Pressure distributions of gaseous slip flow in straight and uniform rectangular microchannels

This paper presents analytical derivations of the pressure distribution in straight and uniform rectangular microchannels in the slip flow regime and new experimental data in those channels. The flow is to be steady state, two-dimensional, isothermal, and to have negligible transverse velocities with a first order slip boundary condition. The measured pressure distributions of airflows are compared with newly derived analytical results. There is close agreement between the measurements and calculation by the slip flow formula. The dimensionless location of the maximum deviation from the linear pressure distribution is found analytically and compared with the measurements. This dimensionless location of the maximum deviation increases with the increasing pressure ratios in the slip flow regime. The effect of several parameters such as the channel aspect ratio and the Knudsen number on the locations of maximum deviation from linearity are investigated. The nonlinearity of the pressure distribution is also discussed.     

A criterion for experimental validation of slip-flow models for incompressible rarefied gases through microchannels

This paper is devoted to analysing the friction factor for incompressible rarefied gas flow through microchannels. A theoretical investigation is conducted in order to underline the conditions for experimentally evidencing rarefaction effects on the pressure drop. It is demonstrated that for a fixed geometry of the microchannel cross-section, it is possible to calculate the minimum value of the Knudsen number for which the rarefaction effects can be observed experimentally, taking into account the uncertainties related to evaluation of the friction factor.     

Thermohydraulics of square-section microchannels following a serpentine path

Fully developed laminar flow and heat transfer behaviour in serpentine channels with a square cross-section has been studied using computational fluid dynamics. Studies were performed up to Re=200, beyond which the flow became unsteady. The effect of geometric configuration was examined in detail for Re=110, 0.525<R _c/d<2 and 3.6<L/d<12 (where d is the side length of the square section, R _c is radius of curvature of the serpentine bends, and L is the half-wavelength of the serpentine path). Simulations were carried out at (Pr=0.7, 6.13 and 100) constant wall heat flux (H2 boundary condition) and constant wall temperature (T boundary condition). Dean vortices formed at the bends promote fluid mixing transverse to the main flow direction. This leads to significant heat transfer enhancement (up to a factor of 8 at high Pr and Re) with relatively small pressure-drop penalty (factor of 1.8 at high Re). Increasing R _c/d mitigates these effects while the effect of increasing L/d decreases the frictional penalty without greatly affecting the heat transfer enhancement.     

Slip effects on MHD mixed convection stagnation point flow of a micropolar fluid towards a shrinking vertical sheet

In the present study, the effects of partial slip on steady boundary layer stagnation point flow of an electrically conducting micropolar fluid impinging normally towards a shrinking sheet in the presence of a uniform transverse magnetic field is investigated. A similarity transformation technique is adopted to obtain the self similar ordinary differential equations and then solved numerically using symbolic software MATHEMATICA 7.0. The features of the flow and heat transfer characteristics for different values of the governing parameters are analyzed and discussed through graphs and tables. Both cases of assisting and opposing flows are considered. The physical aspects of the problem are highlighted and discussed.     

Liquid flow through microchannels with grooved walls under wetting and superhydrophobic conditions

Recent developments in superhydrophobic surfaces have enabled significant reduction in the frictional drag for liquid flow through microchannels. There is an apparent risk when using such surfaces, however, that under some conditions the liquid meniscus may destabilize and, consequently, the liquid will wet the entire patterned surface. This paper presents analytical and experimental results that compare the laminar flow dynamics through microchannels with superhydrophobic walls featuring ribs and cavities oriented both parallel and transverse to the direction of flow under both wetting and non-wetting conditions. The results show the reduction in the total frictional resistance is much greater in channels when the liquid phase does not enter the cavity regions. Further, it is demonstrated that the wetting and non-wetting cavity results represent limiting cases between which the experimental data lie. Generalized expressions enabling prediction of the classical friction factor-Reynolds number product as a function of the relevant governing dimensionless parameters are also presented for both the superhydrophobic and wetting states. Experimental results are presented for a range of parameters in the laminar flow regime.     

Spatially resolved temperature measurement in microchannels

Non-intrusive local temperature measurement in convective microchannel flows using infrared (IR) thermography is presented. This technique can be used to determine local temperatures of the visualized channel wall or liquid temperature near this wall in IR-transparent heat sinks. The technique is demonstrated on water flow through a silicon (Si) microchannel. A high value of a combined liquid emissivity and substrate overall transmittance coupled with a low uncertainty in estimating this factor is important for quantitative temperature measurement using IR thermography. The test section design, and experimental and data analysis procedures that provide increased sensitivity of the detected intensity to the desired temperature are discussed. Experiments are performed on a 13-mm long, 50 μm wide by 135 μm deep Si microchannel at a constant heat input to the heat sink surface for flow rates between 0.6 and 1.2 g min⁻¹. Uncertainty in fluid temperature varies from a minimum of 0.60°C for a Reynolds number (Re) of 297 to a maximum of 1.33°C for a Re of 251.     

