Band spreading in two-dimensional microchannel turns for electrokinetic species transport

Analytical and numerical methods are employed to investigate species transport by electrophoretic or electroosmotic motion in the curved geometry of a two-dimensional turn. Closed-form analytical solutions describing the turn-induced diffusive and dispersive spreading of a species band are presented for both the low and high Peclet number limits. We find that the spreading due to dispersion is proportional to the product of the turn included angle and the Peclet number at low Peclet numbers. It is proportional to the square of the included angle and independent of the Peclet number when the Peclet number is large. A composite solution applicable to all Peclet numbers is constructed from these limiting behaviors. Numerical solutions for species transport in a turn are also presented over a wide range of the included angle and the mean turn radius. On the basis of comparisons between the analytical and numerical results, we find that the analytical solutions provide very good estimates of both dispersive and diffusive spreading provided that the mean turn radius exceeds the channel width. These new solutions also agree well with data from a previous study. Optimum conditions minimizing total spreading in a turn are presented and discussed.     

Heterogeneous surface charge enhanced micromixing for electrokinetic flows

Enhancing the species mixing in microfluidic applications is key to reducing analysis time and increasing device portability. The mixing in electroosmotic flow is usually diffusion-dominated. Recent numerical studies have indicated that the introduction of electrically charged surface heterogeneities may augment mixing efficiencies by creating localized regions of flow circulation. In this study, we experimentally visualized the effects of surface charge patterning and developed an optimized electrokinetic micromixer applicable to the low Reynolds number regime. Using the optimized micromixer, mixing efficiencies were improved between 22 and 68% for the applied potentials ranging from 70 to 555 V/cm when compared with the negatively charged homogeneous case. For producing a 95% mixture, this equates to a potential decrease in the required mixing channel length of up to 88% for flows with Péclet numbers between 190 and 1500.     

Integrated nanopore/microchannel devices for ac electrokinetic trapping of particles

We report integrated nanopore/microfluidic devices in which the unique combination of low pore density, conical nanopore membranes with microfluidic channels created addressable, localized high-field regions for electrophoretic and dielectrophoretic trapping of particles. A poly(ethylene terephthalate) track-etched membrane containing conical pores approximately 130 nm in diameter at the tip and approximately 1 microm in diameter at the base was used as an interconnect between two perpendicular poly(dimethylsiloxane) microfluidic channels. Integration of the nanopore membrane with microfluidic channels allowed for easy coupling of the electrical potentials and for directed transport of the analyte particles, 200 nm and 1 microm polystyrene microspheres and Caulobacter crescentus bacteria, to the trapping region. Square waves applied to the device generated electric field strengths up to 1.3 x 10(5) V/cm at the tips of the nanopores in the microchannel intersection. By varying the applied potentials from +/-10 to +/-100 V and exploring frequencies from dc to 100 kHz, we determined the contributions of electrophoretic and dielectrophoretic forces to the trapping and concentration process. These results suggest that tunable filter elements can be constructed in which the nanoporous elements provide a physical barrier and the applied ac field enhanced selectivity.     

Transient effects on microchannel electrokinetic filtering with an ion-permselective membrane

The electrokinetics and hydrodynamics in a hybrid microfluidic/nanofluidic pore network configuration and its effect on the concentration enrichment of charged analytes are described. A hydrogel microplug, photopolymerized in a microfluidic channel, with negative surface charge serves as a nanoporous membrane and dictates the electrokinetic behavior within the adjoining microchannel compartments. The nanoporous hydrogel with a mean pore size on the order of the electrical double layer thickness imparts ion-permselectivity (cation-selectivity) to the migration of ionic species which, under the influence of an applied electrical field, drives concentration polarization in bulk solution near the interfaces between the two microchannel compartments and the hydrogel-based nanopores. The concentration enrichment efficiency for charged analytes depends on this concentration polarization, which strongly affects the distribution of local electrical field strength. In addition, electroosmotic flow in the device plays a critical role in determining the location of the analyte enrichment zone. A theoretical model and simulations are presented to explain the interplay of concentration polarization and electroosmotic flow with respect to the observed concentration enrichment of negatively charged analytes at the cathodic hydrogel plug-microchannel solution interface.     

