Top Free Surface Research Articles

Dynamics of Substituted Alkyl Monolayers Covalently Bonded to Silicon: A Broadband Admittance Spectroscopy Study

The relaxation dynamics of surface-bound n-alkyl chains was studied by broadband admittance spectroscopy (10 mHz–10 MHz) measured at low temperature (130–300 K) in the reverse bias regime of rectifying Hg//organic monolayer (OML)–n-doped Si tunnel junctions. To obtain molecular-level information on the structure and dynamics of grafted monolayers, carboxyl or amide dipolar moieties were located either at the top free surface with variable acid concentration (0%, 5%, and 100%) or at the inner position in the alkyl backbone (100% amide units). Two classes of dipolar relaxation mechanisms are found with different thermally activated behavior. At low T, only peak A is observed (f ≈ 102–105 Hz) with very small activation energy (EA = 20–40 meV) and pre-exponential factor (f 0A ≈ 103–106 Hz). With increasing T, peak B also appears, with higher values of activation energy, EB = 0.25–0.40 eV, and pre-exponential factor (f 0B ≈ 108–1010 Hz). The bias-independent relaxation mechanism A, with very low activation energy typical of dipole–dipole interaction, is attributed to extrinsic relaxation of adventitious H2O molecules in hydrogen-bond clusters. Mechanism B is attributed to intrinsic relaxation of the alkyl chain assembly. In the acid series, the relative intensity of peak B is consistent with the acid group coverage given by XPS, in contrast with peak A, and its activation energy reveals increased motional constraints in the acid-substituted OML. The shape of dipolar relaxation peaks, discussed in the framework of Dissado-Hill/Jonscher theories for many-body interactions, is useful to discriminate near-substrate and molecular tail relaxations through order/disorder effects.

Distribution of dissipated energy in a multi-size granular system under vertical vibration

We studied the segregation behavior of multi-size granular particles enclosed in a vertically vibrated container. In the experiment, we applied three kinds of 13X molecular sieve particles. The diameters of these particles are approximately 1:3:6 in the multi-size granular system. Under vibration, the system finally forms a stable hybrid convection mode, and the apparent tendency of the Brazilian nut (BN) effect occurs. The dissipated energy distribution model was introduced, in which the multi-size system was compared with the species competition system. We demonstrated that each kind of particle's top free surface area occupation percentage approximately equals its dissipated energy power percentage in the entire system. The results calculated according to the model well matched the experimental results.

Single Crystal Growth of Bi:Sb Alloys

The solid-liquid interface of Bi (1-x) Sb(x) crystal growth is most favorable for investigation of electron and phonon phenomena. Bismuth is a semimetal with high electron and hole mobility. Interest in Bi-Sb material system has recently been stimulated by promise of a new generation of thermoelectric materials based on these alloy. The crystals were grown using zone melting method with 1.0 and 1.5 cm/hour growth velocity and temperature gradient 650C/cm. The surface was determinal on the basis of growth feature profiles under optical microscope. The features observed on the top-free surface of as-grown crystals have also been discussed. The crystals have been characterized by using the powder XRD technique. The optical absorption was measured in the wave number range 510 cm-1 to 4000 cm-1. From the optical absorption through direct inter band transition.The results are reported and discussed in detailed.

Hydrostatic strain enhancement in laterally confined SiGe nanostripes

Strain engineering in SiGe nanostructures is fundamental for the design of optoelectronic devices at the nanoscale. Here we explore a new strategy, where SiGe structures are laterally confined by the Si substrate, to obtain high tensile strain yet avoid the use of external stressors, thus improving the scalability. Spectromicroscopy techniques, finite element method simulations, and ab initio calculations are used to investigate the strain state of laterally confined Ge-rich SiGe nanostripes. Strain information is obtained by tip-enhanced Raman spectroscopy with an unprecedented lateral resolution of \ensuremath{\sim}30 nm. The nanostripes exhibit a large tensile hydrostatic strain component, which is maximal at the center of the top free surface and becomes very small at the edges. The maximum lattice deformation is larger than the typical values of thermally relaxed Ge/Si(001) layers. This strain enhancement originates from a frustrated relaxation in the out-of-plane direction, resulting from the combination of the lateral confinement induced by the substrate side walls and the plastic relaxation of the misfit strain in the (001) plane at the SiGe/Si interface. The effect of this tensile lattice deformation at the stripe surface is probed by work function mapping, which is performed with a spatial resolution better than 100 nm using x-ray photoelectron emission microscopy. The nanostripes exhibit a positive work function shift with respect to a bulk SiGe alloy, quantitatively confirmed by electronic structure calculations of tensile-strained configurations. The present results have a potential impact on the design of optoelectronic devices at a nanometer-length scale.

