Analysis of single droplet dynamics on striped surface domains using a lattice Boltzmann method

In this paper, the behavior of a micron-scale fluid droplet on a heterogeneous surface is investigated using a two-phase lattice Boltzmann method (LBM). The two-phase LBM permits the simulation of the time dependent three-dimensional motion of a liquid droplet on solid surface patterned with hydrophobic and hydrophilic strips. A nearest-neighbor molecular interaction force is used to model the adhesive forces between the fluid and solid walls. The solid heterogeneous wall is a uniform hydrophilic substrate painted with hydrophobic strips. The model is validated by demonstrating the consistency of the simulation results with an exact solution for capillary rise and through qualitative comparison of computed dynamic contact line behavior with experimentally measured surface properties and observed surface shapes of a droplet on a heterogeneous surface. The dependence of spreading behavior on wettability, the width of hydrophobic strip, initial location of the droplet relative to the strips, and gravity is investigated. A decrease in contact angle of the liquid on a hydrophilic surface may lead to breakup of the droplet for certain substrate patterns. The simulations suggest that the present lattice Boltzmann (LB) model can be used as a reliable way to study fluidic control on heterogeneous surfaces and other wetting related subjects.     

Dynamic finite element analysis of a micro lobe pump

The design of a micro lobe pump consisting of four parts in contact is simulated with the finite element method. The contact forces, contact pressures and the stress state in the relevant parts of the micropump are obtained for one turn at full speed. The motion of the rotors results from the numerical simulation based on geometry and contact conditions. The main load is induced by the braking action of the fluid at the outer rotor. The determined maximum tensile stress remains below the fatigue limit by a factor of 1.5 for ceramic and 5.5 for metallic glass. Simulations without fluid have shown that the rotors can get jammed due to higher friction and the lack of damping.The financial support by the BMBF in form of the HGF-Strategiefonds MALVE is gratefully acknowledged. We wish also thank the company HNP for making the CAD data available.     

Effect of substrate flexibility on dynamic wetting: a finite element model

Dynamic wetting plays an important role in coating processes. In this paper, we present a new finite element formulation that can predict the effect of substrate deformation on the location of the dynamic contact line. Our model solves for the fluid-structural interactions between an elastic solid and a viscous liquid with a dynamic contact line that can move across the deformable solid surface. Surface tension forces acting at the dynamic contact line pull outwards on the substrate and cause the formation of capillary ridge. To predict the shape of the capillary ridge and motion of dynamic contact line, our model uses arbitrary Lagrangian Eulerian (ALE) mesh motion in both fluid and solid phases, because ALE decouples the motion of solid and mesh points. In dynamic wetting of rigid solids it is known that a singularity arises at the dynamic wetting line due to a double-valued velocity. The singularity is often relieved by allowing a slip in a small contact region near the dynamic contact line. Dynamic wetting on flexible substrates involves a second singularity, which arises in the solid domain due to a line force acting at the contact line. The line force singularity is relieved by distributing the force over a finite contact region. Two ALE methods of mesh motion are implemented and compared. The variation of dynamic contact line position with respect to various parameters such as downstream pressure, contact angle, capillary number and elasticity number are presented for the finite element model and compared with an analytical model.     

Numerical simulation of droplet impact on textured surfaces in a hybrid state

High Weber number drops are found to undergo a Cassie-to-Wenzel transition on hydrophobic textured surfaces. Previous studies with many-body dissipative particle dynamics (MDPD) method were based on an absolute Cassie state, with no splash. Hence, an MDPD method was developed in this paper, which could simulate drop splashes in a hybrid state. Millimeter-sized drops on hydrophobic substrates were simulated with different solid fraction, and results were compared to experimental results using high-speed photography. The numerical spread diameter, contact time, and splash amount are matched with experiments. Results showed that hydrophobic substrates with lower solid fraction possess better water repellency as compared to those with similar apparent contact angle. However, the influence of the microstructure on superhydrophobic surfaces is much less than that on hydrophobic ones, and surfaces with lower solid fraction did not have better water repellency capabilities. It is believed that the MDPD method proposed in this study can effectively predict relationship between surface topography and water repellency of a material.     

