Acoustically induced bubbles in a microfluidic channel for mixing enhancement

Due to small dimensions and low fluid velocity, mixing in microfluidic systems is usually poor. In this study, we report a method of enhancing microfluidic mixing using acoustically induced gas bubbles. The effect of applied frequency on mixing was investigated over the range 0.5–10 kHz. Under either low frequency 0.5 kHz or high frequency 10 kHz, no noticeable improvement in the present mixer was observed. However, a significant increase in the mixing efficiency was achieved within a window of the frequencies between 1.0 and 5.0 kHz. It was found in our present microfluidic structure, single (or multi-) bubble(s) could be acoustically generated under the frequency ranging from 1.0 to 5.0 kHz by a piezoelectric disc. The interaction between bubble and acoustic field causes bubble oscillation which in turn could disturb local flow field to result in mixing enhancement.     

Magnetophoretic mixing for in situ immunochemical binding on magnetic beads in a microfluidic channel

Functionalized magnetic beads offer promising solutions to a host of micro-total analysis systems ranging from immunomagnetic biosensors to cell separators. Immunochemical binding of functional biochemical agents or target biomolecules serves as a key step in such applications. Here we show how magnetophoretic motion of magnetic microspheres in a microchannel is harnessed to promote in situ immunochemical binding of short DNA strands (probe oligonucleotide) on the bead surface via streptavidin–biotin bonds. Using a transverse magnetic field gradient, the particles are transported across a co-flowing analyte stream containing biotinylated probe oligonucleotides that are labeled with a Cy3-fluorophore. Quantification of the resulting biotin–streptavidin promoted binding has been achieved through fluorescence imaging of the magnetophoretically separated magnetic particles in a third stream of phosphate buffered saline. Both the experimental and numerical data indicate that for a given flow rate, the analyte binding per bead depends on the flow fraction of the co-flowing analyte stream through the microchannel, but not on the fluid viscosity. Parametric studies of the effects of fluid viscosity, analyte flow fraction, and total flow rate on the extent of binding and the overall analyte separation rate are also conducted numerically to identify favorable operating regimes of a flow-through immunomagnetic separator for biosensing, cell separation, or high-throughput applications.     

Influence of angle control on the dynamic behaviour of power systems

Angle stability problems are well known in almost every power system in the form of transient instability or undamped system oscillations. Unlike the frequency stability problems, angle stability has not yet been treated as a control problem, due to limitations in measurements, communication and power equipment. However, new technologies employing satellites, optical fibres and power electronics have overcome these physical problems. The report demonstrates how equipment using power electronics for fast angle control can give a single line the characteristics of a multiple-line interconnection. Further, it is shown how this can completely change the dynamic behaviour of a power system, and in the ideal case even eliminates oscillations following small disturbances. The use of angle control to give DC links the desired properties of AC links is also described.     

Evolution of mixing in a microfluidic reverse-staggered herringbone micromixer

Microfluidic platforms offer a variety of advantages including improved heat transfer, low working volumes, ease of scale-up, and stronger user control on operating parameters. However, flow within microfluidic channels occurs at low Reynolds number (Re), which makes mixing difficult to accomplish. Adding V-shaped ridges to channel walls, a pattern called the staggered herringbone design (SHB), alleviates this problem by introducing transverse flow patterns that enable enhanced mixing. Building on our prior work, we here developed a microfluidic mixer utilizing the SHB geometry and characterized using CFD simulations and complimentary experiments. Specifically, we investigated the performance of this type of mixer for unequal species diffusivities and inlet flows. A channel design with SHB ridges was simulated in COMSOL Multiphysics® software under a variety of operating conditions to evaluate its mixing capabilities. The device was fabricated using soft-lithography techniques to experimentally visualize the mixing process. Mixing within the device was enabled by injecting fluorescent dyes through the device and imaging using a confocal microscope. The device was found to efficiently mix fluids rapidly, based on both simulations and experiments. Varying Re or species diffusion coefficients had a weak effect on the mixing profile, due to the laminar flow regime and insufficient residence time, respectively. Mixing effectiveness increased as the species flow rate ratio increased. Fluid flow patterns visualized in confocal microscope images for selective cases were strikingly similar to CFD results, suggesting that the simulations serve as good predictors of device performance. This SHB mixer design would be a good candidate for further implementation as a microfluidic reactor.     

