Opposite Substrates Research Articles

Reconfiguration of Nematic Disclinations in Plane-Parallel Confinements

We study numerically the reconfiguration process of colliding m=1/2 strength disclinations in an achiral nematic liquid crystal (NLC). A Landau–de Gennes approach in terms of tensor nematic-order parameters is used. Initially, different pairs m1,m2 of parallel wedge disclination lines connecting opposite substrates confining the NLC in a plane-parallel cell of a thickness h are imposed: {1/2,1/2}, {−1/2,−1/2} and {−1/2,1/2}. The collisions are imposed by the relative rotation of the azimuthal angle θ of the substrates that strongly pin the defect end points. Pairs {1/2,1/2} and {−1/2,−1/2} “rewire” at the critical angle θc1=3π4 in all cases studied. On the other hand, two qualitatively different scenarios are observed for {−1/2,1/2}. In the thinner film regime h<hc, the disclinations rewire at θc2=5π4. The rewiring process is mediated by an additional chargeless loop nucleated in the middle of the cell. In the regime h>hc, the colliding disclinations at θc2 reconfigure into boojum-like twist disclinations.

Robust Tipless Positioning Device for Near-Field Investigations: Press and Roll Scan (PROscan).

Scanning probe microscopes scan and manipulate a sharp tip in the immediate vicinity of a sample surface. The limited bandwidth of the feedback mechanism used for stabilizing the separation between the tip and the sample makes the fragile nanoscopic tip very susceptible to mechanical instabilities. We propose, demonstrate, and characterize an alternative device based on bulging a thin substrate against a second substrate and rolling them with respect to each other. We showcase the power of this method by placing gold nanoparticles and semiconductor quantum dots on the two opposite substrates and positioning them with nanometer precision to enhance the fluorescence intensity and emission rate. Furthermore, we exhibit the passive mechanical stability of the system over more than 1 h. Our design concept finds applications in a variety of other scientific and technological contexts, where nanoscopic features have to be positioned and kept near contact with each other.

How the spatial correlation in adhesion properties influences the performance of secondary bonding of laminated composites

Spatial heterogeneity of adhesion properties is known to result in bridging during debonding of secondary bonded composite joints. The ligaments that bridge both substrates have crack-arrest features and thus significantly enhance the fracture resistance of joints. We have investigated the effect of randomly distributed adhesion properties between the adhesive layer and each composite substrate in previous works; however, the spatial correlation of adhesion heterogeneity within or between the substrates and how it effects the failure of secondary bonded composites is still poorly understood. In this current work, we assume that the spatial heterogeneity of adhesion follows a log-normal distribution. The Napierian logarithm of interface toughness Gc and separating strength S can thus be described by a Gaussian Process. We investigate in detail the effect of spatial correlation both within each substrate and between opposite substrates. Finally, we predict the crack resistance of such joints with adhesives of different mechanical properties. We find that the failure strain of adhesives is important through varying the elongation of ligaments and thus the associated extra dissipated energy. This work represents the first critical step for designing tougher secondary bonded composite joints by triggering and controlling adhesive ligament bridging.

Near-field focusing sensor for characterization of void content in thin dielectric layers

A sensor concept is developed and analyzed for in situ characterization of a thin dielectric layer. An array of long, planar electrodes is flush-mounted into opposing faces of two substrates on either side of the dielectric layer. The substrates are oriented such that the lengthwise dimensions of the opposing electrodes are orthogonal. Capacitance is measured between single electrode pairs on opposite substrates while all other electrodes are grounded. The electric field between the active electrodes is sharply focused at their crossing point, resulting in high sensitivity to void content in a square detection zone of the dielectric layer. For a fixed interfacial gap size, direct proportionality of the capacitance with void fraction within the detection zone is poor for high electrode-to-electrode spacing on the substrates, but improves dramatically as this spacing is reduced. Three methods of deriving a simulation-based sensitivity response of measured capacitance to any arbitrary two-dimensional void geometry are investigated. The best method requires data from simulations of an empty air gap and a TIM-filled gap, and uses a reduced-order superposition technique to predict the normalized capacitance value obtained for any void geometry to within 10% of that predicted by a high-fidelity direct simulation. The sensing technique is demonstrated using manually introduced voids of 250 µm–2000 µm diameter in a 254 µm thick interface material layer with a dielectric constant of 4.7. The relationship of the capacitance to the void fraction is shown to fall within the predicted bounds.