Fouling and its mitigation in silicon microchannels used for IC chip cooling

Particulate fouling studies with alumina dispersions in water were performed in rectangular, silicon microchannels having hydraulic diameters between 220 and 225 μm with Reynolds numbers of 17–41. Data show for the most part the absence of particle deposition within the microchannels. The primary reason for this is the relatively high wall shear stress at the microchannel walls of 2.3–3.5 Pa compared to conventional size passageways. In contrast, the headers for the microchannels are quite susceptible to particulate fouling under the same conditions. This is because the shear stress in the header region is lower. Proper adjustment of pH has been identified to effectively mitigate the fouling by controlling the electrostatic forces of repulsion between particle–particle interactions.

Couette–Poiseuille flow of a gas in long microchannels

The flow of a compressible, isothermal gas under slightly rarefied conditions in a 2D planar geometry is considered. The gas is shear driven and is also subject to an applied pressure gradient, which is also known as Couette–Poiseuille (CP) flow. In this paper, the full Navier-Stokes (NS) equations are solved using a perturbation expansion up to the first order. The pressure profile is solved numerically. On the basis of the solutions, effects of rarefaction and compressibility on the flow characteristics are investigated in detail. The results show the parallel flow assumption to be invalid for cases with slight rarefaction. The axial and vertical velocity components are found to depend on the degree of rarefaction, applied pressure gradient and wall velocity. The effects of rarefaction on the occurrence of back flow are also discussed. In addition, the results for the Poiseuille and CP flow with and without rarefaction can be easily obtained from our results.     

An effective localization method for mixed far-field and near-field strictly non-circular sources

In this paper, an effective direction-of-arrival (DOA) and range estimations method for mixed far-field and near-field non-circular sources is proposed based on a large centrosymmetric uniform linear array (ULA). By exploiting the non-circularity of the sources, an extended signal is generated by concatenating the received array data and its conjugate counterparts. Then the DOAs of far-field signals are estimated based on the extended covariance matrix with the traditional MUSIC algorithm. After eliminating the far-field components from the extended signal subspace, the extended covariance matrix of the near-field signals is obtained. Thus a near-field estimator is constructed based on symmetric property of the extended array manifold where the generalized ESPRIT method is adopted to estimate the DOAs of near-field sources. Finally, the range estimator is derived using the DOA estimations of near-field sources. Simulation results are provided to validate that the proposed method has achieved a better performance than existing ones and is quite suitable for massive MIMO (multiple-input multiple-out) system.     

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.     

Methods and preliminary results on enhanced boiling heat transfer in second generation microchannels

This paper reviews literature on conventional scale boiling enhancement techniques by means of reentrant cavities and discusses various avenues the knowledge obtained from that research can be used to enhance boiling in microchannels. Fabrication techniques developed by the Micro Thermal-Fluids Laboratory at Rensselaer Polytechnic Institute together with the Advanced Microsystems Materials Laboratory at McGill University are discussed and preliminary data are given. These results demonstrate the potential for improving boiling heat transfer characteristics in microchannels and introduce next generation microchannel heat transfer technology.     

On the forced convective heat transport in a droplet-laden flow in microchannels

Recently, a great deal of attention has been focused on development of microfabricated devices for manipulating minute amounts of liquids. In particular, an extensive experimental work is devoted to generation, motion and manipulation of drops in microfluidic channels, or digital microfluidics. In the present work the numerical approach based on volume-of-fluid method, combined with the piece-wise linear interface reconstruction scheme, is implemented for modeling of droplet motion and forced heat transport in a droplet-laden laminar flow in a circular microchannel. The simulations show a very good agreement with asymptotic results concerning the motion of spherical and slender drops in confined laminar flows. The effective rates of the forced heat transfer in a droplet-laden flow are found to be superior over that in single-phase Poiseuille flow. The enhancement is anticipated to be a result of the flow disturbance in the carrier fluid due to propagation of a train of translating drops and efficient convective transport within drops due to internal circulation.