基于声表面波技术数字微流体基片间输运研究

复杂的生化分析系统往往很难集成于一个微流基片中,而按功能分别集成于两个或多个基片,为此,需要实现质荷在两基片间输运.提出了采用声表面波技术实现数字微流体在压电基片和玻璃基片间输运的新方法,它在128°YX-LiNbO3基片上光刻一个叉指换能器和一个反射栅,经功率放大器放大后频率为27.5 MHz的RF信号加到叉指换能器上,它激发的声表面波驱动其声路径上的数字微流体,使其按声传播方向快速运动,并到达与其相连接且经疏水处理的弧形聚合物表面,数字微流体由于自身重力克服表面张力作用沿弧形聚合物表面滑落到玻璃基片,实现两基片间输运.实验结果表明弧形聚合物曲率半径和微流体体积的大小影响其在两基片间输运.同时,提出了较小体积的微流体采用不相溶的油作为辅助微流体实现目标数字微流体在两基片间输运.     

Numerical study of a membrane-type micro check-valve for microfluidic applications

In this paper, we present a method to simulate membrane-type micro check-valve using finite element method with fluid structure interaction formulation. The designed micro check-valve which is analogue of semiconductor device, diode, allows the fluid to pass in forward-mode and blocks it in reverse mode. To have a better understanding on the valve design we also studied the effect of design dimensions on the valve performance. Our study shows the valve flow rate-pressure curve is similar to diode current–voltage characterization curve. A series of simulations were carried out by using two-way fully-coupled Fluid Structure Interaction (FSI) analysis. Using Arbitrary Lagrangian–Eulerian (ALE) method in combination with high viscosity region instead of solid valve-seat enables the designer to overcome simulations difficulties and reduces the amount of calculation time by not considering the contact phenomenon of the membrane and the valve seat. The results show the valve can withstand pressures of up to 11 kPa in reverse-mode regardless of membrane’s hole-size and length of chamber-inlet. Also, chamber-inlet size has high effect on valve opening threshold point in a way that increment of chamber-inlet area will reduce opening threshold point and vice versa.

     

Design and experiment of an electromagnetic levitation system for a micro mirror

In this paper, the design and characterization of a contactless electromagnetic levitation and electrostatic driven microsystem is presented, which has applications for example for large scale angle rotation micro mirrors. The proposed design can levitate a fabricated aluminum micro rotor which can incorporate a mirror and control it to rotate around the vertical axis within the range of ± 180°, which enlarges the scanning angle dramatically compared with conventional torsion micro mirrors. The rotation angle of the micro rotor is detected by the change of capacitance and controlled by the electrostatic force produced by variable capacitors. The levitation of the micro rotor is realized by utilizing electromagnetic inductions. The rotation is achieved through electrostatic forces generated by a digital controller. The hybrid system design for a micro rotor, combining magnetic and electrostatic forces is introduced. The digital control strategy is based on a PID controller with bias voltage. The detection interface circuit, which is based on frequency multiplexing, is also presented in this paper. It has been experimentally shown that the proposed design can levitate a 1.65 mm radius and 8 µm thickness aluminum micro rotor to 100 µm height with 20 MHz frequency and 0.5A p-p input current. Square and slope wave input experiments were carried out. The experimental results show that the control principal is in good agreement with the simulation models and this applies as well to the time-response performance and stability.

     

Fabrication of micro sloping structures of SU-8 by substrate penetration lithography

In this paper, three-dimensional (3D) micro sloping structures were fabricated by ordinary mask pattern and diffraction phenomenon. Especially, we fabricated the structures with SU-8 negative photoresist and substrate penetration lithography. In this method, exposure is performed arranging in order of a mask, a substrate and the SU-8 resist. There is a gap that is equal to the thickness of the substrate between resist and mask. In narrow slit of mask, resist is less exposed than usual because of Fraunhofer diffraction. The amount of exposure depends on slit width so that the height of SU-8 resist can be controlled. A 173 μm height of structure was obtained in the case of 27 μm width slit and 24.2 μm height of structure was obtained in the case of 7.4 μm width slit. By using this method, high aspect ratio 3D SU-8 structures with smooth sloping were fabricated in the length of 100–300 μm and in the height of 50–200 μm with rectangular triangle mask pattern. In the same way, there is influence of Fresnel diffraction on edge of aperture so that micro taper structures were fabricated. A lot of taper structures were fabricated by the method to make the surface repellency. The contact angle was achieved more than 160° in this study.     

