Electric-Field Induced Morphological Transitions in Elastic Contact Instability of Soft Solid Films

A soft elastic film, when placed in adhesive proximity with a contactor in a crack-like geometry, spontaneously undergoes a surface instability to form finger patterns with a characteristic wavelength of approximately 4h, where h is the film thickness. We study the morphological evolution and control of this elastic contact instability under the influence of an external electric field. The distinct electric field induced morphological changes, leading to the formation of two-dimensional hexagonally arranged pillars, large-amplitude fingers, and straightening of contact edge, which are studied comprehensively. The conditions for the evolution of morphologically distinct patterns are governed by the film parameters, such as its shear modulus and thickness. A theoretical model and its stability analysis provide an approximate estimate of the critical voltage required for the onset of changes and its scaling with the film parameters (thickness and shear modulus). Further, three-dimensional simulations based on energy minimization are presented to provide important clues regarding the physics of pattern evolution on soft elastic interfaces.     

Soft and Hard Adhesion

The force needed to pull a cylindrical stud from a soft elastomeric film depends on their elastic and geometric properties. For a rigid stud and a thick elastomeric film, the pull-off stress (σ) depends on the elastic modulus (E) of the film and the radius (a) of the stud as σ ∼ (E/a)^1/2 (soft adhesion). However, when the film is very thin, the pull-off stress is significantly higher than the case with thick films, and its value depends on the elastic modulus and the thickness (h) of the film as σ ∼ (E/h)^1/2 (hard adhesion). Here, we study the pull-off behavior of a soft cylindrical stud, one flat end of which is coated with a high modulus thin baseplate. As the flexural rigidity of this baseplate is varied, we observe the transition between the two types of adhesion. We present a simple physical interpretation of the problem, which could be of value in understanding various biofouling and adhesive situations.     

NCD/Si3N4界面接触应力分析

本文运用有限元分析软件MSC.patran/marc对弹性接触状态下NCD/Si3N4进行了接触应力分析,研究了NCD膜弹性模量、膜厚及载荷等因素对NCD/Si3N4界面剪切应力场分布的影响.结果表明:接触状态下NCD/Si3N4最大剪切应力产生于NCD膜与Si3N4基体结合面;弹性模量对NCD膜内部剪切应力场分布、最大剪切应力值及其产生位置影响显著;膜厚影响NCD膜内部剪切应力场分布;载荷与最大剪切应力值约成线形比例关系,对剪切应力场分布无明显影响.     

Critical Confinement and Elastic Instability in Thin Solid Films

When a flexible plate is peeled off a thin and soft elastic film bonded to a rigid support, uniformly spaced fingering patterns develop along their line of contact. Although the wavelength of these patterns depends only on the thickness of the film, their amplitude varies with all material and geometric properties of the film and that of the adhering plate. Here we have analyzed this instability by the regular perturbation technique to obtain the excess deformations of the film over and above the base quantities. Furthermore, by calculating the excess energy of the system, we have shown that these excess deformations, associated with the instability, occur for films that are critically confined. We have presented two different experiments for controlling the degree of confinement: by prestretching the film and by adjusting the contact width between the film and the plate.     

Critical Confinement and Elastic Instability in Thin Solid Films

When a flexible plate is peeled off a thin and soft elastic film bonded to a rigid support, uniformly spaced fingering patterns develop along their line of contact. Although the wavelength of these patterns depends only on the thickness of the film, their amplitude varies with all material and geometric properties of the film and that of the adhering plate. Here we have analyzed this instability by the regular perturbation technique to obtain the excess deformations of the film over and above the base quantities. Furthermore, by calculating the excess energy of the system, we have shown that these excess deformations, associated with the instability, occur for films that are critically confined. We have presented two different experiments for controlling the degree of confinement: by prestretching the film and by adjusting the contact width between the film and the plate.     

Tuning of shear thickening behavior and elastic strength of polyvinylidene fluoride via doping of ZnO-graphene

ZnO rice like nonarchitects are grafted on the graphene carbon core via a rapid microwave synthesis route. The prepared grafted systems are characterized via XRD, SEM, RAMAN, and XPS to examined the structural and morphological parameters. Zinc oxide grafted graphene sheets (ZnO-G) are further doped in β-phase of polyvinylidene fluoride (PVDF) to prepare the polymer nanocomposites (PNCs) via mixed solvent approach (THF/DMF). β-phase confirmation of PVDF PNCs is done by FTIR studies. It is observed that ZnO-G filler enhances the β-phase content in the PNCs. Non-doped PVDF and PNCs are further studied for rheological behavior under the shear rate of 1–100 s⁻¹. Doping of ZnO-G dopant to the PVDF matrix changes its discontinuous shear thickening (DST) behavior to continues shear thickening behavior (CST). Hydrocluster formation and their interaction with the dopant could be the reason for this striking DST to CST behavioral change. Strain amplitude sweep (10⁻³% -10%) oscillatory test reveals that the PNCs shows extended linear viscoelastic region with high elastic modulus and lower viscous modulus. Effective shear thickening behavior and strong elastic strength of these PNCs present their candidature for various fields including mechanical and soft body armor applications.     