The influence of a sloping bottom endwall on the linear stability in the thermally driven baroclinic annulus with a free surface

We present results of a linear stability analysis of non-axisymmetric thermally driven flows in the classical model of the rotating cylindrical gap of fluid with a horizontal temperature gradient [inner (outer) sidewall cool (warm)] and a sloping bottom endwall configuration where fluid depth increases with radius. For comparison, results of a flat-bottomed endwall case study are also discussed. In both cases, the model setup has a free top surface. The analysis is carried out numerically using a Fourier–Legendre spectral element method (in azimuth and in the meridional plane, respectively) well suited to handle the axisymmetry of the fluid container. We find significant differences between the neutral stability curve for the sloping and the flat-bottomed endwall configuration. In case of a sloping bottom endwall, the wave flow regime is extended to lower rotation rates, that is, the transition curve is shifted systematically to lower Taylor numbers. Moreover, in the sloping bottom endwall case, a sharp reversal of the instability curve is found in its upper part, that is, at large temperature differences, whereas the instability line becomes almost horizontal in the flat-bottomed endwall case. The linear onset of instability is then almost independent of the rotation rate.

Direct numerical simulation of rising bubble interaction with free surface using level contour reconstruction method

Bubble rising phenomenon is widely found in many engineering applications including stream generators in power plants. Many experimental and numerical researches have been performed to predict the dynamic behavior of the bubble rising process. Most simulations so far have focused on evolution behavior of the rising bubble itself. Rising bubbles could penetrate through the top free surface which makes the problem much more complicated in addition to the curvature effect on the interface. With top surface, satellite droplets may be generated near breakup region and entrained to rising velocity field. In this paper, the effect of top free surface on rising bubble has been numerically investigated. High-order tetra-marching level contour reconstruction method (LCRM), which is a hybridization of fronttracking and level-set methods has been used to track the gas-liquid interface explicitly. Effects of free surface on the interfacial shape and behavior after breakup are studied with different size and depth of the initial bubble. It has been found that the initial diameter of the bubble is the dominant factor controlling the satellite droplet formation.

Discrete simulation of dense flows of polyhedral grains down a rough inclined plane

The influence of grain angularity on the properties of dense flows down a rough inclined plane are investigated. Three-dimensional numerical simulations using the nonsmooth contact dynamics method are carried out with both spherical (rounded) and polyhedral (angular) grain assemblies. Both sphere and polyhedra assemblies abide by the flow start and stop laws, although much higher tilt angle values are required to trigger polyhedral grain flow. In the dense permanent flow regime, both systems show similarities in the bulk of the material (away from the top free surface and the substrate), such as uniform values of the solid fraction, inertial number and coordination number, or linear dependency of the solid fraction and effective friction coefficient with the inertial number. However, discrepancies are also observed between spherical and polyhedral particle flows. A dead (or nearly arrested) zone appears in polyhedral grain flows close to the rough bottom surface, reflected by locally concave velocity profiles, locally larger coordination number and solid fraction values, smaller inertial number values. This dead zone disappears for smooth bottom surfaces. In addition, unlike sphere assemblies, polyhedral grain assemblies exhibit significant normal stress differences, which increase close to the substrate.

Study of the competing chemical reactions in the initiation and spread of smouldering combustion in peat

Smouldering combustion of natural fuel layers such as peatlands leads to the largest fires on Earth and posses a possible positive feedback mechanism to climate change. In this paper, we use an experimental methodology to study the smouldering combustion of samples of peat under a wide range burning conditions. Vertical samples (30mm deep and 125mm in diameter) are ignited by radiation on the top free surface and the smouldering front propagates downward against a forced flow of oxidizer. By varying the oxygen concentration ([O2]) and the ignition conditions we investigate the competing pyrolysis and oxidation reactions. A reaction framework with two regimes is consistently observed. The measurements show that a char species is formed by the competing pyrolysis and oxidation reactions in the first regime resulting in net char production and in the second regime char oxidation results in conversion of the char to ash. Lower mass loss rates and the larger residual mass at lower [O2] suggest that a wider smouldering front is required to sustain combustion as the [O2] is decreased. These results improve our understanding of smouldering phenomena and the role of the competing chemical reactions.

Visualization and analysis of surface tension and cooling effects on differentially heated cavity using heatline concept

Convection in a square cavity with a free surface and heated from the side is visualized and analyzed by the heatline concept. The temperature gradient is applied along the top free surface and the heat balance at the surface is assumed to obey Newton’s law of cooling. The finite difference method has been used to solve the dimensionless governing equations. The governing parameters considered are the Marangoni number (0⩽Ma⩽1000), the Biot number (0⩽Bi⩽6) and the Prandtl number (0.054⩽Pr⩽6.2). The flows of heat and fluid are of a similar type except at a strong cooling condition. The heat transfer performance presents a minimum at Marangoni number of roughly 100, 200 and 600 for the cavity filled with liquid metal, mixture of noble gases and air, respectively.