Optical properties of hybrid aligned nematic cells with different pretilt angles.

The phase retardation difference, ΔΦ, is calculated for hybrid liquid crystal (LC) cells as a function of LC pretilt angles, θ0(1), θ0(2), on the opposite substrates of the cell for the case of an arbitrary angle of light incidence in the range from 0 to 90°. An LC director configuration is suggested for its application in optical compensators. Design and fabrication methods of hybrid aligned nematic (HAN) cells with an arbitrary LC pretilt angle are described. The LC pretilt angle is measured in the HAN cells with a given planar or vertical LC alignment on one of the substrates.

P‐123: Oblique Light Incidence onto Hybrid Aligned Nematic (HAN) Cells for Optical Compensators

AbstractDesign and fabrication methods of HAN cells with arbitrary LC pretilt angle value are described. Phase retardation △Φ values for the hybrid LC vs. the LC pretilt angles θ0(1), θ0(2) on the opposite cell's substrates are calculated for arbitrary light incidence angle. LC director configuration is suggested for its application in optical compensators. LC pretilt angle value is measured in the HAN cells with given planar or vertical LC alignment on one of the substrates.

Simple and direct measurements of pretilt angles in hybrid-aligned nematic liquid-crystal cells

We have developed a general approach to perform direct measurements of the pretilt angles from 0° to 75° in hybrid-aligned nematic (HAN) liquid-crystal cells whose cell gaps can also be accurately determined with the help of known pretilt angles. In this paper, we have used a Zeeman laser system to measure the angular-dependence phase retardation of the HAN cells and MATLAB mathematical software to carry out theoretical calculations and fit the measured data to derive the pretilt angles. In general, pretilt angles adjacent to opposite substrates of a HAN cell are different. Our measured pretilt angles of the HAN cell were in good agreement with the measured pretilt angles of two accompanying homogenous cells whose alignment methods were the same as applied to opposite substrates of the HAN cell, respectively. The advantage of direct measurement is easily applicable to measure the pretilt angles of aged HAN cells.

Oxidation of Germanium and Silicon surfaces (100): a comparative study through DFT methodology

Density Functional Theory calculations are used to map out the preferential oxygen molecule adsorption sites and oxygen atom incorporation on germanium (100) surface. A comparison with primary oxidation mechanisms encountered in pure silicon and silicon germanium (100) surfaces is presented here. This study highlights opposite substrates behaviors facing oxygen molecule adsorption: 1/ surface germanium atoms move from their crystalline positions to adapt to the approaching oxygen molecule resulting in adsorbed peroxide bridge configuration, whereas oxygen molecule is fully dissociated in strand configuration on a silicon surface 2/ oxygen atoms tend to avoid each other on germanium surface whereas oxide nucleus can be observed on silicon surface even at the early steps of the oxidation process. Results show that germanium surface appears to be less reactive than the silicon substrate towards molecular oxygen species.

Frustration of nanoconfined liquid crystals due to hybrid substrate anchoring

We present Monte Carlo simulations of liquid-crystalline material confined to a nanoscopic slit-pore. The simulations are carried out under isothermal conditions in a specialized isostress ensemble in which N fluid molecules are exposed to a compressional stress τ‖ acting on the fluid in directions parallel with the substrate planes. Fluid-fluid and fluid-substrate interactions are modelled as in our previous work (M. Greschek et al., Soft Matter, 2010, 6, 1898). We study several anchoring mechanisms at the solid substrate by introducing an anchoring function 0 ≤ g(u) ≤ 1 that depends on the orientation u of a fluid molecule relative to the substrate plane; g(u) “switches” the fluid-substrate attraction on or off. Here we focus on various heterogeneous (i.e., hybrid) anchoring scenarios imposing different anchoring functions at the opposite substrates. As in our previous study we compute the isostress heat capacity which allows us to identify states at which the confined fluid undergoes a structural transformation. The isotropic-nematic transformation turns out to be nearly independent of the specific anchoring scenario. This is because the director in the nematic phase assumes a direction that is a compromise between the directions enforced by the competing anchoring scenarios at either substrate. On the contrary, at lower compressional stresses molecules prealign near the slit-pore's midplane in specific directions that depend on details of the anchoring scenario.