Effect of channel geometry on solute dispersion in pressure-driven microfluidic systems

Pressure-driven transport of fluid and solute samples is often desirable in microfluidic devices, particularly where sufficient electroosmotic flow rates cannot be realized or the use of an electric field is restricted. Unfortunately, this mode of actuation also leads to hydrodynamic dispersion due to the inherent fluid shear in the system. While such dispersivity is known to scale with the square of the Peclet number based on the narrower dimension of the conduit (often the channel depth), the proportionality constant can vary significantly depending on its actual cross section. In this article, we review previous studies to understand the effect of commonly microfabricated channel cross sections on the Taylor–Aris dispersion of solute slugs in simple pressure-driven flow systems. We also analyze some recently proposed optimum designs which can reduce the contribution to this band broadening arising from the presence of the channel sidewalls. Finally, new simulation results have been presented in the last section of this paper which describe solutal spreading due to bowing of microchannels that can occur from stresses developed during their fabrication or operation under high-pressure conditions.     

Effects of channel surface finish on blood flow in microfluidic devices

The behaviour of blood flow in relation to microchannel surface roughness has been investigated. Special attention was focused on the techniques used to fabricate the microchannels and on the apparent viscosity of the blood as it flowed through these microchannels. For the experimental comparison of smooth and rough surface channels, each channel was designed to be 10 mm long and rectangular in cross-section with aspect ratios of ≥100:1 for channel heights of 50 and 100 μm. Polycarbonate was used as the material for the device construction. The shims, which created the heights of the channels, were made of polyethylene terephthalate. Surface roughnesses of the channels were varied from R _z of 60 nm to 1.8 μm. Whole horse blood and filtered water were used as the test fluids and differential pressures ranged from 200 to 5,000 Pa. The defibrinated horse blood was treated further to prevent coagulation. The results indicate that a surface roughness above an unknown value lowers the apparent viscosity of blood dramatically due to boundary effects. Furthermore, the roughness seemed to influence both water and whole blood almost equally. A set of design rules for channel fabrication is also presented in accordance with the experiments performed.     

uFlow: software for rational engineering of secondary flows in inertial microfluidic devices

Approaches to abstract and modularize models of fluid flow in microfluidic devices can enable predictive and rational engineering of microfluidic circuits with rapid designer feedback. The shape of co-flowing streams in the inertial flow regime has become of particular importance for new developments in high throughput microscale manufacturing, biological, and chemical research. In a process known as flow sculpting, the cross-sectional distribution of fluid elements is deformed due to the combined effects of diffusion and transverse advection, which are brought on by interaction with velocity gradients induced by sequences of pillar structures. However, the difficulty in solving the Navier–Stokes equations for complex flow-deforming geometries makes design in this space unintuitive, time-consuming, and costly. To mitigate these issues, we have efficiently embedded flow deformation operations previously relegated to high-performance computing into a free, user-friendly, and cross-platform framework called “uFlow”, to bring flow sculpting to the broader community. uFlow computes flow deformation including both advection and diffusion effects from a single pillar in 25 ms on modern consumer hardware, enabling real-time manual design and exploration of microfluidic devices, and fast visualization of 3D particles fabricated via stop flow lithography or optical transient liquid molding. Advanced numerical routines give instant access to a practically infinite set of flow transformations. We showcase uFlow’s design models, describe their implementation and usage, and validate the algorithms which allow real-time feedback with confocal imaging and cutting-edge microfluidic particle fabrication.     

Size-selective separation of magnetic nanospheres in a microfluidic channel

This paper reported an efficient method to size-selective separate magnetic nanospheres using a self-focusing microfluidic channel equipped with a permanent magnet. Under external magnetic field, the magnetophoresis force exerted on particles leads to size-dependent deflections from their laminar flow paths and results in effective particles separation. By adjusting the distance between magnet and main path of channel, we obtained two monodisperse nanosphere samples (Ca. 90 nm, Ca. 160 nm) from polydispersing particles solution whose diameters varied from 40 to 280 nm. Based on the magnetostatic and laminar flow models, numerical simulations were also used to predict and optimize the nanospheres migrations. Two thresholds of particles diameters were obtained by the simulations and diverse at each position of magnet. Therefore, appropriate position of the magnet could be determined at a certain particle sizes’ range when the flow rate of the two inlets remains unchanged.     

