMR dispersion of spin-lattice relaxation in the 2 kHz-90 MHz frequency range. The system exhibits a number of lyotropic liquid crystalline phases, which differ in symmetry and involve motions characterized by a wide frequency scale. The analysis of 2H NMR lineshapes of selectively deuterated DDACl molecules gave us an evidence for local trans-gauche conformational changes in the chains, whereas the dispersion of spin-lattice relaxation times T1 explored by fast field cycling method revealed fast local motions, translational diffusion and collective molecular dynamics of the chains. In particular, we have found that the order director fluctuation mechanism in smectic and nematic phases dominates spin-lattice relaxation below 1 MHz and that local motions and translational diffusion are responsible for the spin-lattice relaxation in the higher Larmor frequency range.     

Direct optical quantification of backflow in a 90 degrees twisted nematic cell

Optical guided mode observations of the transient director profile (optical tensor distribution) during the relaxation of a 90 degrees twisted nematic cell directly reveals backflow. In the first 6 ms of the relaxation process, after a voltage across the cell is removed, the midplane tilt of the director increases, reaching a maximum value of 101 degrees at 1.4 ms. This increase in midplane tilt is attributed to coupling between fluid flow (backflow) and director reorientation. A 270 degrees twisted state of the opposite handedness to the 90 degrees twisted state found at equilibrium is shown to exist during the backflow period. Good fits of theoretical models with experimentally determined time dependent director profiles yield the viscosity coefficients.     

Formation of a temperature gradient in a hybrid-oriented liquid-crystal cell in response to shear stress

The orientational relaxation of the director, velocity, and temperature fields in a hybrid-oriented liquid-crystal cell in response to a shear stress applied to one of the cell surfaces has been investigated. The cases of the complete and partial thermal insulation of one of the cell surfaces have been considered under the condition where a constant temperature is maintained at the other cell surface. The relaxation time and the influence of the velocity field on the reorientation of the director field have been analyzed for a number of hydrodynamic regimes arising in the liquid-crystal cell under the effect of the shear stress and the temperature gradient formed over the cross section of the liquid-crystal cell.     

In-situ transmission electron microscopy observations and molecular dynamics simulations of dislocation-defect interactions in ion-irradiated copper

An in-situ transmission electron microscopy straining technique has been used to investigate the dynamics of dislocation-defect interactions in ion-irradiated copper and the subsequent formation of defect-free channels. Defect removal frequently required interaction with multiple dislocations, although screw dislocations were more efficient at annihilating defects than edge dislocations were. The defect pinning strength was determined from the dislocation curvature prior to breakaway and exhibited values ranging from 15 to 175 MPa. Pre-existing dislocations percolated through the defect field but did not show long-range motion, indicating that they are not responsible for creating the defect-free channels and have a limited contribution to the total plasticity. Defect-free channels were associated with the movement of many dislocations, which originated from grain boundaries or regions of high stress concentration such as at a crack tip. These experimental results are compared with atomistic simulations of the interaction of partial dislocations with defects in copper and a dispersed-barrier-hardening crystal plasticity model to correlate the observations to bulk mechanical properties.     

Williams-watts dielectric relaxation: A fractal time stochastic process

Dielectric relaxation in amorphous materials is treated in a defect-diffusion model where relaxation occurs when a mobile defect, such as a vacancy, reaches a frozen-in dipole. The random motion of the defect is assumed to be governed by a fractal time stochastic process where the mean duration between defect movements is infinite. When there are many more defects than dipoles, the Williams-Watts decaying fractional exponential relaxation law is derived. The argument of the exponential is related to the number of distinct sites visited by the random walk of the defect. For the same reaction dynamics but with more traps than walkers, an algebraically decaying relaxation is found.     

N.M.R. solid echoes in systems of coupled pairs of spins-1/2

Proton solid echo transverse relaxation functions, mapped by measurement of the echo amplitude S _{y x} (t′ = τ) as a function of τ using a P _y (90°)-τ-P _x (90°)-t′ sequence, are reported for a variety of thermotropic mesogens and mesophases (see Table 1). The relationship of the shapes of this function to the structure and dynamics of the mesogen molecule and the mesophase structure is explored. A heuristic approach is adopted. The spin response for an alkyl chain moiety of a mesogen molecule in a uniaxial mesophase is considered to be the sum of the responses for an assembly of representative subsystems each of which consists of two equivalent pairs of spins-½. The response for the latter is calculated by introducing the effects of motional averaging into the expression obtained in Paper I and estimating an ordering matrix from deuterium quadrupole splittings measured on the corresponding deuteriated mesogen. This model, albeit physically unrealistic, qualitatively reproduces the salient features of the observed relaxation functions. The shape functions are found to be rather insensitive to the structure of the mesogen molecule and the type of mesophase involved, but distinguished by the morphological structure of the sample. In homogeneously aligned mesophases, the shape is mainly determined by the distribution in the intersegmental (CH₂) dipolar interactions, whilst in unaligned mesophases the shape is determined by the orientational distribution of the director. A value for M ₂ ^vv(inter-pair) can be obtained from the decay of the relaxation function of unaligned mesophases, but not for aligned ones. There is an exchange process which contributes to the damping of both the FID signal and solid echo relaxation function in unaligned smectic-A mesophases which is absent in the corresponding aligned mesophases. This process must be a consequence of the defect structure of the smectic-A phase and involve rapid diffusion of molecules between regions distinguished by different orientations of the mesophase director.     

The dynamical response to the node defect in thermally activated remagnetization of magnetic dot array

The influence of nonmagnetic central node defect on dynamical properties of regular square-shaped segment of magnetic dot array under the thermal activation is investigated via computer simulations. Using stochastic Landau–Lifshitz–Gilbert equation we simulate hysteresis and relaxation processes. The remarkable quantitative and qualitative differences between magnetic dot arrays with nonmagnetic central node defect and magnetic dot arrays without defects have been found.     

离子辐照对磷烯热导率的影响及其机制分析

通过离子辐照产生缺陷,可以非常有效地调控磷烯诸多物理性质.本文应用分子动力学方法模拟离子辐照磷烯的过程,给出了缺陷的形成概率与入射离子能量、离子种类以及离子入射角度之间的关系,并且应用非平衡态分子动力学计算辐照后磷烯热导率的变化.以缺陷形成概率为切入点,系统地研究了辐照离子的能量、辐照剂量、离子的种类以及离子的入射角度对磷烯热导率的影响.应用晶格动力学方法研究了空位缺陷对磷烯声子参与率的影响,并计算了声子局域模式的空间分布.基于量子微扰和键弛豫理论,指出空位缺陷明显降低磷烯热导率的最重要物理机制是空位缺陷附近的低配位原子对声子强烈散射.本文研究可为缺陷工程调控磷烯的热输运性质提供理论参考.     

Study on inspection method of glass defect based on phase image processing

To solve the problem in projecting grating method, the paper presents an inspection method based on phase image processing for glass defects. In the proposed method, the wrapped phase difference is achieved according to the images of defect-free and defect-containing. The unwrapping phase algorithm, based on jump corrected is used to eliminate jump error. The segmentation of defect region is implemented by integrating grayscale mathematical morphology with high-low threshold segmentation, and the boundary coordinate of connected region is used to calculate the size and location of defect. The results demonstrate that the proposed method provides reliable identification of defects.