Straight Microchannel Research Articles

Monitoring of Non-Ferrous Wear Debris in Hydraulic Oil by Detecting the Equivalent Resistance of Inductive Sensors.

Wear debris in hydraulic oil contains important information on the operation of equipment, which is important for condition monitoring and fault diagnosis in mechanical equipment. A micro inductive sensor based on the inductive coulter principle is presented in this work. It consists of a straight micro-channel and a 3-D solenoid coil wound on the micro-channel. Instead of detecting the inductance change of the inductive sensor, the equivalent resistance change of the inductive sensor is detected for non-ferrous particle (copper particle) monitoring. The simulation results show that the resistance change rate caused by the presence of copper particles is greater than the inductance change rate. Copper particles with sizes ranging from 48 μm to 150 μm were used in the experiment, and the experimental results are in good agreement with the simulation results. By detecting the inductive change of the micro inductive sensor, the detection limit of the copper particles only reaches 70 μm. However, the detection limit can be improved to 48 μm by detecting the equivalent resistance of the inductive sensor. The equivalent resistance method was demonstrated to have a higher detection accuracy than conventional inductive detection methods for non-ferrous particle detection in hydraulic oil.

An experimental study on flow friction and heat transfer of water in sinusoidal wavy silicon microchannels

Sinusoidal wavy microchannels have been known as a more heat transfer efficient heat sink for the cooling of electronics than normal straight microchannels. However, the existing experimental study on wavy silicon microchannels with different phase differences are few. As a result of this, in this paper an experimental study has been conducted to investigate the single phase flow friction and heat transfer of de-ionized water in eight different sinusoidal wavy silicon microchannels (SWSMCs) and one straight silicon microchannel (SMC). The SWSMCs feature different phase differences (α = 0 to π) and different relative wavy amplitudes (β = A/l = 0.05 to 0.4), but the same average hydraulic diameters (Dh = 160 µm). It is found that both flow friction constant fRe and the Nusselt number depend on the phase difference and relative wavy amplitude. For sinusoidal wavy microchannels with a relative wavy amplitude (β = 0.05), the Nusselt number increased noticeably with the phase difference for Re > 250, but the effect was insignificant for Re < 250; however, both pressure drop and apparent flow friction constant fRe increased with the increase in phase difference. For sinusoidal wavy microchannels with 0 phase difference, the increase in relative wavy amplitude obtained by reducing the wavy wave length induced higher pressure drop and apparent friction constant fRe, while the Nusselt number increased with relative wavy amplitude for Re > 300. The results indicate that the thermal resistances of sinusoidal wavy silicon microchannels were generally lower than that of straight silicon microchannels, and the thermal resistance decreased with the increase in relative wavy amplitude. The enhancement of thermal performance is attributed to the flow re-circulation occurring in the corrugation troughs and the secondary flows or Dean vortices introduced by curved channels. It is concluded that silicon sinusoidal wavy microchannels provide higher heat transfer rate albeit with a higher flow friction, making it a better choice for the cooling of high heat flux electronics.

Inertial particle focusing dynamics in a trapezoidal straight microchannel: application to particle filtration

Inertial microfluidics has emerged recently as a promising tool for high-throughput manipulation of particles and cells for a wide range of flow cytometric tasks including cell separation/filtration, cell counting, and mechanical phenotyping. Inertial focusing is profoundly reliant on the cross-sectional shape of channel and its impacts on not only the shear field but also the wall-effect lift force near the wall region. In this study, particle focusing dynamics inside trapezoidal straight microchannels was first studied systematically for a broad range of channel Re number (20 50). Remarkably, an almost complete transition of major focusing from the longer side wall to the shorter side wall was found for large-sized particles of clogging ratio K ~ 0.9 (K = a/Hmin) when Re increased noticeably to ~ 650. Finally, based on our findings, a trapezoidal straight channel along with a bifurcation was designed and applied for continuous filtration of a broad range of particle size (0.3 < K < 1) exiting through the longer wall outlet with ~ 99% efficiency (Re < 100).