On the background design for microscale background-oriented schlieren measurements of microfluidic mixing

Using microfluidic mixing as the benchmark, we assess the influences of the background designs in the accuracy of the microscale background-oriented schlieren measurements in this study. Three parameters are considered, they are as follows: pattern configuration (random dot, random grid, and grid), dot diameter, and area fraction of dot coverage. A photomask covered with the defined pattern is placed on top of the microchannel to serve as the background. When miscible fluids with different refractive indices are mixed in a T-microchannel, light deflects and there are pattern shifts on the acquired image. After a calibration process is carried out to obtain the relationship between the pattern shift and gradient of mass fraction, we are able to evaluate the performance of each background design based on its corresponding uncertainty. Except for the grid configuration, we find that the lowest error level is achieved with a dot diameter of 6 μm, which corresponds to a dot-image diameter of 2.8 pixels. Because a sparse distribution leads to vacant interrogation windows, the optimal random-dot design has the highest area fraction of 0.178 (0.196 for the design value). In contrast, the random-grid design with too many dots becomes comparable to the grid design and has difficulties during the cross-correlation analysis. As a result, the best random-grid background has an area fraction of 0.098. For the grid design, on the other hand, accurate results can be obtained when there is only one dot in each interrogation window. Hence, a dot diameter of 16 μm leads to the lowest uncertainty for the grid design. Once these backgrounds are optimized, we prove that all three configurations are able to deliver satisfactory results for the reconstruction of a concentration field in a T-microchannel and an instantaneous profile of concentration gradient in a microfluidic oscillator.     

基于惯导角度量测的轨道平面最佳线形参数估计算法

轨道线形分段及线形参数优化是铁路轨道既有线复测工作的核心。基于惯导角度量测数据,提出了一种轨道平面线形分段及最佳线形参数估计算法。所提算法根据轨道线形变化规律,利用组合迭代的方法计算轨道的最佳线形参数。该算法将轨道平面线形确定建模成优化问题：首先根据定长曲率曲线最小二乘拟合斜率变化对轨道进行概略分段;然后基于量测数据拟合轨道线形;最后使用组合迭代算法进行精确分段并确定最佳线形参数。仿真算例结果表明,所提算法结果优于现有人工判定算法——基于两组不同分段点的线形参数拟合结果,与穷举法结果更为接近,所提算法均方根误差（RMSE）仅比穷举法高4.93%,但计算量仅为穷举法的0.02%。西安地铁三号线的实测结果也验证了所提算法的有效性。     

Effective mixing in a microfluidic oscillator using an impinging jet on a concave surface

In this study, we present a microfluidic oscillator design that employs an impinging jet on a concave surface to enhance the microscale mixing process. The Coand? effect along with the G?rtler instability proves to incite sustainable flapping motion beyond the obstacle and mixing is profoundly improved. From the flow visualization results, four different regimes are identified and we find that the primary enhancement of mixing performance is always linked to the transition of flow regime. Moreover, incorporating a sudden-expansion confluence provokes flow three dimensionality and elevates the mixing level significantly at low Reynolds numbers. For a Reynolds number as low as 70, the tail flow behind the concave obstacle successfully exhibits a periodic oscillation and Hopf bifurcation is induced, leading to a drastic augmentation in the time-average mixing efficiency. By utilizing the spectrum analysis, the characteristic frequency of flapping motion is found to vary linearly with the throat velocity, resulting in a constant Strouhal number of 3.8?×?10^?5.     

Characterization of DNA hybridization kinetics in a microfluidic flow channel

In this investigation we report on the influence of volumetric flow rate, flow velocity, complementary DNA concentration, height of a microfluidic flow channel and time on DNA hybridization kinetics. A syringe pump was used to drive Cy3-labeled target DNA through a polydimethylsiloxane (PDMS) microfluidic flow channel to hybridize with immobilized DNA from the West Nile Virus. We demonstrate that a reduction of channel height, while keeping a fixed volumetric flow rate or a fixed flow velocity, enhances mass transport of target DNA to the capture probes. Compared to a passive hybridization, the DNA hybridization in the microfluidic flow channel generates higher fluorescence intensities for lower concentration of target DNA during the same fixed period of time. Within a fixed 2 min time period the fastest DNA hybridization at a 50 pM concentration of target DNA is achieved with a continuous flow of target DNA at the highest flow rate and the lowest channel height.     