Hot embossing of transparent high aspect ratio micro parts

Accurate replications of complex, high aspect ratio nano and micro structured parts are still challenging due to the comparatively high surface-volume ratio. The critical process step of automated, undistorted demoulding in high precision replication techniques like hot embossing or thermal nano imprint require good process control at elevated temperatures just below the solidification of the thermoplastic material and higher adhesive forces of the polymer part to the substrate plate than to the structured tool insert. The required increase in interfacial surface to the substrate plate is typically done by a rough substrate plate which results in milky, in-transparent residual layer. We demonstrate a process modification which keeps the advantage of precise automated demoulding and allows for replication of micro structured parts with optically transparent residual layer even for high aspect ratio structures resulting in high demoulding forces.     

A front-tracking method for computational modeling of impact and spreading of viscous droplets on solid walls

A finite-difference/front-tracking method is developed for computational modeling of impact and spreading of a viscous droplet on dry solid walls. The contact angle is specified dynamically using the empirical correlation given by Kistler (1993). The numerical method is general and can treat non-wetting, partially wetting and fully wetting cases but the focus here is placed on the partially wetting substrates. Here the method is implemented for axisymmetric problems but it is straightforward to extend it to three dimensional cases. Grid convergence of the method is demonstrated and the validity of the dynamic contact angle method is examined. The method is first tested for the spreading and relaxation of a droplet from the initial spherical shape to its final equilibrium conditions for various values of Eotvos number. Then it is applied to impact and spreading of glycerin droplets on wax and glass substrates and, the results are compared with experimental data of Sikalo et al. (2005). The numerical results are found in a good agreement with the experimental data. Finally the effects of governing non-dimensional numbers on the spreading rate, apparent contact angle and deformation of the droplet are investigated.     

Lagrangian particle model for multiphase flows

A Lagrangian particle model for multiphase multicomponent fluid flow, based on smoothed particle hydrodynamics (SPH), was developed and used to simulate the flow of an emulsion consisting of bubbles of a non-wetting liquid surrounded by a wetting liquid. In SPH simulations, fluids are represented by sets of particles that are used as discretization points to solve the Navier-Stokes fluid dynamics equations. In the multiphase multicomponent SPH model, a modified van der Waals equation of state is used to close the system of flow equations. The combination of the momentum conservation equation with the van der Waals equation of state results in a particle equation of motion in which the total force acting on each particle consists of many-body repulsive and viscous forces, two-body (particle-particle) attractive forces, and body forces such as gravitational forces. Similar to molecular dynamics, for a given fluid component the combination of repulsive and attractive forces causes phase separation. The surface tension at liquid-liquid interfaces is imposed through component dependent attractive forces. The wetting behavior of the fluids is controlled by phase dependent attractive interactions between the fluid particles and stationary particles that represent the solid phase. The dynamics of fluids away from the interface is governed by purely hydrodynamic forces. Comparison with analytical solutions for static conditions and relatively simple flows demonstrates the accuracy of the SPH model.     

旋转Timoshenko输流管道的固有频率和稳定性分析

基于Timoshenko梁模型,本文研究了旋转输流管道在自由振动状态下的流固耦合振动特性.考虑流体压力、重力、初始轴应力作用,基于Hamilton原理和欧拉角转换,推导得到了旋转Timoshenko输流管道的偏微分方程.根据Galerkin截断法将运动方程进行离散,通过求解系统的特征方程即可得到输流管一阶复频率的实部和虚部,实部代表固有频率,虚部代表能量变化.在流速较高时,研究发现必须考虑4阶及以上Galerkin截断,才能得到稳定的结果.通过与EulerBernoulli梁模型对比,验证了本文的结果正确性.研究发现针对短粗型管道,Timoshenko梁模型更加精确.此外研究了多种参数对旋转Timoshenko输流管道固有频率和振动稳定性的影响.研究结果表明质量比、流速、剪切系数对Timoshenko输流管道流固耦合振动的稳定性影响显著,而转动惯量、重力、流体压力和初始轴应力在一定程度上也会影响管道振动的频率和稳定性.转速的出现将管道频率分为两个量值,但转速并不影响系统能量变化.     