Hydrodynamic shrinkage of liquid CO2 Taylor drops in a straight microchannel

Hydrodynamic shrinkage of liquid CO2 drops in water under a Taylor flow regime is studied using a straight microchannel (length/width ~100). A general form of a mathematical model of the solvent-side mass transfer coefficient (ks) is developed first. Based on formulations of the surface area (A) and the volume (V) of a general Taylor drop in a rectangular microchannel, a specific form of ks is derived. Drop length and speed are experimentally measured at three specified positions of the straight channel, namely, immediately after drop generation (position 1), the midpoint of the channel (position 2) and the end of the channel (position 3). The reductions of drop length (Lx, x = 1, 2, 3) from position 1 to 2 and down to 3 are used to quantify the drop shrinkage. Using the specific model, ks is calculated mainly based on Lx and drop flowing time (t). Results show that smaller CO2 drops produced by lower flow rate ratios () are generally characterized by higher (nearly three times) ks and Sherwood numbers than those produced by higher , which is essentially attributed to the larger effective portion of the smaller drop contributing in the mass transfer under same levels of the flowing time and the surface-to-volume ratio (~104 m−1) of all drops. Based on calculated pressure drops of the segmented flow in microchannel, the Peng–Robinson equation of state and initial pressures of drops at the T-junction in experiments, overall pressure drop (ΔPt) in the straight channel as well as the resulted drop volume change are quantified. ΔPt from position 1–3 is by average 3.175 kPa with a ~1.6% standard error, which only leads to relative drop volume changes of 0.3‰ to 0.52‰.

Thermal performance of flat micro heat pipe with converging microchannels

Optimizing the groove size of flat micro heat pipes is crucial for improving their thermal performance. In this study, we developed a grooved converging microchannel array for use in a flat micro heat pipe to enhance the capillary force. A simplified theoretical analysis was used to optimize the groove size for given operating conditions of converging microchannels and straight microchannels. The evaporation section of the grooved microchannel was hydrophilic and had a smaller hydraulic diameter than the hydrophobic condensation section. The smaller diameter of the evaporation section enabled the condensed working fluid to be effectively drawn back to the same section. Experiments were performed to measure the thermal performance of the micro heat pipes under the analyzed operating conditions. Compared to a heat pipe with a straight microchannel, and a heat pipe with an unoptimized converging microchannel, the micro heat pipe with the optimized converging microchannel was confirmed to yield a higher thermal performance. Capillary-driven flow experiments at room temperature and atmospheric pressure were also used to investigate the capillary forces of the different microchannels. The optimized converging microchannel was once again observed to generate the largest capillary force.

Thermal-hydraulic performance of interrupted microchannel heat sinks with different rib geometries in transverse microchambers

The thermal-hydraulic performance of microchannel heat sinks with ribs in the interrupted transverse microchambers is studied using a three-dimensional conjugated heat transfer model and considering entrance effect, viscous heating and temperature-dependent thermophysical properties. Five different configurations of ribs and four lengths along the flow direction for every rib configuration are selected to analyze the effects of rib geometry on the thermal-hydraulic performance. The five rib configurations are rectangular, backward triangular, diamond, forward triangular and ellipsoidal, and the rib geometry parameters include expansion-constriction profile, ratio and length. The effects of rib geometry on thermal-hydraulic performance are firstly examined by the variations of friction factor and Nusselt number with Reynolds number, and corresponding correlations are proposed. Then, the conductive, convective and fluid capacitive thermal resistances are analyzed to obtain some insight into the basic heat transfer mechanism. Next, the entropy generation rates due to heat transfer and fluid friction are investigated for the analysis of the lost available work and irreversibility in the heat transfer process. Finally, the performance evaluation criteria is calculated to comprehensively assess the performance of such interrupted microchannel heat sinks with different rib geometry. For the studied operation parameters and rib geometries, the interrupted microchannel heat sinks with ribs in the transverse microchambers show a 4–31% decrease in the total thermal resistance, a 4–26% decrease in the total entropy generation rates, the maximum value 1.39 in performance evaluation criteria, compared with the straight microchannel heat sink.

