(大连理工大学精密与特种加工教育部重点实验室，辽宁 大连，116024)

基于微注塑成型中熔体充模流动理论,对微尺度下熔体与壁面间的传热系数对熔体充模流动影响进行了研究.在分析两相间传热机理的基础上,讨论了宏观尺度流道和微观尺度流道中传热系数取值对熔体温度分布的影响.在引入传热系数模型的基础上,对正方形截面分别为500μm、300μm和200μm的三种长方形微流道中熔体充模流动进行三维数值模拟.通过与已有实验数据相比较,验证了模型的合理性,并分析了不同的模具温度和熔体温度下,采用常数传热系数和传热系数模型得到的熔体温度分布及其随微流道特征尺度变化规律.     

聚合酶链式反应微流控生物芯片的数值模拟

微通道内具有一定流速的DNA反应混和物能否达到聚合酶链式反应(PCR)指定的温度PCR微流控芯片研究的关键问题.本文应用有限体元法(FEV)数值模拟该芯片上3个恒温区的直型、弯型、逶迤型三类微通道内,微流体的温度场和速度场.结果表明:对于宽100 μm深50μm的微通道,速度在0.002 m/s～0.02 m/s范围内,180.的弯道以及温度场、温度梯度的存在对其速度场分布无影响,微流体仍旱现为层流;微流体大约经过60μm的距离,其温度场达到稳态,其速度场充分发展为层流;采用宽4 mm深2 mm的空气隔热槽能起到隔热的效果.     

基于交流电场驱动的微流体运动特性模拟研究

为研究在交流电场驱动下微沟道中流体的运动特性,采用有限元的方法建立了非对称电极驱动流体的数学模型.微管道高为40μm,出口在交流微电极以后100μm处,通过多物理场耦合软件研究了交流微电极对数、驱动电压、频率与微沟道出口平均速度之间的关系,其中交流微电极对数、驱动电压与出口速度成正比,对于几何尺寸一定的微管道,在特定频率下可以得到最大出口速度.实例中微管道在1V电压下,最佳驱动频率为3500 Hz,可获得的最大出口速度为9.6×10-8 m/s.     

微/纳米流体控制用冰阀器件执行过程的数值模拟

冰阀是一种利用微/纳米流道内流体自身的冻结/融化相变过程来实现关断或开启作用的新型流体元件.由于无运动部件、无泄漏、不会引起流体污染且易于实现电控等优点,该器件在微/纳米流体系统的控制中具有重要的应用前景.针对冰阀的工作原理,基于相应的流体力学及传热学数学模型,从数值计算角度对微流道冰阀器件的执行过程进行了模拟,并开展了相应的参数化研究,在此基础上可对冰阀的工作状态及控制过程更好地加以理解和剖析.     

微流控阵列光开关中微流体流动特性的研究

介绍了微流控光开关的基本结构与工作原理。对微流体的性质作了分析,介绍了压力驱动流体流动的数学模型。对微流控阵列光开关中流体流动特性进行研究。利用ANSYS仿真软件,对控制单元中微流道和储液小槽的尺寸变化对微流体流速的影响进行仿真和分析,得出微流道高度及长度、储液小槽高度及半径与微流体流动速度的关系变化情况。根据仿真结果,微流道的宽度、高度和长度,以及储液小槽的高度和半径最佳状态的比例为1∶2∶4∶4∶1,此时微流体的速度与稳定性达到最佳。     

绝缘体介电电泳细胞分离器件数值分析和优化

绝缘体介电电泳(iDEP)利用微流体管道中均匀绝缘柱阵列产生介电电泳(DEP)所需要的非均匀电场,实现微全分析系统(μTAS)和芯片实验室(LOC)中细胞等生物粒子的分离.对这种新型的iDEP器件进行理论分析,建立了数值计算方法.基于有限元数值计算理论,利用Ansys软件,计算了细胞的介电电泳力,验证了iDEP的富集原理,对iDEP器件的介质形状、尺寸、间距和微管道长度等参数进行了优化,提出了设计准则.选用间距10 μm,圆形,微管道长度3550 μm的5×5绝缘柱阵列,器件性能最优.     