Faceted design of channels for low-dispersion electrokinetic flows in microfluidic systems

A novel methodology for designing microfluidic channels for low-dispersion, electrokinetic flows is presented. The technique relies on trigonometric relations that apply for ideal electrokinetic flows, allowing faceted channels to be designed using common drafting software and a hand calculator. Flows are rotated and stretched along the abrupt interface between adjacent regions having differing specific permeability--a quantity with dimensions of length that we introduce to derive the governing equations. Two-interface systems are used to eliminate hydrodynamic rotation of bands injected into channels. Regions bounded by interfaces form faceted flow "prisms" with uniform velocity fields that can be combined with other prisms to obtain a wide range of turning angles and expansion ratios. Lengths of faceted prisms can be varied arbitrarily, simplifying chip layout and allowing the ability to reduce unwanted effects such as transverse diffusion and Joule heating for a given faceted prism. Designs are demonstrated using two-dimensional numerical solutions of the Laplace equation.     

A variational multiscale method for particle-cloud tracking in turbomachinery flows

We present a computational method for simulation of particle-laden flows in turbomachinery. The method is based on a stabilized finite element fluid mechanics formulation and a finite element particle-cloud tracking method. We focus on induced-draft fans used in process industries to extract exhaust gases in the form of a two-phase fluid with a dispersed solid phase. The particle-laden flow causes material wear on the fan blades, degrading their aerodynamic performance, and therefore accurate simulation of the flow would be essential in reliable computational turbomachinery analysis and design. The turbulent-flow nature of the problem is dealt with a Reynolds-Averaged Navier–Stokes model and Streamline-Upwind/Petrov–Galerkin/Pressure-Stabilizing/Petrov–Galerkin stabilization, the particle-cloud trajectories are calculated based on the flow field and closure models for the turbulence–particle interaction, and one-way dependence is assumed between the flow field and particle dynamics. We propose a closure model utilizing the scale separation feature of the variational multiscale method, and compare that to the closure utilizing the eddy viscosity model. We present computations for axial- and centrifugal-fan configurations, and compare the computed data to those obtained from experiments, analytical approaches, and other computational methods.     

Particle dispersion in turbulent sedimentary duct flows

A simplified geometric model of a circular tube with a sedimentary layer is proposed and named as sedimentary duct. Based on h = 1/4d (h is the thickness of sedimentary layer and d is the pipe diameter), the flow field (Re = 40000) and particle distribution (5, 10, 50 μm and St = 0.6, 2.5, 63) in the sedimentary duct are simulated using Large Eddy Simulation (LES) coupled with Lagrange Particle Tracking (LPT) method. As a result, four streamwise eddies are found in the sedimentary duct as distributed in pairs near the corner. The eddy center near ceiling is found to be farther from the corner than that from the floor. Small particles (5 μm, St = 0.6) tend to move with the secondary flow as their upward movements distribute in both sides. Their centripetal movement is near the floor and preferential distribution near the top. For large particles (50 μm, St = 63), it is the drag force that dominates their motion while for medium particles (10 μm, St = 2.5) lift force may have significant influence on their motion. This study is the first work to investigate the characteristics of particle behavior in turbulent sedimentary duct flows.     

Computer simulations of electrokinetic injection techniques in microfluidic devices

Computer simulations are used to study electrokinetic injections on microfluidic devices (microchips). The gated and pinched injection techniques are considered. Each injection technique uses a unique sequence of steps with different electric field distributions and field magnitudes in the channels to effectuate a virtual valve. The goal of these computer simulations is to identify operating parameters providing optimal valve performance. In the pinched injection, the conditions of both loading and dispensing steps were analyzed to reach a compromise between the sample plug spatial extent and its concentration. For the gated injection, the condition of leakage free valve operation was found for the sample loading step. The simulation results for the gated valve are compared with experimental data.     

Modeling of flows in a microchannel based on the boltzmann lattice equation

An approach to the analysis of micro- and macroflows with the use of the method of Boltzmann lattices is considered. The velocity profiles of flows in the entry sections of microchannels under no-slip and slip conditions on the wall have been obtained. The influence of the Knudsen number on the hydrodynamic entrance length of a microchannel and its hydrodynamical resistance have been analyzed.     