Heat transfer by nanofluids in wavy microchannels

Pumping coolants through microchannels with well-defined structures along micro-electronic devices is a typical approach to remove the heat. It has been found recently that so-called nanofluids, i.e. dilute water-Cu or water-Al2O3 suspensions with a particle diameter of 100–150 nm, are highly efficient coolants. Numerical simulations can help to optimize the microchannel structures and typically homogenous single-phase models are applied. However, these underestimate the experimental results. An alternative approach is two-phase models based on an Eulerian approach for the base fluid and a Lagrangian description of the suspended particles. In this paper we follow that route and solve the three-dimensional governing equations including continuity, Navier-Stokes and energy equations with the well-known SIMPLE method. The governing equations for particles are solved by a 4th order Runge-Kutta algorithm. We focus on a wavy microchannel structure and demonstrate that the disagreement between the two simulation approaches is due to the non-homogeneous particle distribution in the domain. We also find that the Nusselt number increases with the increase in volume fraction and the decrease in particle diameter and that it. is about three times higher for a nanofluid in a wavy microchannels as compared to water in a straight microchannels.

Transparent and Waterproof Ionic Liquid-Based Fibers for Highly Durable Multifunctional Sensors and Strain-Insensitive Stretchable Conductors.

Ionic liquids (ILs) are regarded as ideal components in the next generation of strain sensors because their ultralow modulus can commendably circumvent or manage the mechanical mismatch in traditional strain sensors. In addition to strain sensors, stretchable conductors with a strain-insensitive conductance are also indispensable in artificial systems for connecting and transporting electrons, similar to the function of blood vessels in the human body. In this work, two types of ILs-based conductive fibers were fabricated by developing hollow fibers with specific microscale channels, which were then filled with ILs. Typically, the ILs-based fiber with straight microchannels exhibited a high strain sensitivity and simultaneously rapid responses to strain, pressure, and temperature. The other ILs-based fiber with helical microchannels exhibited a good strain-isolate conductance under strain. Due to the high transparency of ILs along with the sealing process, the as-prepared ILs-based fibers are both highly transparent and waterproof. More importantly, owing to the low modulus of ILs and the core-shell structure, both conductive fiber prototypes demonstrated a high durability (>10 000 times) and a long-term stability (>4 months). Ultimately, the ILs-based fibrous sensors were successfully woven into gloves, flaunting the ability to detect human breathing patterns, sign language, hand gestures, and arm motions. The ILs-based strain-insensitive fibers were successfully applied in stretchable wires as well.

Collective spreading of red blood cells flowing in a microchannel

Due to recent advances in micro total analysis system technologies, microfluidics provides increased opportunities to manipulate, stimulate, and diagnose blood cells. Controlling the concentration of cells at a given position across the width of a channel is an important aspect in the design of microfluidic devices. Despite its biomedical importance, the collective spreading of red blood cells (RBCs) in a microchannel has not yet been fully clarified. In this study, we experimentally investigated the collective spreading of RBCs in a straight microchannel, and found that RBCs initially distributed in one side of the microchannel spread to the spanwise direction during downstream flow. Spreading increased considerably as the hematocrit increased, though the flow rate had a small effect. We proposed a scaling argument to show that this spreading phenomenon was diffusive and mainly induced by cell–cell interactions. The dispersion coefficient was approximately proportional to the flow rate and the hematocrit. These results are useful in understanding collective behaviors of RBCs in a microchannel and in microcirculation.

Effects of obstacle lengths on the asymmetric breakup of a droplet in a straight microchannel

Rheological behaviors of a droplet moving in the micro-channel with a linear obstacle off the channel axis are investigated numerically by employing Volume of Fluid methods. Five different hydrodynamic patterns of the rheological behaviors of the droplet have been observed. The effects of the length ratio L of the mother droplet to the linear obstacle on the critical capillary number of the droplet breakup have been particularly studied. When L is less than a critical value Lmin, the larger the droplet is, the smaller the critical capillary number Cacr is; when L is greater than Lmin, the larger the droplet is, the greater Cacr is. The curve of Cacr as a function of L has a minimum point at Lmin, which means that the droplet with L = Lmin could be broken up most easily. This is caused by the competition of the enhancing (positive) factors and the hindering (negative) factors on the droplet breakup, which has been analyzed carefully in this paper. When other parameters are fixed, it is very interesting to find out that the change of the obstacle length does not change the value of Lmin, which is fixed to 0.66 for the conditions discussed in this paper. The results obtained here might enlighten potential applications for the design of the micro-channel with obstacles and the packing rings in columns.

