Polydimethylsiloxane material as hydrophobic and insulating layer in electrowetting-on-dielectric systems

Open and closed electrowetting-on-dielectric (EWOD) systems based on a spin coated polydimethylsiloxane (PDMS) layer are presented. The PDMS layer acts as both insulation and hydrophobic material. Characterization, through sessile drop experiments, shows the hydrophobic behaviors of the PDMS and saturation of the contact angle at negative bias voltage applied to the droplet. This behavior is ascribed to trapped carrier in the PDMS layer and explains the movement of the droplet toward the grounded electrode found in the EWOD experiments.An electronic board controls all the signals needed for the actuation and sensing functionalities of the EWOD systems. Detection of drop position along the electrode array is successfully achieved by implementing the time-constant method, which evaluates the variation of electrode capacitance induced by the droplet presence on the PDMS surface corresponding to the metal electrode.The microfluidic operations (movement, dispensing and splitting) in both open and closed configurations have been verified and accomplished at voltages around 200 V.     

介质上电润湿液滴输运、合并及振荡实验研究

利用MEMS技术制作了平板电润湿芯片,建立了可视化实验研究平台,运用高速CCD对分裂式电极施加交流电压后的液滴传输和合并过程以及相邻电极接地式芯片上液滴的交流振荡过程进行了可视化实验研究。除了对液滴形状的观测外,还重点关注了接触线的运动规律。研究发现,接触角滞后现象将导致接触线在液滴输运、合并以及振荡过程中出现停顿现象,在液滴振荡过程中,只有施加电压足够大时,才能克服这种迟滞现象。当液滴初始位置偏离电极对中心时,在一定条件下交流电作用将造成液滴形态左右摇摆、接触线非对称运动的非对称振荡现象。     

Optimization of the dielectric layer for electrowetting on dielectric

In this work the minimum actuation voltage V_min for droplet actuation with electrowetting on dielectric (EWOD) is analyzed. At first, the theoretical background of drop transport with EWOD is shown. Then the impact of thin dielectric films deposited with atomic layer deposition (ALD) and super-hydrophobic layers on the minimum required actuation voltage for drop transport are presented. To this, contact angles are measured on both Al₂O₃ and fluorinated DLC each with a drop of water. For verify the hypothesis, the analytically calculated values for the minimum actuation voltage V_min are compared with numerical simulation results using COMSOL Multiphysics^®. The results show that the actuation voltage value from the simulation is lower than calculated analytically.     

Droplet Energy Harvesting Is Reverse Phenomenon of Electrowetting on Dielectric

Electric energy is generated when water droplets slide down electrodes coated with a hydrophobic dielectric layer. The principle of energy generation needs to be clarified for the optimization and scalable design of the energy-harvesting system. In this study, it is shown that droplet energy harvesting is the reverse phenomenon of voltage-driven droplet actuation or electrowetting-on-dielectric (EWOD). For this reverse EWOD, the interfacial energy difference generated between the three-phase contact line of the advancing and receding part of the droplet is the cause of the generation of electric energy. To prove the effect of interfacial energy on energy harvesting, the wetting property of the dielectric layer is controlled by nanopatterning while maintaining the chemical properties. The width and gap of the electrodes and the droplet size determine whether the harvested voltage waveform is monophasic or biphasic. The energy conversion efficiency is determined by the wetting properties of the surface, and the maximum value is as high as 40%.     

基于红外光强的数字微流控系统定位

基于介电润湿(EWOD)效应的数字微流控(DMF)系统是一种能够实现多个独立的微液滴控制的新兴技术。该技术具有芯片结构简单、控制方法易于实现、样品消耗小和无流体运动死区等优点。为了满足DMF系统液滴精准驱动与控制的迫切需求,提出了一种基于红外光强的DMF系统定位方法,该方法可以在驱动控制液滴的同时,实现无输入参数的液滴精准定位。进一步基于该定位方法,实现了DMF系统的反馈控制。最终实现了一套具备液滴驱动、定位和反馈功能的DMF控制系统。实验结果表明提出的方案有利于提高DMF系统应用的成功率和可靠性。在同样的芯片参数结构下,成功率提高了约38%。同时设计的控制系统可以为其他研究基于EWOD的DMF系统的研究团队提供一个可靠的自动化实验平台。     

Decreasing the Saturated Contact Angle in Electrowetting‐on‐Dielectrics by Controlling the Charge Trapping at Liquid–Solid Interfaces

