Meniscus and viscous forces during normal separation of liquid-mediated contacts

Menisci form between two solid surfaces with the presence of an ultra-thin liquid film. Meniscus and viscous forces contribute to an adhesive force when two surfaces are separated. The adhesive force can be very large and can result in high friction, stiction and possibly high wear. The situation may become more pronounced when the contacting surfaces are ultra-smooth and the normal load is small, as is common for micro-/nanodevices. In this study, equations for meniscus and viscous forces during separation of two flat surfaces, and a sphere and a flat surface, are developed, and the corresponding adhesive forces contributed by these two types of forces are examined. The geometric meniscus curvatures and break point are theoretically determined, and the role of meniscus and viscous forces is evaluated during separation. The influence of separation distance, liquid thickness, meniscus area, separation time, liquid properties and contact angles are analyzed. Critical meniscus areas at which transition in the dominance of meniscus to viscous forces occurs for different given conditions, i.e.?various initial liquid thicknesses, contact angles and designated separation time, are identified. The analysis provides a fundamental understanding of the physics of separation process, and insights into the relationships between meniscus and viscous forces. It is also valuable for the design of the interface for various devices.     

在制氧机开车状态下处理导叶阀电机故障

由于进口导叶阀电机突然损坏 ,冷水机组出口水温升高 ,使制氧工况恶化 ,在制氧机不能停机的状况下 ,处理了这一起故障 ,使制氧机得以连续运行。     

Meniscus and viscous forces during separation of hydrophilic and hydrophobic surfaces with liquid-mediated contacts

Adhesion due to the formation of meniscus bridges has been of interest since the early 20th century. Extensive studies have been carried out analytically and numerically. Adhesive or repulsive forces contributed by meniscus and adhesive viscous forces can be significant and become one of the main reliability issues when the contacting surfaces are smooth and/or when the normal load is small, as is common for micro/nanodevices. Previous numerical studies mainly focus on static meniscus analysis for hydrophilic surfaces. More recently, analysis of meniscus and viscous forces during separation of both hydrophilic and hydrophobic surfaces with symmetric and asymmetric contact angles have been carried out. These studies are useful to understand the relative roles of meniscus and viscous forces during the separation process. In this paper, a comprehensive review of analytical and numerical modeling of the meniscus and viscous forces are presented. The analyses for both forces during normal and tangential separation of hydrophilic and hydrophobic smooth or rough surfaces with symmetric and asymmetric contact angles, and viscous forces during tangential separation are presented. The analyses provide a fundamental understanding of the physics of the separation process and insight into the relationships between meniscus and viscous forces. Implications of these analyses in macro/micro/nanotechnologies are discussed.     

主动混合式砂轮-电主轴系统自动平衡装置研究综述

高速磨削过程中,砂轮-电主轴的不平衡影响工件精度和主轴寿命。因此,自动平衡装置在高速精密加工场合应用广泛。主动混合式自动平衡头是当今应用最多的一类自动平衡装置,阐述了其动平衡原理,并对其按结构特点进行了分类介绍。最后,对自动平衡装置未来发展作了展望,提出了平衡头的设计要求     

随机激励作用下硬涂层薄板振动有限元分析及减振预估

硬涂层减振是一项新兴的阻尼减振技术,硬涂层复合结构在随机激励载荷作用下的振动特性建模与分析方法还未开展研究.将有限元建模方法和虚拟激励法(pseudo excitation method,PEM)相结合,实现了随机激励作用下硬涂层悬臂薄板结构振动建模及减振性能预估.首先,基于等效单层法创建了双面涂敷硬涂层的薄板结构振动...     

A comparison of spectrum estimation techniques for nonstationary signals in induction motor drive measurements

This paper compares different spectrum estimation techniques applied to nonstationary signals in order to determine which is the most suitable in analyzing the spectral harmonic content of signals in a very close frequency band. The comparison examines a specific application in which spectral analysis is applied to nonstationary signals, such as currents or voltages in induction motor drives. A test signal and some performance parameters are defined. Experimental tests were carried out by applying different spectral analysis algorithms in order to compare their behavior in detecting defined harmonic frequencies. The results show that the chirp-Z transform outperforms other techniques especially when a restricted frequency band has to be analyzed.     

FEM analysis to predict development of structure during extrusion and subsequent solution soak cycle

Abstract

Materials which form the surface and subcutaneous layers of an extrudate experience large deformations when they traverse the die land, which, added to the inhomogeneous caused by the dead metal zone, leads to considerable modifications to the deformation parameters when compared with the remainder of the extrusion. The distribution of structure is therefore greatly inhomogeneous. Reference to both empirical and physical models of the recrystallisation process indicates that nucleation and growth will differ at these locations in those alloys that are usually solution treated and aged subsequent to the deformation process. Since static recrystallisation (SRX) has a significant influence on many of the properties of the extrudate, it is therefore essential to provide the methodology to predict these variations. In the present work, a physical model based on dislocation density, subgrain size and misorientation is modified and integrated into the commercial FEM codes, FORGE2 and FORGE3 to study the changes of the microstructure. Axisymmetrical and shape extrusion are presented as examples. The evolution of the substructure influencing SRX is studied. The metallurgical behaviours of axisymmetric extrusion and that of shape extrusion are compared. The predicted results show an agreement with the experimental measurement. The distribution of equivalent strain, temperature compensated strain rate and temperatures is also presented to aid in interpretation. Importantly the properties of hard alloys improve with the increase in the temperature of the extrusion. This phenomenon is discussed and theoretically justified.     