人字形波纹板式换热器性能数值模拟的研究

基于简化模型的计算结果难以准确描述换热器内完整的流体流动和换热特性.为此,本文建立与人字形波纹板片完全相同的,含分配区和传热区冷热双流道换热的计算模型,用计算流体力学软件Fluent 6.3,数值模拟4组不同名义速度下流体的流动和换热情况.分析流道内速度场和温度场发现,进口分配区对流体流动分布和换热都有显著影响,还将流体在流道内的流动情况详细描述.两侧流体的压降和进出口温差的计算值与实验值的误差小于6%,较准确地反映了换热器内整体的流动和换热特性,可直接用于研究板式换热器的性能,具有一定的工程实际意义.     

基于N-S方程的真实感流体仿真

本文以耐维-斯托克斯(Navier-Stokes)方程这一计算流体力学领域的经典方程为基础,通过对其进行离散化求解,实现对二维及三维流体的真实感实时仿真.这种基于物理的模型能够更真实的反映流体的细节信息,并且可以方便的实现流体与固体的实时交互以及其它一些复杂场景的仿真.这里进一步引入了多种技术来解决求解过程中出现的数值离散问题.     

基于OpenGL的流体交互式仿真

为避免二维波方程中的大量计算,使用二维波动方程的中心差分近似来模拟流体运动.在该方法上实现了基于物理的浮力和阻力模型,模拟物体在流体表面的漂浮.在OpenGL上实现上述方法,实验证明该方法实现简单,大大降低了运算代价.     

基于机器视觉的复杂形状模具尺寸测量

采用扫描仪与Matlab图像处理技术相结合的方法,对织针针坯模具进行了快速、准确地非接触式测量;首先,分析比较了摄像机和扫描仪在成像原理上的不同点,最终选择线阵CCD扫描仪来获取模具图像;采用数字图像处理技术,对扫描图像进行图像分割、边缘提取和特征检测等操作;通过计算二值图中边界区域的二维矩阵,得到边界点坐标;利用圆的hough变换检测出圆特征,计算得出圆心坐标和半径值;根据图像分辨率得出实际尺寸与像素尺寸之间的数值关系,计算并标注模具的特征尺寸;基于Matlab图像处理模块设计了用于测量模具和零件二维尺寸的图形界面;测量结果表明,该软件的测量精度可以达到3μm,满足测量的精度要求;测量时间明显低于游标卡尺和三坐标测量仪等传统测量工具,能有效提高测量效率。     

Experimental characterization of electrical current leakage in poly(dimethylsiloxane) microfluidic devices

Poly(dimethylsiloxane) (PDMS) is usually considered as a dielectric material and the PDMS microchannel wall can be treated as an electrically insulated boundary in an applied electric field. However, in certain layouts of microfluidic networks, electrical leakage through the PDMS microfluidic channel walls may not be negligible, which must be carefully considered in the microfluidic circuit design. In this paper, we report on the experimental characterization of the electrical leakage current through PDMS microfluidic channel walls of different configurations. Our numerical and experimental studies indicate that for tens of microns thick PDMS channel walls, electrical leakage through the PDMS wall could significantly alter the electrical field in the main channel. We further show that we can use the electrical leakage through the PDMS microfluidic channel wall to control the electrolyte flow inside the microfluidic channel and manipulate the particle motion inside the microfluidic channel. More specifically, we can trap individual particles at different locations inside the microfluidic channel by balancing the electroosmotic flow and the electrophoretic migration of the particle.     