Contact angle saturation in electrowetting‐on‐dielectrics (EWOD) has restricted the tuning range of the wettability of a solid surface, which has generally limited the performances of EWOD devices such as digital microfluidics, lab‐on‐chip, electronic displays, and so forth. Here, a method is proposed for decreasing the saturated contact angle by controlling the behavior of charge trapping at the liquid–solid interface. An unexpected phenomenon is uncovered: for a short time the contact angle reaches smaller values before it retreats to its saturation value, which is caused by charge trapping at the liquid–solid interface. Experimental results suggest that the trapped charges can be repeatedly detrapped and retrapped when the polarity of the applied voltage is periodically reversed, which results in contact angles retreating periodically. By applying a well‐modulated voltage signal with reversing polarity, the retreat movement of the contact angle can be controlled to obtain a value significantly smaller than the previously possible threshold. As a specific application of this method, an economic two‐step process which is developed potentially suitable for mass‐producing large‐area flexible microlens arrays with controllable curvatures and wide fields of view.     

Nanograssed Micropyramidal Architectures for Continuous Dropwise Condensation

Engineering the dropwise condensation of water on surfaces is critical in a wide range of applications from thermal management (e.g. heat pipes, chip cooling etc.) to water harvesting technologies. Surfaces that enable both efficient droplet nucleation and droplet self‐removal (i.e. droplet departure) are essential to accomplish successful dropwise condensation. However it is extremely challenging to design such surfaces. This is because droplet nucleation requires a wettable surface while droplet departure necessitates a super‐hydrophobic surface. Here we report that these conflicting requirements can be satisfied using a hierarchical (multiscale) nanograssed micropyramid architecture that yield a gobal superhydrophobicity as well as locally wettable nucleation sites, allowing for ?65% increase in the drop number density and ?450% increase in the drop self‐removal volume as compared to a superhydrophobic surface with nanostructures alone. Further we find that synergistic co‐operation between the hierarchical structures contributes directly to a continuous process of nucleation, coalescence, departure, and re‐nucleation enabling sustained dropwise condensation over prolonged periods. Exploiting such multiscale coupling effects can open up novel and exciting vistas in surface engineering leading to optimal condensation surfaces for high performance electronics cooling and water condenser systems.     

单片机在表面粗糙度测量系统中的应用

设计了一种以单片机和光纤传感器为基础构成的表面粗糙度测量系统。该系统由光纤传感器、信号检测电路、单片机处理电路和显示装置组成。其特点是系统性能稳定、适应性强、精度高,可以对不同加工方法的工件实现非接触和在线测量。重点给出了该系统的主要硬件和软件及实现方法。     

The effect of thermal constriction on heat management in a microelectronic application

Thermal contact constriction between a chip and a heat sink assembly of a microelectronic application is investigated in order to access the thermal performance. The finite element model (FEM) of the electronic device developed using ANSYS software was analysed while the micro-contact and micro-gap thermal resistances were numerically analysed by the use of MATLAB. In addition, the effects of four major factors (contact pressure, micro-hardness, root-mean-squared (RMS) surface roughness, and mean absolute surface slope) on thermal contact resistance were investigated. Two lead-free solders (SAC305 and SAC405) were used as thermal interface materials in this study to bridge the interface created between a chip and a heat sink. The results from this research showed that an increase in three of the factors reduces thermal contact resistance while the reverse is the case for RMS surface roughness. In addition, the use of SAC305 and SAC405 resulted in a temperature drop across the microelectronic device. These results might aid engineers to produce products with less RMS surface roughness thereby improving thermal efficiency of the microelectronic application.     

Electrically Adjustable,Super Adhesive Force of a Superhydrophobic Aligned MnO2 Nanotube Membrane

A superhydrophobic membrane of MnO₂ nanotube arrays on which a water droplet can be immobilized by application of a small DC bias, despite a large contact angle, is reported. For a 3 μL water droplet, the measured adhesive force increases monotonically with increasing negative voltage, reaching a maximum of 130 μN at 22 V, 25 times the original value. It follows that the nearly spherical water droplet can be controllably pinned on the substrate, even if the substrate is turned upside down. Moreover, the electrically adjustable adhesion is strongly polarity‐dependent: only a five‐fold increase is found when a positive bias of 22 V is applied. This remarkable electrically‐controlled adhesive property is ascribed to the change in contact geometry between the water droplet and MnO₂ nanotube array, on which water droplets exhibit the different continuities of three‐phase contact line. As the modulation in this manner is in situ, fast, efficient and environmentally‐friendly, this kind of smart material with electrically adjustable adhesive properties has a wide variety of applications in biotechnology and in lab‐on‐chip devices.     

