An efficient adaptive multigrid algorithm for predicting thin film flow on surfaces containing localised topographic features

Gravity-driven continuous thin film flow over a plane, containing well-defined single and grouped topographic features, is modelled as Stokes flow using lubrication theory. The associated time-dependent, nonlinear, coupled set of governing equations is solved using a full approximation storage (FAS) multigrid algorithm by employing automatic mesh adaptivity, the power, efficiency and accuracy of which is demonstrated by comparing the results with corresponding global fine-mesh solutions. These show that automatic grid refinement effectively restricts the use of fine grids to regions of rapid flow development which, for the topographies considered, includes the topography itself, the upstream Capillary ridge, downstream surge region, and the characteristic bow wave. It is shown that for the accurate solution of the associated lubrication equations, adaptive multigrid offers increased flexibility together with a significant reduction in memory requirement. This is further demonstrated by solving the problem of transient flow over a trench topography, generated by a sinusoidally varying inlet condition.     

Thin film flow on surfaces containing arbitrary occlusions

The flow of a continuous liquid film, only several microns in thickness, on a plane surface containing occlusions, is modelled using lubrication theory. The resultant time-dependent non-linear equation set is discretised using both finite difference and finite element analogues; the former is solved using a very efficient full approximation storage (FAS) multigrid algorithm employing both automatic spatial and temporal adaptivity, the latter exploits the COMSOL suite of software to solve a weak form of the governing equations. The efficiencies and accuracies of both approaches are compared and a variety of problems, of increasing complexity, explored. Although the adaptive multigrid approach is clearly the most efficient, with the two sets of solutions indistinguishable, COMSOL offers an attractive alternative to the non-specialist user. The results, the first of their kind, further demonstrate: (i) the power of using adaptive multigriding, in time as well as space, for the simultaneous control of both spatial and temporal errors; (ii) the significant reduction in memory requirements and CPU time that can be achieved. It is revealed that occlusions lead to many of the features inherent in the flow of thin liquid films over fully submerged micro-scale topographic features; namely, the presence of capillary ridges linked to “bow-wave” plus “comet-tail” free-surface disturbances.     

Fabrication flaws and reliability in MEMS thin film polycrystalline flow sensor

A micro hot wire anemometer sensor has been constructed. The process consists in depositing a thin doped polycrystalline silicon layer on silicon substrate, using a micro-machined technique. This paper discusses the reliability and the fabrication flaws of this sensor. The different steps of fabrication are oxidation, deposit, photolithography, chemical attack, ionic implantation and annealing. An additional step, allowing the release of the suspended structures, is added. With each technological process step, a certain number of problems can be met. Each of these problems can potentially give rise to a defect of the final structure. Various tests are carried out on the final structure to make a first approach of the micro flow sensor flaws.     

Fluid flow and heat transfer in the evaporating thin film region

The evaporating thin film region is an extended meniscus beyond the apparent contact line at a liquid/solid interface. Thin film evaporation plays a key role in a highly efficient heat pipe. A detailed mathematical model predicting fluid flow and heat transfer through the thin film region is developed. The model considers the effects of inertial force, disjoining pressure, surface tension, and curvature. Utilizing the order analysis, the model is simplified and can be numerically solved for the thin film profile, interfacial temperature, meniscus radius, heat flux distribution, velocity distribution, and mass flow rate in the evaporating thin film region. The prediction shows that while the inertial force can affect the thin film profile, interfacial temperature, meniscus radius, heat flux distribution, velocity distribution, and mass flow rate, in particular, near the non-evaporating region, the effect can be neglected. It is found that a maximum velocity, a maximum heat flux, and a maximum curvature exist for a given superheat, but the locations for these maximum values are different.     

Single-walled carbon nanotube network/poly composite thin film for flow sensor

A fabrication method about single-walled carbon nanotube (SWCNT) network and polydimethylsiloxane (PDMS) based composite thin film is reported, which can be used as flow sensor cell. This composite thin film is immersed in deionized water and salt solution with different flow rate tests. The morphology of SWCNTs on the surface of the composite thin film is characterized by scanning electron microscopy, revealing the SWCNTs are coated by PDMS chains. The induced voltage generates along the direction of the flowing liquid and depends significantly on the ionic concentration and flow velocity. Since the SWCNTs are fixed into PDMS matrix, the I–V curves of the composite thin film before and after several flow velocity measurements are exactly coincident, and the repeating flow-induced voltage experiment shows the composite thin film has a reliable electric characteristic and wide potential of device application.     