Experimental study of falling film evaporation heat transfer outside horizontal tubes

The heat transfer process of falling film horizontal evaporation includes evaporation outside tubes and condensation inside tubes, the heat transfer coefficient of the former is about 50% of that of the latter. So the overall heat transfer coefficient is influenced mainly by the falling film evaporation outside tubes. An experimental study of falling film heat transfer outside horizontal tubes was carried out in order to show how the heat transfer coefficient is affected by different parameters such as flow density evaporation temperatures, temperature difference between wall and saturation water, and mass concentration of the seawater. Experiments were conducted using 14 mm outer diameter Al-brass tubes heated by internal electric heaters so that a uniform heat flux was generated on the outside surface of tubes. The results show that when flow density Γ varies between 0.013 kg/ms < Γ< 0.062 kg/ms, the heat stransfer coefficient of falling film evaporation outside horizontal tubes h increases with the increase in liquid feeding, evaporation boiling temperature and heat flux. h also increases with an increase in distributor height, however there is a maximum height in which any height above this. Besides, the amount of non-condensing gas has significant effect on h. The difference of heat transfer coefficient between freshwater and seawater is small. These results contribute to further improving the performance of heat transfer process and developing new evaporator.     

粘结层对压电纤维复合材料机电响应行为的影响

本文从功能材料力-电耦合的角度分析了粘结层参数对压电纤维复合材料(macro fiber composites, MFC)机电响应行为的影响。通过有限元模拟计算发现,减少粘结层厚度及增大其介电常数有利于缓解MFC介电失配现象,提高有效电场加载,从而获得高压电性。试验制备了MFC并进行了驱动及传感性能表征,试验结果与模拟仿真一致。减少粘结层厚度和弹性模量,及增大其介电常数,能有效增大MFC尖端位移和输出电压,提高驱动和传感性能。该研究对驱动和传感用MFC的设计提供了指导。     

浮法玻璃在线镀复合膜的力学性能

用化学气相沉积法在浮法玻璃上制备了复合薄膜.用台阶仪、纳米压痕和X射线衍射分别测定了薄膜厚度、弹性模量、纳米硬度和室温时薄膜的残余应力.实验结果表明:薄膜厚度为560.7nm,弹性模量为48.33GPa,纳米硬度值为10.65GPa,计算得到薄膜的残余应力值为-0.19GPa.对膜厚、气体的配比、流量、玻璃板的厚度、玻璃基体的化学成分等因素对薄膜残余应力的影响进行了讨论.     

Biaxial elastic modulus of very thin diamond-like carbon (DLC) films

The biaxial elastic modulus of very thin diamond-like carbon (DLC) films was measured by the recently suggested free overhang method. The DLC films of thickness ranging from 33 to 1100 nm were deposited on Si wafers by radio frequency plasma-assisted chemical vapor deposition (r.f.-PACVD) or by the filtered vacuum arc (FVA) process. Because the substrate was partially removed to obtain sinusoidal free overhang of the DLC film, this method has an advantage over other methods in that the measured value is not affected by the mechanical properties of the substrate. This advantage is more significant for a very thin film deposited on a substrate with a large difference in mechanical properties. The measured biaxial elastic moduli were reasonable values as can be judged from the plane strain modulus of thick films measured by nanoindentation. The biaxial elastic modulus of the film deposited by r.f.-PACVD was 90±3 GPa and that of the film deposited by FVA process was 600±50 GPa. While the biaxial elastic modulus of the film deposited by FVA is independent of the film thickness, the film deposited by r.f.-PACVD exhibited decreased elastic modulus with decreasing film thickness when the film is thinner than 500 nm. Although the reason for the different behavior could not be clarified at the present state, differences in structural evolution during the initial stage of film growth seem to be the reason.     