Experiment and simulation of mixed flows in a trapezoidal microchannel

This paper presents experimental and numerical results of mixed electroosmotic and pressure driven flows in a trapezoidal shaped microchannel. A micro particle image velocimetry (μPIV) technique is utilized to acquire velocity profiles across the microchannel for pressure, electroosmotic and mixed electroosmotic-pressure driven flows. In mixed flow studies, both favorable and adverse pressure gradient cases are considered. Flow results obtained from the μPIV technique are compared with 3D numerical predictions, and an excellent agreement is obtained between them. In the numerical technique, the electric double layer is not resolved to avoid expensive computation, rather a slip velocity is assigned at the channel surface based on the electric field and electroosmotic mobility. This study shows that a trapezoidal microchannel provides a tapered-cosine velocity profile if there is any pressure gradient in the flow direction. This result is significantly different from that observed in rectangular microchannels. Our experimental results verify that velocity distribution in mixed flow can be decomposed into pressure and electroosmotic driven components.     

Effect of finite reservoir size on electroosmotic flow in microchannels

In electrokinetically driven microfluidic applications, reservoirs are indispensable and have finite sizes. During operation processes, as the liquid level in reservoirs keeps changing as time elapses, a backpressure is generated. Thus, the flow in microfluidic channels actually exhibits a combination of the electroosmotic flow and the time-dependent induced backpressure-driven flow. In this paper, a model is presented to describe the effect of the finite reservoir size on electroosmotic flow in a rectangular microchannel. Important parameters that describe the effect of finite reservoir size on flow characteristics are discussed. A new concept termed as “effective pumping period” is introduced to characterize the reservoir size effect. The proposed model identifies the mechanisms of the finite-reservoir size effects and is verified by experiment using the micro-PIV technique. The results reported in this study can be used for facilitating the design of microfluidic devices.     

Manipulation of microfluidic flow pattern by optically controlled electroosmosis

Light is used to dynamically control the pattern of electroosmotic flow in a microfluidic channel. One wall of the channel is formed by a photoconductor film in a specific geometry. Illumination of this surface results in an increase of the conductivity that modifies the structure of the electric field inside the channel. A dramatic change of the electroosmotic flow pattern can be achieved. This approach provides useful capabilities for the manipulation of fluids in microfluidic systems like directing flow and mixing.     

Electroosmotic flow control in complex microgeometries

Numerical simulation results for pure electroosmotic and combined electroosmotic/pressure driven Stokes flows are presented in the cross-flow and Y-split junctions. The numerical algorithm is based on a mixed structured/unstructured spectral element formulation, which results in high-order accurate resolution of thin electric double layers with discretization flexibility for complex engineering geometries. The results for pure electroosmotic flows in cross-flow junctions under multiple electric fields show similarities between the electric and velocity fields. The combined electroosmotic/pressure driven flows are also simulated by regulating the flowrate in different branches of the cross-flow junctions. Flow control in the Stokes flow regime is shown to have linear dependence on the magnitude of the externally applied electric field, both for pure electroosmotic and combined flows. This linear behavior enables utilization of electroosmotic forces as nonmechanical means of flow control for microfluidic applications     

Numerical simulation of the capillary flow in the meander microchannel

Pumping in microfluidic devices is an important issue in actuating fluid flow in microchannel, especially that capillary force has received more and more attractions due to the self-driven motion without external power input. However, less 2D simulation was done on the capillary flow in microchannel especially the meander microchannel which can be used for mixing and lab-on-a-chip (LOC) application. In this paper, the numerical simulation of the capillary flow in the meander microchannel has been studied using computer fluid dynamic simulation software CFD-ACE⁺. Different combinations of channel width in the X-direction denoted as W_x and Y-direction denoted as W_y were designed for simulating capillary flow behavior and pressure drop. The designed four types of meander microchannels (W_x × W_y) were 100 × 100 μm, 100 × 200 μm, 50 × 200 μm, and 50 × 400 μm. In this simulation results, it is found that the capillary pumping speed is highly depending on the channel width. The large speed change occurs at the turning angle of channel width change from W_x to W_y. The fastest pumping effect is found in the meander channel of 100 × 100 μm, which has an average pumping speed of 0.439 mm/s. The slowest average flow speed of 0.205 mm/s occurs in the meander channel of 50 × 400 μm. Changing the meander channel width may vary the capillary flow behavior including the pumping speed and the flow resistance as well as pressure drop which will be a good reference in designing the meander microchannels for microfluidic and LOC application.     