SiC晶圆背面激光退火工艺研究

降低芯片背面金属-半导体欧姆接触电阻是有效提高器件性能的方式之一。采用650 V SiC肖特基势垒二极管(SBD)工艺,使用波长355 nm不同能量的脉冲激光进行退火实验,利用X射线衍射(XRD)和探针台对晶圆背面镍硅合金进行测量分析,得出最佳能量为3.6 J/cm²。退火后采用扫描电子显微镜(SEM)观察晶圆背面碳团簇,针对背面的碳团簇问题,在Ar;气氛下对晶圆进行了表面处理,使用SEM和探针台分别对两组样品的表面形貌和电压-电流特性进行了对比分析。实验结果表明,通过表面处理可以有效降低表面的碳含量,并且使器件正向压降均值降低了6%,利用圆形传输线模型(CTLM)测得芯片的比导通电阻为9.7×10^-6Ω·cm²。器件性能和均匀性都得到提高。     

Accurate Magneto-Driven Multi-Dimensional Droplet Manipulation

Droplet manipulation has gradually drawn worldwide attention through diverse potential applications such as microfluidics, and medical diagnostic test. Whereas, the high-precision liquid manipulation on an open surface that is under control at will is still a huge challenge, especially in 3D. Herein, the novel magnetic micropillars array (MMA) is developed for multi-dimensional droplet manipulation, depending on huge symmetric bending deformation under the low magnetic field. In situ observation demonstrated the droplet's behavior and the driving force acted on the droplet is derived from these micropillar's deformation. Two modes, that are, propelling mode and rolling mode are found in horizontal transport that determined by the relative position of crest and droplets and can be transported with excellent accuracy and rapidity. The recombination of the contact liquid between droplets and micropillars occurs in swinging to dynamically adjust the length of the three-phase contact line, which is the main reason for capture-release behavior. Theoretical models of multi-dimensional droplet manipulation are systematically established to demonstrate the underlying mechanism. Finally, several MMA-based operating platforms are built to validate its feasibility in accurate 3D droplet manipulation and exhibit great potential in chemical micro-reactions, bioassays, and the medical field.     

A dynamic model for predicting the motion of solder droplets during assembly

The dynamic motion of a solder droplet during assembly is a complex, unsteady, free surface problem involving surface tension and viscous effects. The motion of the droplet is coupled with the motion of the component or chip to be assembled and involves dynamic contact lines. A methodology based on a non-uniform rational b-spline (NURBS) discretization has been developed for the dynamic analysis of the droplet motion. A surface energy based formulation has been developed to incorporate the surface tension effects. The developed methodology leads to an updated Lagrangian scheme with a Galerkin in space and Least square in time formulation. The NURBS representation used for the spatial discretization enables the method to handle problems involving complex droplet geometries. The ability of the NURBS representation to provide both global and local control, along with the least square method used in this methodology, enables us to develop an unconditionally stable time integration scheme which can be optimized to achieve desired accuracy and numerical dissipation efficiently. A sample problem of droplet shape evolution has been solved to demonstrate the path prediction capability of the proposed methodology. In future, the method can be applied to solve various real world dynamic motion problems associated with droplets.     

Energy minimization-based analysis of electrowetting for microelectronics cooling applications

Electrowetting (EW)-induced droplet motion has been studied over the last decade in view of its promising applications in the field of microfluidics. The objective of the present work is to analyze the physics underlying two specific EW-based applications for microelectronics thermal management. The first of these involves heat absorption by liquid droplets moving on the surface of a chip under EW actuation. Droplet motion between two flat plates under the influence of an electrowetting voltage is analyzed. An energy minimization framework is employed to predict the actuation force on a droplet. This framework, in combination with semi-analytical models for the forces opposing droplet motion, is used to develop a model that predicts transient EW-induced droplet motion. The second application is targeted at hot-spot thermal management and relies on the control of droplet states on artificially structured surfaces through an applied EW voltage. The influence of an electrowetting voltage in determining and altering the state of a static droplet resting on a rough surface is analyzed. An energy minimization-based modeling approach reveals the influence of interfacial energies, surface roughness parameters and electric fields in determining the apparent contact angle of a droplet in the Cassie and Wenzel states under the influence of an EW voltage. The model is used to establish preliminary criteria to design rough surfaces for use in the hot-spot mitigation application. The concept of an electrically tunable thermal resistance switch for hot-spot cooling applications is introduced and analyzed.     