Anisotropic Self-Diffusion in Nanofluidic Structures

By means of equilibrium molecular dynamics simulations, we investigate self-diffusion in a “simple” fluid confined to nanoscopic slit-pores. The pore walls are decorated with wettable and nonwettable chemical “stripes” that alternate in the x direction and are assumed infinitely long in the y direction. We consider the impact of pore width as well as variations of the width of the wettable stripes dwet. Depending on these model parameters and the thermodynamic conditions, the confined fluid may exist as one of three morphologically distinct phases: a thin fluid film adsorbed by the wettable stripes, a fluid bridge spanning the gap between the (aligned) stripes on the two opposite substrates, or a nanostructured liquid where molecules occupy the entire space between the substrate surfaces. By analyzing mean square displacements, velocity autocorrelation functions, and their power spectra, a detailed picture of mass transport and its relation to substrate decoration emerges. In particular, we find that the...

Charge control of the water monolayer/Pd interface

Using an ab initio electrochemical approach, we show that the water monolayer on Pd(1 1 1) undergoes a first order transition as the surface charge is tuned, with a phase equilibrium between two modified ice structures on opposite charged substrates. These findings refine the classical H-down and H-up configurations by invoking a permanent charged surface. We explore the experimental consequences of this phase behaviour on scanning tunneling microscopy (STM) with STM simulations. A strong dependence of the topographic contrast with surface charge (or equivalently STM potential) is exhibited allowing to reinterprete recent experimental observations.

Thin Single Crystals of Organic Insulators, Metals, and Superconductors by Confined Electrocrystallization

The confined electrocrystallization technique, first described by Thakur et al., involves forcing electrocrystallization to occur within the interface of two opposite substrates. This technique is further demonstrated and extended herein to the synthesis of a variety of thin crystalline films of conducting and insulating molecular materials alike, regardless of their structural dimensionality and shape, form, or habit of the corresponding bulk phases grown independently by standard electrocrystallization in solution. Given that in a confined environment the molecular species will progress much more slowly, or barely at all, to the vicinity of the active electrode, the deposition of neutral π-donor molecules in the form of a 1 μm thick layer on the surface of a glass slide, by sublimation under vacuum, proved to be a major experimental improvement, and also yielded better crystal quality in more reasonable amounts of time. The effect of temperature and the typically high current densities commonly achieved under these conditions, are discussed along with the experimental conditions used to grow thin crystals of (TMTTF)2ReO4 and (TMTSF)2ReO4, κ-(BEDT–TTF)2Cu(NCS)2 (25 °C), (BEDT–TTF)Cu2(NCS)3 (60 °C), (EDT–TTF)3[PW12O40]·(CH3CN)· (CH2Cl–CHCl2), β″-(BEDT–TTF)4·(guest)·[Re6Se6Cl8] and β″-(BEDT–TTF)4·(guest)·[Mo6Cl14]. It is concluded that the confined electrocrystallization technique appears to be suitable for growing thin crystals of radical ion salts of essentially all the phases previously obtained in bulk crystal form by classical electrocrystallization in solution.

Influence of the Alignment Process on the Switching of High Contrast Antiferroelectric Liquid Crystal Displays

In this paper we investigate the causes of poor alignment in antiferroelectric liquid crystal devices making use of spin coated and rubbed polyimide. We found that poor alignment is due to the deviation of the layer normal from the rubbing direction. By optimising the skew between the rubbing direction on opposite substrates we found that a high quality and stable alignment can be obtained. Quantitative experimental results showing a static extinction ratio higher than 3000:1 and dynamic contrast ratio higher than 2000:1 are presented.