压／滑觉传感器及其信号检测

ＦＰＳＲ＾ＴＭ系列传感器元是美国Ｉｎｔｅｒｌｉｎｋ公司最新工艺制造的，对采用它所研制开发的压滑觉传感器进行了介绍，并阐述了其信号的检测方法。     

Capillary flow in microchannels

The surface of microchannels, especially polymer channels, often needs to be treated to acquire specific properties. This study investigated the capillary flow and the interface behavior in several glass capillaries and fabricated microchannels using a photographic technique and image analysis. The effect of air plasma treatment on the characteristics of capillary flow in three types of microfluidic chips, and the longevity of the acquired surface properties were also studied. It was observed that the dynamic contact angles in microchannels were significantly larger than those measured from a flat substrate and the angle varied with channel size. This suggests that dynamic contact angle measured in situ must be used in the theoretical calculation of capillary flow speed, especially for microfabricated microchannels since the surface properties are likely to be different from the native material. This study also revealed that plasma treatment could induce different interface patterns in the PDMS channels from those in the glass and PC channels. The PDMS channel walls could acquire different level of hydrophilicity during the plasma treatment, and the recovery to hydrophobicity is also non-homogeneous.     

Analysis of combined pressure-driven electroosmotic flow through square microchannels

In this paper three-dimensional single-phase liquid flow through microchannels with a square-shaped cross-section driven by simultaneous application of pressure gradient and electroosmotic pumping mechanism is studied. The governing system of equations consists of the electric potential field and flow field equations. The solution procedure involves three steps. First, the net charge distribution on the cross-section of the microchannel is computed by solving two-dimensional Poisson–Boltzmann equation using the finite element method. Then, using the computed fluid’s charge distribution, the magnitude of the resulting body force due to interaction of an external electric field with the charged fluid particles is calculated along the microchannel. In the third step, the flow equations are solved by considering three-dimensional Navier–Stokes equations with an electrokinetic body force. The computations reveal that the flow pattern in the microchannel is significantly different from the parabolic velocity profile of the laminar pressure-driven flow. The effect of the liquid bulk ionic concentration and the external electric field strength on flow patterns through the square-shaped microchannels is also investigated.     

Comprehensive model of electrokinetic flow and migration in microchannels with conductivity gradients

A comprehensive model of electrokinetic flow and transport of electrolytes in microchannels with conductivity gradients is developed. The electrical potential is modeled by a combination of an electrostatic and an electrodynamic approach. The fluid dynamics are described by the Navier–Stokes equations, extended by an electrical force term. The chemistry of the system is represented by source terms in the mass transport equations, derived from an equilibrium approach. Moreover, the interaction between ionic species concentration and physicochemical properties of the microchannel substrate (i.e. zeta-potential) is taken into consideration by an empirical approach. Approximate analytical solutions for all variables are found which are valid within the electrical double layer. By using the method of matched asymptotic expansions, these solutions provide boundary conditions for the numerical simulation of the bulk liquid. The models are implemented in a Finite-Element-Code. As an example, simulations of an electrophoretic injection/separation process in a micro-electrophoresis device are performed. The results of the simulations show the strong coupling between the involved physicochemical phenomena. Simulations with a constant and a concentration-depend zeta-potential clarify the importance of a proper modeling of the physicochemical substrate characteristics.     

Gaseous slip flow in long microchannels

An analytic and experimental investigation into gaseous flow with slight rarefaction through long microchannels is undertaken. A two-dimensional (2-D) analysis of the Navier-Stokes equations with a first-order slip-velocity boundary condition demonstrates that both compressibility and rarefied effects are present in long microchannels. By undertaking a perturbation expansion in ϵ, the height-to-length ratio of the channel, and using the ideal gas equation of state, it is shown that the zeroth-order analytic solution for the streamwise mass flow corresponds well with the experimental results. Also, the effect of slip upon the pressure distribution is derived, and it is obtained that this slip velocity leads directly to a wall-normal migration of mass. The fabrication of wafer-bonded microchannels that possess well-controlled surface structure is described, and a means for accurately measuring the mass how through the channels is presented. Experimental results obtained with this mass-flow measurement technique for streamwise helium mass flow through microchannels 52.25-μm wide, 1.33-μm deep, and 7500-μm long for a pressure range of 1.6-4.2 atmospheres (outlet pressures at atmospheric) are presented and shown to compare favorably with the analysis     

一种基于PLC控制的滑片自动分选系统

针对目前国内仍大量存在手工对滑片的检测这一状况,开发了一种基于PLC控制的滑片自动分选系统。本文阐述了滑片自动分选系统的整体结构及主要功能,以计算机和S7—200PLC为控制核心,并通过应用现代气动元件和基于机电控制和电气控制相结合的自动化技术,实现了高精度、高速度、高效率、低差错率对滑片进行自动分选。