Study of miscible and immiscible flows in a microchannel using magnetic resonance imaging

Magnetic resonance imaging (MRI) is a noninvasive technique that can be used to visualize mixing processes in optically opaque systems in up to three dimensions. Here, MRI has been used for the first time to obtain both cross-sectional velocity and concentration maps of flow through an optically opaque Y-shaped microfluidic sensor. Images of 23 micromx23 microm resolution were obtained for a channel of rectangular cross section (250 micromx500 microm) fed by two square inlets (250 micromx250 microm). Both miscible and immiscible liquid systems have been studied. These include a system in which the coupling of flow and mass transfer has been observed, as the diffusion of the analyte perturbs local hydrodynamics. MRI has been shown to be a versatile tool for the study of mixing processes in a microfluidic system via the multidimensional spatial resolution of flow and mass transfer.     

Thermoswitchable electrokinetic ion-enrichment/elution based on a poly(N-isopropylacrylamide) hydrogel plug in a microchannel

This paper reports a novel protocol consisting of the thermomodulated electrokinetic enrichment, elution, and separation of charged species based upon a thermoswitchable swelling-shrinking property of a poly(N-isopropylacrylamide), PNIPAAm, hydrogel. A 0.2-1 mm long PNIPAAm hydrogel plug was photopolymerized inside a glass microfluidic channel to produce a composite device consisting of the PNIPAAm hydrogel plug and the glass microchannel (abbreviated as plug-in-channel). After voltage was applied to the composite device, anions, such as FITC, could be enriched at the cathodic end of the PNIPAAm plug when the temperature of the plug was kept below its lower critical solution temperature (LCST, ～32 °C). The concentrated analytes could then be eluted by electroosmotic flow when the temperature of the plug was heated above the LCST. The mechanism of the thermoswitchable ion enrichment/elution process was studied with the results presented. The analytical potential of the composite device was demonstrated for the temperature-modulated preconcentration, elution, and separation of FITC-labeled amino acids.     

Particle tracking microrheology of cancer cells in living subjects

Cumulative evidence shows that microenvironmental conditions play a significant role in the regulation of cell functions, and how cells respond to these conditions are of central importance to regenerative medicine and cancer cell response to therapeutics. Here, we develop a new method to examine cell mechanical properties by analyzing the motion of nanoparticles in living in mice, combining particle tracking with intravital microscopy. This method directly examines the mechanical response of breast carcinoma cells and normal breast epithelial cells under intravital microenvironments. Our results show both carcinoma and normal cells display significantly reduced compliance (less deformability) in vivo compared to the same cells cultured in 2D, in both sparse and confluent conditions. While the compliance of the normal cells remains steady over time, the compliance of carcinoma cells decreases further as they form tumor-like architectures. Integrating the cancer cells into spheroids embedded in 3D collagen matrices in part redirected the mechanical response to a state closer to the in vivo setting. Overall, our study demonstrates that the microenvironment is a crucial regulator of cell mechanics and the intravital particle tracking method can provide novel insights into the role of cell mechanics in vivo.     

基于最大后验概率密度的粒子过滤器跟踪算法

Kalman滤波的弱点是它无法解决非线性、非高斯问题的跟踪。为此提出了一种新型的跟踪算法,粒子过滤器算法。该算法采用加权的粒子集模型表示状态的分布,迭代跟踪状态的变化。其优点是它可以适应复杂环境的重叠和遮挡情况,且能同时跟踪多目标。采用最大后验概率模型确保了状态判断和估计的准确性。对重采样的分析减少了算法对噪声的敏感。并把样本安排在目标可能出现的区域。在眼睛跟踪系统上实现了该算法。仿真结果表明MAP模型在精度上与传统的方法比较提高7%。眼睛跟踪的结果证实了仿真的结果。     

Dissipative Particle Dynamics investigation of parameters affecting planar nanochannel flows

The method of Dissipative Particle Dynamics is applied to investigate the effect of the parameters involved in a nano-channel Poisseuille flow. The parameters considered here include (a) fluid/wall interactions, (b) wall material, (c) range of interaction of fluid particles and wall particles, and (d) external applied force. The computed macroscopic quantities include density, velocity, pressure and temperature profiles. Fluid particle localization near the solid wall is affected by the conservative force (fluid/wall interactions), the wall number density, and the range of atomic interactions (cut-off radius). The external driving force magnitude does not affect the number density distribution. Fluid velocity increases as the conservative force and the wall density increase and the cut-off radius decreases. Pressure distribution is mainly affected by the conservative force and the interaction cut-off radius. Temperature is uniform across most of the channel but presents an increase close to the solid walls especially when increasing the external driving force. We believe that the detailed knowledge of the fluid behaviour under variation of the system parameters obtained from the DPD simulations could be helpful in the design of nanodevices such as lab-on-a-chip devices and nanomixers.     