Thermomechanical properties of confined fluids exposed to a shear strain

The properties of a molecularly thin film of spherically symmetric molecules confined to a chemically heterogeneous slit-pore were investigated by Monte Carlo simulations in a grand mixed stress-strain ensemble. The slit-pore comprises two identical plane-parallel solid substrates, each of which consists of alternating strips of solid of two types: strongly adsorbing (width ds) and weakly adsorbing. Under favourable thermodynamic conditions the confined film consists of fluid bridges - that is, a high(er)-density fluid over the strongly attractive strip surrounded by a low(er)-density fluid supported by the (outer) weakly attractive strips. By misaligning the opposite substrates, bridge phases can be exposed to a shear strain sx (0½, sx the side length of the simulation cell) and the associated shear stress Tzx of (fluidic) bridge phases can be calculated from molecular expressions. The stress curve Tzx( sx) is qualitatively similar to the one characteristic of solidlike films confined between atomically structured substrates in that the initial response to small shear strains is Hookean, and this is followed by an increasingly nonlinear regime up to the yield point where Tzx( sx) assumes its maximum. We also investigated the influence of chemical corrugation cr: = ds/sx on Tzx( sx). With increasing cr, yield strain and stress increase at first up to a maximum and decline thereafter. By employing the theory of corresponding states, Tzx( sx) is renormalized by yield stress and strain such that the results can be represented uniquely by a master curve independent of any system-dependent parameters.

Structure of a hard-sphere fluid in hard wedges

We investigate local structural properties of a hard-sphere fluid exposed to periodic arrays of parallel hard wedges by means of grand canonical ensemble Monte Carlo simulations. The corrugated substrate is characterized by the dihedral angle $\ensuremath{\gamma}$ of the grooves, the angle $2\ensuremath{\pi}\ensuremath{-}\ensuremath{\gamma}$ of the tips, and the lateral periodicity length ${s}_{x}$ in the $x$ direction; $\ensuremath{\gamma}$ is varied over the range $\ensuremath{\pi}/2<~\ensuremath{\gamma}<~\ensuremath{\pi}$ including a planar wall $(\ensuremath{\gamma}=\ensuremath{\pi})$ as a special case. In the second lateral direction $y$ periodic boundary conditions are used, whereas confinement in the normal direction $z$ is accomplished by two opposite substrates sufficiently far apart. We analyze the ordering of the fluid within the grooves and at the tips, respectively, in terms of the number density $\ensuremath{\rho}(x,z;\ensuremath{\gamma})$. The crossover between these two regions exhibits pronounced density oscillations. From the density distribution we extract the excess coverage $\ensuremath{\Gamma}(\ensuremath{\gamma})$, which up to now has been known only for $\ensuremath{\gamma}=\ensuremath{\pi}$; in this special case we find excellent agreement with previous work. $\ensuremath{\Gamma}(\ensuremath{\gamma})$ is composed of surface and line contribution whose relative magnitude permits us to quantify corrugation effects vs planar confinement.

Analysis of close-paced brush-fiber interfaces for spacecraft thermal management

A new interpenetrating-brush thermal interface is analyzed and evaluated for application to space-based thermal management. The interface consists of fibers attached to oppositely facing substrates. Each fiber is attached to one or the other substrate but not both. Heat transfer occurs through conduction and radiant exchange between interpenetrating fibers of opposite substrates. Linearization of the radiation terms results in an analytical temperature distribution along the fibers. The thermal performance of the interface is expressed in terms of a thermal effectiveness and an effective conductance-length ratio, both of which depend on the radiation/conduction number. For thin enough fibers, the heat transfer approaches that of the case in which all fibers are attached to both substrates. A fiber length (parameter-dependent) for optimum heat transfer is identified and agrees with previously published results. Such interfaces offer both high thermal effectiveness and compliance and are well suited for internal and external spacecraft applications. Nomenclature A = total jc-sectional area of fibers on one side, m2 Ah = base (substrate) area, m2 a = single fiber (fin) cross-sectional area, m2 d = fiber diameter, m £ = effectiveness factor F = view factor T = transfer function h = unit surface conductance, W/m2 • K K '= thermal conductance, kA/Ld, W/K