Numerical Simulation of Taylor Couette Flow in Bioreactor

Taylor Couette flow in bioreactor at different Re values is researched numerically by means of establishing a simplified model. Numerical results indicate that vortex appears earlier in the vicinity of bottom wall than in the vicinity of top free surface. As Re number increases, vortexes generate from both ends towards middle area of flow field and will fill the whole space when critical Re value is reached. Although all vortexes look the same in structure, intensities of them are widely divergent. The strength of vortex at bottom wall is more intensive than that at top free surface, while the vortex at middle position is the weakest.

Stacking fault in Bi2Te3 and Sb2Te3 single crystals

The structural characterizations of the Bi2Te3 and Sb2Te3 single crystals grown by the Zone melting method have been carried out by XRD and Chemical Etching process. The top free surface of as-grown Bi2Te3 and Sb2Te3 single crystal were observed under optical microscope. The particle size was calculated for a number of reflections using Scherrer’s formula by X-ray diffraction method. By the use of XRD data, the growth and deformation fault probability has been estimated for the grown single crystals. The presence of stacking fault in the lattice structure of the grown crystals was verified by the probability of growth and dislocation fault. Also new dislocation etchant has been developed by the successive trial–error method. Dislocation etching was achieved on (111) crystal plane examined using a saturated solution of citric acid and nitric acid as the etchant. The dislocation etchant has been found to give reproducible etch pits on the cleavage surface for an appropriate etching time 30s. The use of citric acid and nitric acid proved to be especially advantageous for the basal plane, producing etched pits suitable for dislocation etch pit counting.

Analysis of the densification of a vibrated sand packing

Two techniques have been used to analyze the densification of silica sand by horizontal sinusoidal vibrations of frequency f = 50 Hz for relative accelerations between 0 and 6: the quantitative analysis of motion observed through the transparent wall, and the altitude map of the free top surface of the sample. The first technique was used to analyze the transient regime: during the first 10 s, slight densification occurs at the bottom of the powder bed, while the upper part enters convective motion and the intermediate part reaches densities higher than 66%. The second technique allowed to quantify the evolution of the overall density vs. acceleration Γ during the steady regime (dynamic density) and after the vibrations (relaxed density): a maximum density is observed in both cases for an optimal acceleration which depends on the initial height of the powder bed. These results are analyzed and discussed.

Contact problems for several transversely isotropic elastic layers bonded to an elastic half‐space

AbstractThe idea of generalized images, first used by the author for the case of crack problems, is applied here to solve a contact problem for n transversely isotropic elastic layers, bonded to each other and to an elastic half‐space, made of a different transversely isotropic material. A rigid punch of arbitrary shape is pressed against the top layer's free surface. The governing integral equation is derived for the case of two layers; it is mathematically equivalent to that of an electrostatic problem of an infinite row of coaxial charge disks in the shape of the domain of contact. This result is then generalized for an arbitrary number of layers. As a comparison, the method of integral transforms is also used to solve the problem. The main difference of our integral transform approach with the existing ones is in separating of our half‐space solution from the integral transform terms. It is shown that both methods lead to the same results, thus giving a new interpretation to the integral transform as a sum of an infinite series of generalized images.

Thermal effects on Rayleigh–Marangoni–Bénard instability in a system of superposed fluid and porous layers

Thermal effects of the heat transfer at free surface (represented by Biot number) on the Rayleigh–Marangoni–Bénard instability in a system of liquid–porous layers with top free surface are investigated numerically. The results indicate that this thermal effect can evidently lead to the mode transition of convection, which is overlooked in previous works.

Clay smear processes in mechanically layered sequences — Results of water-saturated model experiments with free top surface

The aim of this study is to improve our knowledge of the processes that lead to clay smear during faulting of a layered sand-clay sequence in an analogue sandbox model. We carefully characterized mechanical properties of the materials used by a series of geotechnical tests. Displacement field was quantified using PIV (Particle Image Velocimetry). The model is water-saturated to allow the deformation of wet clay and sand in one experiment comprising a sand package with a horizontal layer of clay above a predefined rigid basement fault. The thickness and rigidity of the clay layer are the parameters varied in this study. The model shows a range of structures that are related to competence contrast between sand and different clay types. Results show ductile shearing of soft clay with a transition to brittle fracturing of stiff clay accompanied by the formation of rotating clay blocks in the fault zone. Localized deformation is observed through time showing (i) the propagation of one active fault migrating laterally through the sediment package, and (ii) the formation of a stable prism between two or more active faults that gets progressively smaller with minor rotation of the hanging wall fault. Continuous clay smear is observed resulting from the lateral injection of clay as well as from a reworked mixture of sand and clay.