Unloading angular momentum for inertial actuators of a spacecraft in the pitch channel

The problem of gravitational unloading of the angular momentum of inertial actuators of a spacecraft in the pitch channel for circular and elliptic orbits is considered using the band theory of modal control. Control laws for gravitational unloading and stabilization of a given spacecraft position unambiguously determined by the object parameters and given coefficients of characteristic equation are obtained.     

多轴联动的风洞变角度机构同步协调控制

为解决风洞尾撑变角度机构运动过程中多轴之间的同步协调控制问题,本文提出了一种结合并行控制和交叉耦合控制的同步控制方法.首先,将并行控制应用于多轴联动控制中,根据前后侧滑轴定位时间一致以及速度变化时间一致的原则,开展了提高多轴同步性能的研究.然后,采用了基于同步误差传递函数的交叉耦合控制方法,对并行控制给定的运动参数进行修正补偿.最后,通过机构运动进行了实验验证,实验结果表明该控制方法可有效减小横向同步误差,提高同步性能,满足风洞试验控制要求.该方法也为类似机构的同步控制提供了参考.     

Characterizing effects of humidity and channel size on imbibition in paper-based microfluidic channels

Paper-based microfluidic devices have gained an increasing amount of interest over the last few years. As such devices continue advancing toward more complex and sophisticated functions, obtaining accurate and consistent fluid imbibition under different conditions will become increasingly important. This study presents a series of controlled imbibition experiments investigating effects of relative humidity and channel width in paper-based microfluidic channels. The obtained imbibition data highlighted the importance in accounting for the effects of these design and environmental parameters. Additionally, fitting of the experimental data to three relevant imbibition models revealed evaporation, not water saturation, to be the main mechanism of the observed relative humidity effect. The current study has created a library of paper-specific, imbibition-related properties for commonly used filter and chromatography papers for the first time. Collectively, the presented imbibition data and the discovered relationships are expected to help researchers design more precise and reproducible paper-based microfluidic devices.     

微沟道内两相流速比对液滴形成的影响

研究了流聚焦微沟道内不同两相流速比对单分散性液滴形成的影响.不相混容的两相系统能够快速、周期性地形成液滴,液滴的形态和生成率随着油相速度的变化而改变.研究采用模拟软件COMSOL Mutiphysics,模拟了不同流速下液滴的形成过程,并通过实验进行了验证,从流体动力学的角度解释了其物理机制.所得结论为在流聚焦微沟道内获得分散性良好且大小可控的液滴提供了理论和数据支持.     

Influence of back angle on the quality of sleep in seats

Nocturnal sleep was assessed electroencephalographically in 9 males aged between 29 and 48 (mean 36·5) years in bed and in three seats with back angles to the vertical of 49·5° (sleeperette), 37·0° (reclining seat) and 17·5° (armchair).

Sleep in the sleeperette did not differ from that in bed, but in the reclining seat the duration of sleep was reduced and the amount of awake activity was increased. Sleep in the armchair was markedly worse than in any of the other three conditions. Total sleep time was shorter and awake activity was increased with more awakenings. Sleep efficiency was also reduced.

It would appear that adequate sleep may be obtained in seats as long as the back angle with the vertical approaches 40°.     

A numerical design study of chaotic mixing of magnetic particles in a microfluidic bio-separator

A two-dimensional numerical investigation into the mixing of magnetic microparticles with bio-cells in a chaotic micromixer is carried out by using a multiphysics finite element analysis package. Fluid and magnetic problems are simulated in steady-state and time-dependent modes, respectively. Intensity of segregation is utilized as the main index to examine the efficiency of the mixer. Trajectories of the particles are used in order to detect chaos in their motion and quantify its extent. Moreover, probability of the collision between particles and target bio-cells is examined as a supplemental index to study the effects of driving parameters on the mixing process. Simulation results reveal that while in some ranges of operating conditions all indices are in good agreement, there are some ranges where they appear to predict contradicting results which is discussed in details. It is found that optimum operating conditions for the system is obtained when the Strouhal number is less than 0.6, which corresponds to the efficiency of about 85% in a mixing length of 500 μm (The mixer design described here is patent pending).