Robust统计在主动声纳分裂波束目标方位走向估计中的应用

主动声纳通常采用分裂波束双通道互谱法进行目标方位估计，但信号处理过程中目标回波脉冲长度往往和信号处理帧周期长度不匹配，或者在整个信号处理帧周期的信噪比不一致，造成瞬时方位估计目标走向算法具有较大的误差，不利于目标参量估计与分类。采用Robust统计对瞬时方位估计数据进行处理，能够有效降低目标方位走向估计的误差。本文通过对Huber估计进行了分析并提出了改进Huber估计，给出了其工程实现算法，实验验证该Robust统计在降低目标方位走向估计误差上具有稳健收敛的性能。     

Influence of friction and adhesion on the onset of plasticity during normal loading of spherical contacts

Increasing contact loading causes early transformation from elastic to elastic–plastic deformations in many conventional systems as well as micro/nano-electro-mechanical systems. The load required for yielding and the location of the onset of plasticity is critical in the robustness of systems with contacts. For frictionless (such as fully-lubricated) contacts, inception of plastic yielding occurs beneath the contact surface. However, frictional slip (contact shear) and adhesion push the inception of plastic yielding toward the contact surface. The influence of elastic mismatch, shear tractions and adhesive normal tractions on the subsurface stress field is studied analytically by superposition of the Hertzian stress field and the stress field created by the shear and additional (due to adhesion) normal tractions. Specifically, three contact conditions have been studied in this work: (i) frictionless, (ii) finite friction, and (iii) infinite friction (full stick). Also, a finite-element model is developed to verify certain assumptions in the analytical solution for the contact with finite friction. The results obtained are applied to two sets of in situ nanoindentation experiments to explain the change in the yielding behavior of submicrometer polycrystalline aluminum grains.     

Preparation of highly conductive adhesives by in situ generated and sintered silver nanoparticles during curing process

The lower resistivity (7.5 × 10⁻⁵ Ω cm) of nano-electrically conductive adhesives (nano-ECAs) with silver flakes and in situ formed and sintered silver nanoparticles was developed. At room temperature,the silver nanoparticles (Ag NPs) could not be generated in ECAs due to no reaction between silver nitrate and N,N-dimethyl-4-aminobenzaldehyde (DABA). However, during curing process, Ag NPs were immediately generated through reducing silver nitrate by DABA in absence of stabilizing agents. At the same time, the increased viscosity of epoxy due to the curing could prevent the agglomerates of Ag NPs. Morphology studies showed that most Ag NPs have been attached onto the surfaces of silver flakes due to the good affinity between them, resulting in more effectively interconnecting with silver flakes by the sintered Ag NPs. Thus, the lower bulk resistivity was obtained. On the other hand, DABA, containing a tertiary amine, can stabilize contact resistance of nano-ECAs by effectively preventing galvanic corrosion at the interface between nano-ECAs and Sn surfaces due to the fact that amines can strongly bond to a Sn surface.     

Detection of damage in fibre reinforced plastics using thermal fields generated during resonant vibration

The presence of damage in fibre reinforced plastics results in the formation of cracks and crazes. When cyclic stresses are applied to a damaged composite material, relative motion (damping) takes place between the sides of the assorted cracks, resulting in the generation of heat. The change in the overall level of damping in a structure is small for many forms of serious but localised damage, while the damping in this small, local area may be large.By using resonant vibration it was possible to achieve sufficiently high cyclic stresses without using large and heavy testing machines, such as are needed for non-resonant testing. Also, by using higher frequencies than can be achieved with these large machines, the cyclic stresses did not cause propagation of the damage, so that the test was truly non-destructive.     

High-energy radiation generated during electric explosion of a conductor in a longitudinal magnetic field

Our recent experiments [1] with the electric explosion of conductors in vacuum with an applied strong longitudinal magnetic field showed that this process was accompanied by radiation capable of penetrating through metal layers of considerable thickness. The high intensity of this radiation allowed it to be reliably detected outside the vacuum chamber using a photographic film with an amplifying luminescent (ZnS) screen situated in a paper envelope at a distance above 3 m from the region of explosion. In the same series of experiments, it was established that elements of the setup situated in the vicinity of the explosion exhibited residual radioactivity. This radioactivity was manifested, in particular, by flare spots on a photographic film in contact with an amplifying phosphor screen contained in a paper cassette that was placed inside the solenoid gap upon the discharge. No such explosion-induced radioactivity was observed in analogous experiments performed without an applied magnetic field. This paper presents the first results of investigations of the induced radioactivity, in which the activation detector was a gold ring situated at a distance of about 0.2 m from the axis of the exploding conductor.     