Application of network identification by deconvolution method to the thermal analysis of the pump-probe transient thermoreflectance signal

The paper discusses the possibility to apply network identification by deconvolution (NID) method to the analysis of the thermal transient behavior due to a laser delta pulse excitation in a pump-probe transient thermoreflectance experiment. NID is a method based on linear RC network theory using Fourier's law of heat conduction. This approach allows the extraction of the thermal time constant spectrum of the sample under study after excitation by either a step or pulse function. Furthermore, using some mathematical transformations, the method allows analyzing the detail of the heat flux path through the sample, starting from the excited top free surface, by introducing two characteristic functions: the cumulative structure function and the differential structure function. We start by a review of the theoretical background of the NID method in the case of a step function excitation and then show how this method can be adjusted to be used in the case of a delta pulse function excitation. We show how the NID method can be extended to analyze the thermal transients of many optical experiments in which the excitation function is a laser pulse. The effect of the semi-infinite substrate as well as extraction of the interface and thin film thermal resistances will be discussed.

Unsteady transitional swirling flow in the presence of a moving free surface

Unsteady incompressible viscous flows of a fluid partly enclosed in a cylindrical container with an open top surface are presented in this article. These moving free-surface flows are generated by the steady rotation of the solid bottom end wall. Such type of flows belongs to a group of recirculating lid-driven cavity flows with geometrical axisymmetry. The top surface of the cylindrical cavity is left open so that the free surface can freely deform. The Reynolds regime corresponds to unsteady transitional flows with some incursions in the fully laminar regime. The approach taken here revealed new nonaxisymmetric flow states that are investigated based on a fully three-dimensional solution of the Navier–Stokes equations for the free-surface cylindrical swirling flow without resorting to any symmetry property unlike all other results available in the literature. The results are compared with those of Bouffanais and Lo Jacono [“Transitional cylindrical swirling flow in presence of a flat free surface,” Comput. Fluids 38, 1651 (2009)] corresponding to the exact same parameters but with a flat-and-fixed top free surface. These solutions are obtained through direct numerical simulations based on a highly accurate Legendre spectral element method combined with a moving-grid technique.

Inhibiting Shear Instability Induced by Large Amplitude Internal Solitary Waves in Two‐Layer Flows with a Free Surface

We consider a strongly nonlinear long wave model for large amplitude internal waves in two‐layer flows with the top free surface. It is shown that the model suffers from the Kelvin–Helmholtz (KH) instability so that any given shear (even if arbitrarily small) between the layers makes short waves unstable. Because a jump in tangential velocity is induced when the interface is deformed, the applicability of the model to describe the dynamics of internal waves is expected to remain rather limited. To overcome this major difficulty, the model is written in terms of the horizontal velocities at the bottom and the interface, instead of the depth‐averaged velocities, which makes the system linearly stable for perturbations of arbitrary wavelengths as long as the shear does not exceed a certain critical value.

Polymerization Shrinkage Behavior of Light Cure Resin Composites in Cavities

The polymerization shrinkage of dental restorative resin composites in cavities was studied using a digital image correlation method. The cylindrical and semi-cylindrical cavities in bovine teeth were used to examine the shrinkage behavior on the top free surface and the simulated cross section, respectively. The cavities filled with the resin after spreading a bonding agent were irradiated using a visible-light curing unit and photographed with a CCD camera as a function of time. The two images photographed were correlated to study the deformation during the polymerization process. Experiments were also performed on the cavities without the bonding agent, and its effect on the bonding strength at the interface between the resin and tooth structure was examined.

Analytical and Numerical Study of Combined Effects of a Magnetic Field and an External Shear Stress on Soret Convection in a Horizontal Porous Enclosure

The fluid flow induced by combined actions of Soret effect and shear stress applied on the top horizontal free surface (the lower one being rigid) in a horizontal porous layer, under an external magnetic field, is studied analytically and numerically. The horizontal walls of the porous layer are subject to uniform heat fluxes. The porous layer is sparsely packed then the flow is governed by the Brinkman model assuming the Boussinesq approximation. The governing parameters are the thermal Rayleigh number, RT, the Lewis number, Le, the separation parameter, ϕ, the effective Darcy number, Da, the Hartmann number Ha, the dimensionless shear stress, τ, and the aspect ratio of the enclosure, Ar. An analytical solution is derived on the basis of the parallel flow approximation, assuming enlarge aspect ratio layer, and validated numerically using a finite-difference method. The critical Rayleigh numbers for the onset of stationary, subcritical, and oscillatory convection are determined explicitly as functions of the governing parameters for infinite layers with a zero shear stress, τ = 0. The codimension-2 point is identified and different flow behaviors are observed and discussed. The effects of the governing parameters on the fluid flow intensity and heat and mass transfer characteristics are also discussed.