Numerical analysis of non Newtonian fluid flow in a low voltage cascade electroosmotic micropump

In microfluidic devices, many fluids have non-Newtonian behaviors, especially biofluids. The viscosity of these fluids mostly depends on the shear rate. Sometimes the non-Newtonian fluids should be transferred by micropumps in lab-on-chip devices. Previous researchers investigated the flow rate in simple electroosmotic flow micropumps which have a simple channel geometry. In the present study, the effects of non-Newtonian properties of fluid in a low voltage cascade electroosmotic micropump are numerically investigated using the power law model. The micropump is modeled in two dimensional with one symmetric step and has a more complex geometry than previous studies. The numerical results show that, the non-Newtonian behavior of fluid affects flow rate in the micropump. The flow rate decreases if the fluid is dilatant. Also, it increases if the fluid is pseudoplastic. Moreover, the pressure which is needed to stop the electroosmotic flow rate in the micropump is calculated. Results show that, the back pressure has a slight change as the fluid has non-Newtonian behavior.     

基于Poisson-Nernst-Planck模型的微通道电渗流数值模拟

采用由电解质溶液离子输运Nernst-Planck方程、流体运动Navier-Stokes方程和电场Possion方程建立的Possion-Nernst-Planck模型,应用有限元分析方法研究二维光滑微通道电渗流输运特性和离子分布。对比分别基于Possion-Nernst-Planck模型和Poisson-Boltzmann模型数值模拟结果,结果表明:Possion-Nernst-Planck模型能更准确地模拟计算微通道中的电渗流输运特性和离子分布。     

On the thin-film-dominated passing pressure of cancer cell squeezing through a microfluidic CTC chip

Detection of circulating tumor cells (CTCs) shows strong promise for early cancer diagnosis, and cell-deformation-based microfluidic CTC chips have been playing an important role. For the design and optimization of high-throughput CTC chips, the dynamic pressure drop in the microfluidic chip during the CTC passing process is a key parameter related to the device sensitivity and filtering performance and has to be given very serious consideration. Although insights have been provided by previous researches, there is still a lack of understanding of the fundamental physics and complex interplay between viscous tumor cell and the flow inside the microfluidic filtering channel. In this paper, the process of the viscous cell squeezing through a microchannel is modeled by solving the governing equations of microscopic multiphase flows, with the tumor cell modeled by a droplet model and the immiscible cell–blood interface tracked by the volume-of-fluid method. Detailed dynamics regarding the filtering process is discussed, including the cell deformation, flow characteristics, passing pressure characteristics as well as the relationship between the pressure drop across the device and the thin film formed in the filtration channel. Current simulation shows a good agreement with analytic results, and an analytical formula is proposed to predict the passing pressure in the microchannel. Our study provides insights into the fluid physics of a viscous cell passing through a constricted microchannel, and the proposed formula can be readily applied to the design and optimization of cell-deformation-based microchannels for CTC detection.     

Electroosmotic flow in microchannels with prismatic elements

Fundamental understanding of liquid flow through microchannels with 3D prismatic elements is important to the design and operation of lab-on-a-chip devices. In this paper, we studied experimentally and theoretically the electroosmotic flow (EOF) in slit microchannels with rectangular 3D prismatic elements fabricated on the bottom channel wall. The average electroosmotic velocity measured by the current-monitoring technique was found lower than that in a smooth microchannel. This velocity reduction becomes larger in microchannel with larger but less number of the prisms even though the space taken by the prisms are identical. The velocity distribution and streamlines on two typical horizontal planes in the microchannel are measured and visualized by a particle-based technique. These experimental observations are in good agreement with the numerical simulation. The comparison of streamlines near the prisms in the pressure-driven flow with that in the EOF showed that the EOF was more sensitive to the local geometry.