Soft and Hard Adhesion

The force needed to pull a cylindrical stud from a soft elastomeric film depends on their elastic and geometric properties. For a rigid stud and a thick elastomeric film, the pull-off stress (σ) depends on the elastic modulus (E) of the film and the radius (a) of the stud as σ ～ (E/a)^1/2 (soft adhesion). However, when the film is very thin, the pull-off stress is significantly higher than the case with thick films, and its value depends on the elastic modulus and the thickness (h) of the film as σ ～ (E/h)^1/2 (hard adhesion). Here, we study the pull-off behavior of a soft cylindrical stud, one flat end of which is coated with a high modulus thin baseplate. As the flexural rigidity of this baseplate is varied, we observe the transition between the two types of adhesion. We present a simple physical interpretation of the problem, which could be of value in understanding various biofouling and adhesive situations.     

Micromechanical Characterization of Electrophoretic-Deposited Green Films

Low-load indentation experiments have been performed on electrophoretic-deposited films of SiC particles on a graphite substrate. Films with thicknesses between 60 and 300 µm prepared at two current intensities and subsequently dried under different humidities were indented with spherical indenters with nominal radii of 10, 50, and 150 µm. Force-displacement data were analyzed to determine contact pressure and elastic modulus versus depth results. The modulus and contact pressure behavior with depth exhibited opposite trends with indenter radius: the modulus increase was least for the 10 µm and greatest for the 150 µm, whereas the contact pressure was the inverse. The results may be rationalized by plotting modulus normalized to the ratio "contact radius/film thickness" ( a / t ), whereas the contact pressure results at small a / t could be normalized when plotted versus contact strain, i.e., contact radius divided by indenter radius ( a / R ). These approaches enabled the properties of the variously deposited films to be compared. Additional interesting microstructural and cracking behavior patterns are also reported.     

Surface coating by means of velocity shear instability in plasma

In this paper the effect of different parameters like magnetic field, homogenous direct-current electric field, shear scale length, temperature anisotropy, inhomogeneity in direct-current electric field and density gradient on ions velocity is discussed. A mathematical model for ions/micron size particles velocity is discussed and its values are calculated by taking experimental parameters and by applying computer technique. A model of plasma spray machine is also suggested, which contains plasma production with velocity shear instability in laboratory, powder injection and mass and momentum transfers between particles. The coating process by means of velocity shear instability in plasma has possibility to spray hard and arduous material (alloy) with minimum defects and maximum technical and economic efficiency.     

Substrate effects on the mechanical properties and contact damage of diamond-like carbon thin films

Substrate effects on the mechanical properties and contact damage of diamond-like carbon (DLC) films have been investigated. To that end, a DLC film of 1 μm thickness was deposited on two different substrates: soda–lime glass (compliant and soft), and single-crystal silicon (stiff and hard). The elastic modulus and hardness were measured by means of nanoindentation. Quasi-static and sliding contact configurations were simulated by means of ultra-micro indentation, and surface and cross-sectional damage were assessed using a focused ion beam (FIB) miller. It was found that a compliant and soft substrate enhances crack initiation on the film surface in the form of ring/cone cracks. On the other hand, a stiff and hard substrate delays crack initiation in the film, but is more susceptible to fracture in the form of median and lateral cracks due to increased brittleness. The results have implications for the reliability of DLC-coated systems.     

Tip Deflection and Blocking Force of Soft PZT-Based Cantilever RAINBOW Actuators

A piezoelectric/electrostrictive RAINBOW actuator is a monolithic bending device consisting of an electromechanically active layer and a reduced passive layer formed in a high-temperature reduction treatment. When the piezoelectric or electrostrictive layer is driven under an electric field or when the environmental temperature changes, bending deflection is produced because of the constraint of the reduced inactive layer or because of the thermal expansion coefficient difference of the two layers. In this study, general analytical expressions relating tip deflection, blocking force, and equivalent moment with an applied electric field and temperature change are derived for a cantilevered RAINBOW actuator. It is shown that optimal actuator performance can be achieved in the RAINBOW actuator by choosing a suitable thickness ratio of the reduced layer to the PZT layer. A series of RAINBOW cantilever actuators have been experimentally prepared from high-density, soft, lead zirconate titanate (PZT) ceramics. Different reduction layer thickness is obtained by adjusting the processing parameters, such as reduction temperature and time. The measured results on tip deflection and blocking force agree well with theoretical prediction under a weak electric field. However, when a high driving electric field is used, deviation is observed, which can be attributed to a nonlinear piezoelectric response and a nonlinear elastic behavior associated with soft PZT materials under high driving electric fields.     