Correspondenceless stereo and motion: planar surfaces

It is shown that a binocular observer can recover the depth and three-dimensional motion of a rigid planar patch without using any correspondences between the left and right image frames (static) or between the successive dynamic frames (dynamic). Uniqueness and robustness issues are studied with respect to this problem and experimental results are given from the application of the theory to real images     

ReigSAC: fast discrimination of spurious keypoint correspondences on planar surfaces

Various methods were proposed to detect/match special interest points (keypoints) in images and some of them (e.g., SIFT and SURF) are among the most cited techniques in computer vision research. This paper describes an algorithm to discriminate between genuine and spurious keypoint correspondences on planar surfaces. We draw random samples of the set of correspondences, from which homographies are obtained and their principal eigenvectors extracted. Density estimation on that feature space determines the most likely true transform. Such homography feeds a cost function that gives the goodness of each keypoint correspondence. Being similar to the well-known RANSAC strategy, the key finding is that the main eigenvector of the most (genuine) homographies tends to represent a similar direction. Hence, density estimation in the eigenspace dramatically reduces the number of transforms actually evaluated to obtain reliable estimations. Our experiments were performed on hard image data sets, and pointed that the proposed approach yields effectiveness similar to the RANSAC strategy, at significantly lower computational burden, in terms of the proportion between the number of homographies generated and those that are actually evaluated.     

Maxwell stress generated long wave instabilities in a thin aqueous film under time-dependent electro-osmotic flow

In this study the dynamics and stability of thin and electrically conductive aqueous films under the influence of a time-periodic electric field are explored. With the help of analytical linear stability analysis for long wavelength disturbances, the stability threshold of the system as a function of various electrochemical parameters and transport coefficients is presented. The contributions of parameters like surface tension, disjoining pressure, electric double layer (Debye length and interfacial zeta potential), and unsteady Maxwell and viscous stresses are highlighted with the help of appropriate dimensionless groups. The physical mechanisms affecting the stability of thin films are detailed with the above-mentioned forces and parametric dependence of stability trends is discussed.     

Development of a piezoelectric lead titanate thin film process on silicon substrates by high rate gas flow sputtering

Polycrystalline lead titanate (PT) thin films in the range of 3–6 μm were crack and void free deposited on silicon substrates in a high rate gas flow sputtering process. Gas flow sputtering uses the hollow cathode effect which results into high deposition rates of about 120 nm/min. (1 1 1) Textured platinum was used as bottom electrode to assist the nucleation of PT.Material properties of the PT thin films as well as the Pt bottom electrode like topography, morphology, chemical composition, and structure are evaluated. The sputtered PT layers show clearly Perovskite traces in XRD patterns, even the (1 1 1) texture of the Pt is partial transferred. The most difficult part is to fulfil the empirical formula PbTiO₃. This problem is solved by stabilising the process parameters. It was shown that the temperature has got enormous influence at the stoichiometry.     

Temperature sensor using ferrofluid thin film

A brand new design of temperature sensor using ferrofluid thin film is proposed in this paper. When magnetic field parallel to the plane of the ferrofluid thin film is applied, magnetic chains form in the same direction of the magnetic field, which results in the suppressing of optical transmission. It is observed that the optical transmission is changed by the ambient temperature, so that temperature sensor can be constructed by measuring the transmission power of a laser. The physics and the sensitivity of the temperature sensor are also analyzed.     

Materials development for thin film actuators

In the development of microsystems, actuators are required which can be triggered in various different ways. The actuating principles to be used are magnetostriction, the inverse piezoelectric effect, the shape memory effect, and the bimetallic effect. The variables triggered in these cases are magnetic fields, electric fields, or temperature changes. Thin film actuators are of special interest for the development of microsystems, as they can be easily scaled down to the m-range and as their manufacturing is compatible to microsystem fabrication processes. The common property of these materials is their ability to transform electrical into mechanical energy by the effects mentioned above. Of special interest are magnetostrictive or piezoelectric materials as they allow energy transformation in both directions. These inverse effects can therefore be employed as sensoric mechanism for mechanical sensors (e.g. for stress, pressure, torque) as well. The report contains a discussion of various PVD techniques sucessfully used for producing magnetostrictive films (TbFe, TbDyFe, SmFe), piezoelectric films (PbtiO₃, ZnO, AlN), shape memory films (TiNi, TiNiPd, TiPd) and bimetallic film composites (e.g. FeNi₂₀Mn₆-FeNi₄₂). The properties of these layers are presented and compared. Possible applications and future development are outlined.     

Diagnostic techniques for the dynamics of a thin liquid film under forced flow and evaporating conditions

Motivated by quantification of micro-hydrodynamics of a thin liquid film which is present in industrial processes, such as spray cooling, heating (e.g., boiling with the macrolayer and the microlayer), coating, cleaning, and lubrication, we use micro-conductive probes and confocal optical sensors to measure the thickness and dynamic characteristics of a liquid film on a silicon wafer surface with or without heating. The simultaneous measurement on the same liquid film shows that the two techniques are in a good agreement with respect to accuracy, but the optical sensors have a much higher acquisition rate up to 30 kHz which is more suitable for rapid process. The optical sensors are therefore used to measure the instantaneous film thickness in an isothermal flow over a silicon wafer, obtaining the film thickness profile and the interfacial wave. The dynamic thickness of an evaporating film on a horizontal silicon wafer surface is also recorded by the optical sensor for the first time. The results indicate that the critical thickness initiating film instability on the silicon wafer is around 84 μm at heat flux of ~56 kW/m². In general, the tests performed show that the confocal optical sensor is capable of measuring liquid film dynamics at various conditions, while the micro-conductive probe can be used to calibrate the optical sensor by simultaneous measurement of a film under quasi-steady state. The micro-experimental methods provide the solid platform for further investigation of the liquid film dynamics affected by physicochemical properties of the liquid and surfaces as well as thermal-hydraulic conditions.     