Computational techniques and validation of 3-dimensional viscous/turbulent codes for internal flows

Computational techniques and codes developed for the prediction of three-dimensional turbulent flows in internal configurations and rotor passages are described. Detailed calibration and validation of the flow fields in 90° curved ducts, cascades, end-wall flows and turbomachinery rotors are presented. Interpretation and comments on accuracy, level of agreement with various turbulence models and limitations of the codes are described. The single pass space-marching code is found to be efficient for curved duct and two-dimensional cascade flows. Multipass space-marching, time-marching and zonal methods are found to be accurate for complex situations. The efficiency and accuracy of a zonal technique, with considerable saving in computational time, is demonstrated. Paper presented atagard Symposium “Validation of Computational Fluid Dynamics,” May 2–5, 1988, Lisbon, Portugal The research work on computation was sponsored by the David Taylor Naval Ship Research and Development Center with Dr D Fuhs as the technical monitor, and NASA Lewis Research Center with Dr P Sockol as the technical monitor.     

Asymptotic modelling of flows in microchannel by using Navier-Stokes or Burnett equations and comparison with DSMC simulations

The modelling of isothermal gas flows driven by pressure drops, in coplanar microchannels is investigated. The goal is to construct an asymptotic model deduced from Navier-Stokes or Burnett equations for this type of flow, assuming slip boundary conditions along the walls. The dimensionless balance equations are written, taking into account a geometrical parameter introduced in this study. The application of the Principle of Least Degeneracy produces models with small Mach numbers and small or moderate Knudsen numbers and allows the development of asymptotic models. The first and second approximations associated with the Navier-Stokes or Burnett equations are presented and discussed. Navier-Stokes and Burnett second approximations reduce the mass flow rates.Additionally, first asymptotic solutions as against Direct Simulation Monte Carlo (DSMC) simulations show overall satisfactory agreements.     

Measuring pressure of pulsed plasma flows by quadrature interferometry techniques

We have developed a new method for measuring the gaskinetic pressure of corpuscular flows in pulsed plasma that employs a probe in the form of an acoustic rod incorporated in the optical scheme of a laser interferometer. This method has been used to study the temporal dynamics of the pressure of corpuscular flows emerging from a micropinch discharge of the low-inductance vacuum spark type. Due to a large dynamic range (～10⁵), the proposed method can be implemented in a large variety of plasma setups with a broad range of parameters.     

PIV血流场显示测速技术

通过分析多普勒测速技术与粒子图像测速技术的区别,从一个新角度把PIV全流场测速技术应用于血液流场的研究中。用激光片光源照亮血流粒子场,再计算确定实验系统光学参数,以获得最佳流场图片。对流场分析常用的互相关算法进行改进,辅以曲面拟合和误差修正,获得了亚像素级的全流场速度的大小和方向,并进一步计算出血流场的涡量分布和剪切率分布。为了验证改进的算法,对日本视频协会提供的PIV-STD序列标准图像进行仿真计算和误差分析,与原算法相比其速度矢量图的误差降低了2个百分点,流场速度值的平均误差小于±1%。该结果表明文中建立的方法是有效的,并可推广用于其它的流场分析。     

Magnetic microparticle-polydimethylsiloxane composite for reversible microchannel bonding

In this study, an iron oxide magnetic microparticles and poly(dimethylsiloxane) (MMPs-PDMS) composite material was employed to demonstrate a simple high-strength reversible magnetic bonding method. This paper presents the casting of opaque-view (where optical inspection through the microchannels was impossible) and clear-view (where optical inspection through the microchannel was possible) MMPs-PDMS. The influence of the microchannel geometries on the casting of the opaque-view casting was limited, which is similar to standard PDMS casting. Clear-view casting performance was highly associated with the microchannel geometries. The effects of the microchannel layout and the gap between the PDMS cover layer and the micromold substrate were thoroughly investigated. Compared with the native PDMS bonding strength of 31 kPa, the MMPs-PDMS magnetic bonding experiments showed that the thin PDMS film with an MMPs-PDMS layer effectively reduced the surface roughness and enhanced MMPs-PDMS reversible magnetic bonding strength. A thin PDMS film-coated opaque-view MMPs-PDMS device exhibited the greatest bonding strength of 110 kPa, and a clear-view MMPs-PDMS device with a thin PDMS film attained a magnetic bonding strength of 81 kPa.