Mesoscale simulations of particulate flows with parallel distributed Lagrange multiplier technique

Fluid particulate flows are common phenomena in nature and industry. Modeling of such flows at micro and macro levels as well establishing relationships between these approaches are needed to understand properties of the particulate matter. We propose a computational technique based on the direct numerical simulation of the particulate flows. The numerical method is based on the distributed Lagrange multiplier technique following the ideas of Glowinski et al. [16] and Patankar [30]. Each particle is explicitly resolved on an Eulerian grid as a separate domain, using solid volume fractions. The fluid equations are solved through the entire computational domain, however, Lagrange multiplier constrains are applied inside the particle domain such that the fluid within any volume associated with a solid particle moves as an incompressible rigid body. Mutual forces for the fluid-particle interactions are internal to the system. Particles interact with the fluid via fluid dynamic equations, resulting in implicit fluid-rigid body coupling relations that produce realistic fluid flow around the particles (i.e., no-slip boundary conditions). The particle-particle interactions are implemented using explicit force-displacement interactions for frictional inelastic particles similar to the DEM method of Cundall et al. [10] with some modifications using a volume of an overlapping region as an input to the contact forces. The method is flexible enough to handle arbitrary particle shapes and size distributions. A parallel implementation of the method is based on the SAMRAI (Structured Adaptive Mesh Refinement Application Infrastructure) library, which allows handling of large amounts of rigid particles and enables local grid refinement. Accuracy and convergence of the presented method has been tested against known solutions for a falling particle as well as by examining fluid flows through stationary particle beds (periodic and cubic packing). To evaluate code performance and validate particle contact physics algorithm, we performed simulations of a representative experiment conducted at the U.C. Berkeley Thermal Hydraulic Lab for pebble flow through a narrow opening.     

Contact line characteristics of liquid–gas interfaces over grooved surfaces

Surface wetting is an important phenomenon in many industrial processes including micro- and nanofluidics. The wetting characteristics depend on the surface tension forces at the three-phase contact line and can be altered by introducing patterned groove structures. This study investigates the effect of the grooves on the transition in the wetting behavior between the Cassie to Wenzel regimes. The experiments demonstrate that the wettability on a patterned surface depends on the spacing factor (S = channel depth/channel width). The spacing factor influences the contact angle, contact angle hysteresis, and the transition characteristics between the Cassie and Wenzel states. It was noted that under certain conditions (S > 1) the droplet behaved as a Cassie droplet, while exhibiting Wenzel wetting the rest of the time on the silicon microchannels tested. This criterion was used to design the groove structures on the sidewall of the proton exchange membrane fuel cell gas channel to remove the water effectively. The water coming from the land region into the gas channel is pulled by the grooves to the top wall where the airflow aided in its removal. Also, the contact angles measured on the surfaces were compared with the classical models that use wetted area, and the contact line model that uses the three-phase contact line length. It was found in our experiments that the contact line model predicts the contact angle on the patterned groove surfaces more accurately than the classical models.     

Euler–Lagrange coupling with damping effects: Application to slamming problems

During a high velocity impact of a structure on a nearly incompressible fluid, impulse loads with high-pressure peaks occur. This physical phenomenon called ‘slamming’ is a concern in shipbuilding industry because of the possibility of hull damage. Shipbuilding companies have carried out several studies on slamming modeling using FEM software with added mass techniques to represent fluid effects. In the added mass method inertia effects of the fluid are not taken into account and are only valid when the deadrise angle is small. This paper presents the prediction of the local high pressure load on a rigid wedge impacting a free surface, where the fluid is represented by solving Navier–Stokes equations with an Eulerian or ALE formulation. The fluid–structure interaction is simulated using a coupling algorithm; the fluid is treated on a fixed or moving mesh using an ALE formulation and the structure on a deformable mesh using a Lagrangian formulation.A new coupling algorithm is developed in the paper. The coupling algorithm computes the coupling forces at the fluid–structure interface. These forces are added to the fluid and structure nodal forces, where fluid and structure are solved using an explicit finite element formulation. Predicting the local pressure peak on the structure requires an accurate fluid–structure interaction algorithm. The Euler–Lagrange coupling algorithm presented in this paper uses a penalty based formulation similar to penalty contact in Lagrangian analyses. Both penalty coupling and penalty contact can generate high frequency oscillations due to the nearly incompressible nature of the fluid. In this paper, a damping force based on the relative velocity of the fluid and the structure is introduced to smooth out non-physical high frequency oscillations induced by the penalty springs in the coupling algorithm.     