涡旋真空泵运行工况流场的计算机模拟研究

在相关试验的基础上,通过分析软件ANSYS和C++语言编程对涡旋真空泵工作过程的流场进行了模拟分析,获得了涡旋真空泵运行工况下个月牙形封闭压缩腔内压力场分布和排气口速度场分布情况.相关结果表明,所进行的分析和研究基本上可以反映涡旋真空泵运行过程中流场分布及变化情况,从而为其设计参数及相关因素的确定提供有效的分析手段.     

施工与运营初期厚表土立井井壁受力实测研究

采用井筒内外井壁现场埋设传感器的方法,建立由计算机控制的自动监测系统,实测大屯矿区某混合井井壁施工过程及之后一段时间的受力和变形演变规律。对实测结果进行分析,研究内外井壁的综合变形规律,可以预测井壁的长期安全性,为井壁破裂的预防和治理提供科学决策的依据。     

Self-adjustment of Young's modulus in biomedical titanium alloys during orthopaedic operation

In spinal fixation devices, the Young's modulus of the metallic implant rod should be not only sufficiently low to prevent stress shielding for the patient but also sufficiently high to suppress springback for the surgeon. This paper proposes a novel function of biomedical titanium alloys—self-adjustment of Young's modulus. Deformation-induced ω phase transformation was introduced into β-type titanium alloys so that the Young's modulus of only the deformed part would increase during operation, while that of the non-deformed part would remain low. The Young's modulus increase by deformation was investigated for a binary Ti-12Cr alloy. This alloy successfully underwent deformation-induced ω phase transformation and exhibited the increase in the Young's modulus by deformation.     

Far infrared radiation generated during femtosecond laser excitation of a semiconductor surface in magnetic field

The effect of a magnetic field on the generation of electromagnetic radiation pulses in the terahertz frequency range from a semiconductor surface excited by an ultrashort laser pulse is considered within the framework of a hydrodynamic model. The appearance of a photocurrent component in the Hall direction leads to elliptic polarization of the microwave radiation and to a severalfold increase in the generation efficiency. This is consistent with the results of Monte Carlo modeling of a self-consistent field and photogenerated carrier dynamics.     

An analysis of the forces and bending moments generated during the notched beam impact test

An analysis is presented which predicts the hammer force and specimen bending moment-time responses during impact from the dynamics of the system and the fracture characteristics of the material. The theoretical predictions are compared to available experimental observations and good agreement is obtained for brittle materials.

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Zusammenfassung Es wurde eine Analyse präsentiert, die die Hamerkraft and die Reaktion des Moments der Modelbiegung während des Aufschlages von der Dynamik des Systems and den Bruchmerkmalen des Materials bestimmte. Theoretische Vorausbestimmungen wurden mit vorhandenen experimentellen Beobachtungen verglichen and gute Übereinstimmung wurde für brüchige Materiale erhalten.

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Résumé L'analyse qui est présentée permet de prédire, à partir de la dynamique du système et des caractéristiques de rupture du matériau, l'évolution dans le temps de la force du mouton et du moment de flexion dans l'éprouvette.On compare ces predictions théoriques avec les observations recueillies au, cours d'essais, et l'ón conclut à un accord satisfaisant dans le cas des matériaux fragiles.

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Notation a Notch depth - A _g Cross sectional area of gross section - A Beam cross sectional area - B ₁ Portion of the boundary over which displacements are prescribed - B ₂ Portion of the boundary over which tractions are prescribed bBeam thickness - d Unbroken ligament depth - E Young's modulus - F Hammer force - Fi Body force vector - fi - G Shear modulus - G _I Mode I fracture toughness - h Beam half height - H Heaviside unit step function - I Cross sectional moment of inertia about the c.g. - I _g Moment of inertia of the gross section - k ₂ Constant in the Hertz theory of contact forces - k ₄ Constant in the assumed linear contact theory LBeam length - M Beading moment - m ₁ Hammer mass - m ₂ Anvil mass - p Applied load per unit length on the top surface of the beam; also the Laplace transform parameter - R Bounded region of Euclidean 3-space - T Prescribed surface traction on B ₂ - û _i Prescribed displacement vector on B ₁ - u _i Displacement vector - u _y Beam deflection referred to the embedded coordinates x _i - V Shear force - V _i Hammer velocity referred to the fixed coordinates X _i - V ₂ Rigid body beam velocity referred to the fixed coordinates X _i - V ₀₁ V ₀₂ Initial hammer or rigid body beam velocity     

Predicted modifications in the direct and indirect gaps of Ge

The charge density of Ge was studied at various k-points and for various bands, by the ab initio pseudopotential method, using additionally the interstitial sites. The lowest X_c conduction-band points were found to be unique in having a high charge density in the interstitial site. It has been therefore predicted and verified that the X_c points move up in energy relative to the Γ_c point when closed-shell atoms (like H) are substituted at the interstitial sites. The calculations also indicate the change of the band-gap for HGeH.