红外焦平面探测器芯片应力降低方法研究

以1280×1024红外焦平面探测器为例,利用三维可视化实体模拟软件建立了包含冷指部件、陶瓷框架、探测器芯片的三维模型,并利用ANSYS仿真软件对模型(仅球形冷台结构与常规冷台不同,其余零件均相同)进行了仿真对比.研究结果表明,球形冷台结构通过增加冷台与制冷机接触面的面积可以实现更低的芯片热应力以及更小的芯片热变形,进...     

Motion of Drops with Different Viscosities on Micro‐Nanotextured Surfaces of Varying Topography and Wetting Properties

Superhydrophobic surfaces are extensively investigated in the literature, yet the phenomenon of drop motion on such surfaces and the corresponding friction properties of surfaces with different topography are not sufficiently analyzed. Here, drop motion on hydrophobic and superhydrophobic surfaces with different size topography is investigated for drops of largely varying viscosity (i.e., water and glycerol). The threshold force required to initiate drop movement is probed, the drop motion (velocity and acceleration) is analyzed, and the friction force on each surface is calculated. It is evident that as roughness increases, the threshold force to initiate 20 µL drop motion decreases; the lowest value for water is 17.9 ± 4.0 µN. For glycerol, the lowest threshold force value is 22.3 ± 5.9 µN. The results also indicate that this threshold force required for the initiation of the drop motion seems to be higher than that when the drop starts moving. Finally, this force (being proportional to the contact line) is expected to be about half smaller for 5 µL droplets. Water drops obtain higher velocities and accelerations by an order of magnitude compared to glycerol drops, which is attributed to the combinational effect of the higher hysteresis and the larger contact line of glycerol drops.     

Printing Patterned Fine 3D Structures by Manipulating the Three Phase Contact Line

The preparation of fine 3D microstructures is an attractive issue; however, it is limited at large‐area fabrication process and fineness morphology manipulation. Here, we propose a strategy to fabricate controllable 3D structures and morphologies from one single droplet via ink‐jet printing. Based on the surface energy difference between the hydrophilic patterns and hydrophobic surface, the three phase contact line of a droplet contained nanoparticles is forced to pin on the patterned hydrophilic points and asymmetrically dewets on the hydrophobic surface, which leads to various morphologies. Through the regulation of pinning patterns and solution properties, the 3D morphology can be well manipulated. This strategy to control the 3D morphology of nanoparticle assembly based on hydrophilic patterns would be of great importance for fabricating controllable 3D structures.     

激光二极管合束模块整体散热热阻分析

半导体激光器散热是在热源至热沉之间尽可能提供一条低的热阻通路。其主要目的是降低外热阻(即激光器芯片至散热空间的热阻),使发热激光器芯片与被冷却表面之间保持一个低的温度梯度和良好的热接触。对于接触热阻冷却方法,人们往往根据自身的研究对象,用实验方法来解决接触热阻的问题。通过对单管合束模块整体热阻逐步进行分析,通过软件模拟和结合频率红移法对激光二极管热阻进行测量,得出单管合束模块整体散热热阻小于0.25 ℃/W。此散热模块可以满足百瓦级半导体激光器的散热要求。     

Fretting in copper-to-copper contacts under AC and DC currentconditions

The effect of fretting on the contact resistance behavior of copper-to-copper wire-plate combinations under AC and DC current carrying conditions was investigated. The fretting conditions were as follows: frequency 1 Hz, slip amplitude 100 μm and load 400 g (4N). The current level in both AC and DC conditions was 50 mA. In addition to contact resistance measurements, SEM and EDX were used to examine the surface damage in the fretted contact zones. The results indicate that the overall contact resistance behavior of copper-to-copper wire-plate couples subjected to the same fretting conditions but under AC and DC currents was practically the same. The characteristic feature of the samples under AC current conditions is a pronounced distortion of the waveforms of the contact voltage. The results of SEM surface analysis of the contact zones indicates that the surface damage resulting from fretting under AC current conditions was different from that under DC current conditions     

用微机串口实现对交流伺服电机的控制

交流伺服电机是自动控制系统中较为常用的部件。本文介绍了一种采用微机串口控制交流伺服电机的方法。选用LM628运动芯片，简化了系统软硬件设计。采用本方法可以低成本实现对交流伺服电机的控制。