Field emission of polycrystalline diamond films grown by microwave-plasma chemical vapor deposition. I. Effects of surface morphology of diamond

The effects of surface morphology on the field emission of non-doped polycrystalline diamond films with thicknesses ranging from 5 to 55 μm were studied. Diamond films grown by a microwave-plasma chemical vapor deposition technique had both the diamond and non-diamond components with pyramidal and angular crystalline structures. Although the average crystallite size increased with increasing the film thickness (d), the volume fraction of the non-diamond components in the films was insensitive to the film thickness. However, the turn-on electric field, F_T, (defined as the low-end electric field to emit electrons) showed a U-shape dependence on the film thickness. This U-shape dependence was explained by a model in which the emission current was controlled by Fowler–Norheim tunneling of electrons at surface of the pyramids when d was thinner than 20 μm and by carrier transport in the polycrystalline diamond film when d was thicker than 20 μm. The lowest field of 4 V/μm was obtained in the film with 20 μm thick.     

电场对竖直微槽润湿及毛细流动特性影响

竖直微槽群毛细结构广泛应用在重力热管、蒸发器等散热设备内,但受重力等因素影响易达到毛细极限。引入电场的主动强化方式来提高竖直微槽的毛细极限,并通过实验和建立数学模型研究电场对竖直微槽内液体润湿及毛细流动特性的影响。结果表明,电场可以提高竖直微槽内液体润湿高度,当电场为5.0 kV时与无电场时相比,润湿高度强化比可达到30.0%。同时,电场作用下流体在微槽道内的毛细润湿流动呈分段效应：润湿流动初期,润湿高度与时间的1/2次方呈线性关系,即h-t^1/2,润湿速率与润湿高度的倒数呈线性关系,即v-1/h;润湿流动中后期,润湿高度与时间的1/3次方呈线性关系,即h-t^1/3,润湿速率与润湿高度平方的倒数呈线性关系,即v-1/h²,且润湿速率随时间呈下降趋势。     

电场对竖直微槽润湿及毛细流动特性影响

竖直微槽群毛细结构广泛应用在重力热管、蒸发器等散热设备内,但受重力等因素影响易达到毛细极限。引入电场的主动强化方式来提高竖直微槽的毛细极限,并通过实验和建立数学模型研究电场对竖直微槽内液体润湿及毛细流动特性的影响。结果表明,电场可以提高竖直微槽内液体润湿高度,当电场为5.0 kV时与无电场时相比,润湿高度强化比可达到30.0%。同时,电场作用下流体在微槽道内的毛细润湿流动呈分段效应：润湿流动初期,润湿高度与时间的1/2次方呈线性关系,即h-t^1/2,润湿速率与润湿高度的倒数呈线性关系,即v-1/h;润湿流动中后期,润湿高度与时间的1/3次方呈线性关系,即h-t^1/3,润湿速率与润湿高度平方的倒数呈线性关系,即v-1/h²,且润湿速率随时间呈下降趋势。     

平板浸涂过程中的液膜排液影响因素

针对一端与活性剂溶液池相接的竖直平板浸涂液膜排液过程，基于润滑理论建立了液膜厚度、活性剂浓度及液膜表面速度的演化方程组，通过数值计算分析了Marangoni效应和表面黏度对液膜排液特征的影响。结果表明，Marangoni效应是影响液膜排液过程的重要因素，可导致液膜厚度增加，随Marangoni效应增强可得到更均匀的液膜。液膜表面速度随表面黏度增大而减小，较大的表面黏度将推迟表面速度逆流现象的发生，但其对膜厚均匀度几乎没有影响。     

Kinetics of Adhesion Interaction of Polyolefins with Metals Under Conditions of Contact Thermooxidation

The formation of gradients of macromolecular transformations at contact oxidation in the adhesive layer during formation of adhesive joints of pure and peroxide-containing polyolefins with steel was studied. Indirect methods were used, such as measurement of fluidity in adhesive samples, measurement of shear creep and shear modulus of a polymer melt layer for different adhesive thickness, and some others. Effective viscosity, μ, was calculated from the experimental shear creep curves as a function of the thickness, b, of a cross-linking PE melt layer for several levels of contact time. The experimentally-measured values of viscosity were regarded as the complex average values of a certain viscosity gradient. It was found that in the case of catalytically-inactive substrate (Cellophane), there was no gradient. The dependence of the shear modulus on the layer thickness was determined. The measurements of fluidity of adhesive samples scraped at different distances from the interface for various contact times confirmed that the gradient of macromolecular transformations also originated when steel was in adhesion contact with polyethylene and ethylene-vinylacetate copolymer which did not contain peroxide. In the adhesive layers near the adhesive-steel interface, the catalyzed oxidative cross-linking prevailed. Within the adhesive layers located far enough from the interface, the oxidative destruction was dominant. For a catalytically-inert substrate (Teflon), the oxidative destruction prevailed over the whole thickness of the polymer layer.