The effects of variable viscosity and thermal conductivity on a thin film flow over a shrinking/stretching sheet

The effects of variable viscosity and thermal conductivity on the flow and heat transfer in a laminar liquid film on a horizontal shrinking/stretching sheet are analyzed. The similarity transformation reduces the time independent boundary layer equations for momentum and thermal energy into a set of coupled ordinary differential equations. The resulting five-parameter problem is solved by the homotopy perturbation method. The results are presented graphically to interpret various physical parameters appearing in the problem.     

Reconstruction of planar surfaces behind occlusions in range images

Analysis and reconstruction of range images usually focuses on complex objects completely contained in the field of view; little attention has been devoted so far to the reconstruction of simply shaped wide areas like parts of a wall hidden behind furniture pieces in an indoor range image. The work presented in the paper is aimed at such reconstruction. First of all, the range image is partitioned based on depth discontinuities and fold edges. Next, the planes best fitting each of the regions constituting the partition of the image are determined. A third step locates potentially contiguous surfaces, while a final step reconstructs the hidden regions. The paper presents results for reconstruction of the shape of planar surfaces behind arbitrary occluding surfaces. The system proved to be effective and the reconstructed surfaces appear to be reasonable. Some examples of results are presented from the Bornholm church range images     

An analysis method for atomistic abrasion simulations featuring rough surfaces and multiple abrasive particles

We present a molecular dynamics (MD) model system to quantitatively study nanoscopic wear of rough surfaces under two-body and three-body contact conditions with multiple abrasive particles. We describe how to generate a surface with a pseudo-random Gaussian topography which is periodically replicable, and we discuss the constraints on the abrasive particles that lead to certain wear conditions. We propose a post-processing scheme which, based on advection velocity, dynamically identifies the atoms in the simulation as either part of a wear particle, the substrate, or the sheared zone in-between. This scheme is then justified from a crystallographic order point of view. We apply a distance-based contact zone identification scheme and outline a clustering algorithm which can associate each contact atom with the abrasive particle causing the respective contact zone. Finally, we show how the knowledge of each atom’s zone affiliation and a time-resolved evaluation of the substrate topography leads to a break-down of the asperity volume reduction into its components: the pit fill-up volume, the individual wear particles, the shear zone, and the sub-surface substrate compression. As an example, we analyze the time and pressure dependence of the wear volume contributions for two-body and three-body wear processes of a rough iron surface with rigid spherical and cubic abrasive particles.     

Dry-spot nucleation in thin liquid films on chemically patterned surfaces

We systematically study the influence of chemical patterning on the instability of thin liquid films induced by chemical heterogeneities on a flat, horizontal, and partially wetting substrate. We consider common geometric shapes like wedges, circles, and stripes and determine the time required for nucleation of a dry-spot as a function of film thickness, contact angle, pattern dimensions, and geometry. Moreover, we characterized the resulting liquid distribution and identified conditions that avoid the formation of residual droplets on the less wettable regions, which is usually undesirable in technological applications.     

Performance of thin film silicon MEMS on flexible plastic substrates

The fabrication and characterization of thin film silicon MEMS microbridges on flexible polyethylene terephthalate substrates are described. Surface micromachining using an aluminum sacrificial layer and a maximum processing temperature of 110 °C was used for device fabrication. These microbridges are electrostatically actuated and their deflection at resonance and at low frequencies is measured optically. Quasi-DC deflection with a quadratic dependence of the actuation voltage is observed, and resonance frequencies up to 2 MHz and quality factors of around 500 are measured in vacuum. Bending measurements are performed by subjecting these devices to tensile and compressive strain. The low frequency response (bridge deflection as a function of the applied voltage) was measured in air before bending and after every bending step. Under tensile strain, 16.6% of the devices survive the maximum bending with a radius of curvature of 1 cm, equivalent to a tensile strain 1.25%. In contrast, for compressive strain, 50% of the devices survive the bending corresponding to a radius of curvature of −0.5 cm, equivalent to a compressive strain of −2.5%. Thin film silicon microresonators on flexible plastic substrates can withstand more compressive strain than tensile.     

基于MEMS的镍薄膜加热器催化传感器研究

设计了基于微机电系统(MEMS)工艺,以镍薄膜作为加热体的新型催化传感器。通过磁控溅射沉积、刻蚀等工艺制成镍薄膜加热器,并对其进行了稳定化热处理以及防氧化聚合物涂层处理。传感器性能测试结果表明:镍薄膜加热器具有良好的抗高温氧化能力;气氛老化可缩短50%的老化时间;甲烷气体浓度与灵敏度呈良好的线性关系;对甲烷的灵敏度约为30 m V/1%Vol;传感器功耗约为150 m W,零点和灵敏度年漂移小于1. 5%LEL。