微机械陀螺的刻蚀误差特性分析

微陀螺仪结构上的腐蚀凹槽或腐蚀腔可以由深层反应离子刻蚀技术得到,加工过程中存在的刻蚀误差对微陀螺的固有频率、输出精度和稳定性有重要的影响.采用有限元分析软件ANSYS建立了一种梳状微机械陀螺的有限元分析模型,采用解析的方法并通过Matlab数学软件进行仿真,研究了由于加工误差导致微梁过度刻蚀对微陀螺驱动模态、检测模态、固有频率、带宽、灵敏度的影响.结果表明,微梁刚度和微陀螺固有频率随着刻蚀角度的增大而增大;最大过度刻蚀角度为±2度时,其驱动模态和检测模态的固有频率的变化率均超过了14%;刻蚀误差会导致微陀螺工作模态降阶,以及干扰模态介于与驱动和检测模态之间且与驱动模态频率相近,这会严重影响微陀螺的输出精度;带宽随过度刻蚀夹角增大而减小,灵敏度随过度刻蚀夹角的变化而发生不规律变化;当刻蚀角度介于0°~1.5°时,微陀螺的灵敏度将高于无刻蚀误差时微陀螺的灵敏度.     

Prediction of part orientation error tolerance of a robotic gripper

This paper presents a model which predicts the part orientation error tolerance of a three-fingered robotic gripper. The concept of “self-alignment” is introduced, where the gripper uses the grasping process to bring the workpiece into its final state of orientation. The gripper and part are represented mathematically, and initial contact locations upon grasp closure determined. This information is used to solve for the contact forces present, and criteria are developed to determine if beneficial part motion resulting in self-alignment is expected. The results are visualized via a boundary projected on a reference plane below the part. The model is validated experimentally with a number of part configurations with favorable results. This method presents a useful tool by which the mechanical designer can quantitatively predict the performance of an intuitively designed gripping system.     

Fluorescence detection in a micro flow cytometer without on-chip fibers

This paper describes a novel concept of integrated on-chip fiber free laser-induced fluorescence detection system. The poly-dimethylsiloxane (PDMS) chip was fabricated using soft lithography and was bonded with a glass substrate of 150 μm thickness that reduced the distance of channel-to-sidewall to less than 180 μm. The cells and particles detection was conducted by an external single fiber close to the glass substrate that transmitted laser light for simultaneous excitation and receipt of the emission light signals. The performance of the proposed device was demonstrated using fluorescence beads, stained white blood cells, and yeast cells. The experimental results showed the simplicity and flexibility of the proposed device configuration which can provide convenient on-chip integration interface for fast, high throughput, and low-cost laser-induced fluorescence detection micro flow cytometer.     

Design and manufacturing of micro milling tools

The actual geometrical design of micro milling tools has been adopted from macro tools, assuming that the effects during the milling process are analogical. Experience has also proved that micro tools respond to influences in a very different way than macro tools do. So it is very important to achieve a comprehensive understanding of the entire process by taking a structural, mechanical and cutting technological approach to micro milling tools in order to be able to optimize them. Oftentimes, structural details such as the rake angle and the twist angle impede further miniaturization and are impossible to achieve with conventional manufacturing techniques. This paper deals with an alternative method to manufacture structural optimized milling tools, namely Electro Discharge Machining (EDM). The present state of research already puts the deficits of the currently available tools on display. Manufacturing tolerances of ±10 μm on a micro tool are insufficient to ensure constant cutting conditions for the commonly used lateral infeed or feed per tooth of a few micrometers. Sometimes, only one cutting edge is engaged, which results in increased wear, increased cutting forces, minor surface quality and a higher risk of milling cutter breakage. That is why a single-edged micro milling tool has been developed. It guarantees clear adjustment of the process parameters, feed per edge and lateral infeed. For that purpose, stability analyses of simple stylus geometries have been conducted by means of FEM simulations. Corresponding to the results of the simulation the geometry of this tool was optimized in several steps. The resulting tool with a diameter down to 30 μm was machined on the EDM-machine (Sarix SX 100) at the wbk—Institute of Production Science. Initial tests have been carried out and showed the ability of these tools to optimize cutting process.