Reconstruction of directed neuronal networks in a microfluidic device with asymmetric microchannels.

Microfluidic devices for controlling neuronal connectivity in vitro are extremely useful tools for deciphering pathological and physiological processes occurring in neuronal networks. These devices allow the connection between different neuronal populations located into separate culture chambers through axon-selective microchannels. In order to implement specific features of brain connectivity such as directionality, it is necessary to control axonal growth orientation in these devices. Among the various strategies proposed to achieve this goal, one of the most promising and easily reproducible is the use of asymmetric microchannels. We present here a general protocol and several guidelines for the design, production and testing of a new paradigm of asymmetric microchannels geometries based on a "return to sender" strategy. In this method, axons are either allowed to travel between the emitting and receiving chambers within straight microchannels (forward direction), or are rerouted toward their initial location through curved microchannels (reverse direction). We introduce variations of these "arches" microchannels and evaluate their respective axonal filtering capacities. Importantly, one of these variants presents an almost complete filtration of axonal growth in the non-permissive direction while allowing robust axonal invasion in the other one, with a selectivity ratio as high as 99.7%.

Continuous-flow trapping and localized enrichment of micro- and nano-particles using induced-charge electrokinetics.

In this work, we report an effective microfluidic technique for continuous-flow trapping and localized enrichment of micro- and nano-particles by using induced-charge electrokinetic (ICEK) phenomena. The proposed technique utilizes a simple microfluidic device that consists of a straight microchannel and a conducting strip attached to the bottom wall of the microchannel. Upon application of the electric field along the microchannel, the conducting strip becomes polarized to introduce two types of ICEK phenomena, the ICEK flow vortex and particle dielectrophoresis, and they are identified by a theoretical model formulated in this study to be jointly responsible for the trapping of particles over the edge of the conducting strip. Our experiments showed that successful trapping requires an AC/DC combined electric field: the DC component is mainly to induce electroosmotic flow for transporting particles to the trapping location; the AC component induces ICEK phenomena over the edge of the conducting strip for particle trapping. The performance of the technique is examined with respect to the applied electric voltage, AC frequency and the particle size. We observed that the trapped particles form a narrow band (nearly a straight line) defined by the edge of the conducting strip, thereby allowing localized particle enrichment. For instance, we found that under certain conditions a high particle enrichment ratio of 200 was achieved within 30 seconds. We also demonstrated that the proposed technique was able to trap particles from several microns down to several tens of nanometer. We believe that the proposed ICEK trapping would have great flexibility that the trapping location can be readily varied by controlling the location of the patterned conducting strip and multiple-location trapping can be expected with the use of multiple conducting strips.

Accurate Multi-Physics Numerical Analysis of Particle Preconcentration Based on Ion Concentration Polarization

This paper studies the mechanism of preconcentration of charged particles in a straight microchannel embedded with permselective membranes by numerically solving the coupled transport equations of ions, charged particles and solvent fluid without any simplifying assumptions. It is demonstrated that trapping and preconcentration of charged particles are determined by the interplay between drag force from the electroosmotic fluid flow and the electrophoretic force applied through the electric field. Several insightful characteristics are revealed, including the diverse dynamics of co-ions and counter ions, replacement of co-ions by focused particles, lowered ion concentrations in particle-enriched zone, and enhanced electroosmotic pumping effect, etc. Conditions for particles that can be concentrated are identified in terms of charges, sizes and electrophoretic mobilities of particles and co-ions. Dependences of enrichment factor on cross-membrane voltage, initial particle concentration and buffer ion concentrations are analyzed and the underlying reasons are elaborated. Finally, post priori condition for the validity of decoupled simulation model is given based on the charges carried by focused particles and buffer co-ions. These results provide an important guidance in the design and optimization of nanofluidic preconcentration and other related devices.

"From the Edge to the Center": Viscoelastic Migration of Particles and Cells in a Strongly Shear-Thinning Liquid Flowing in a Microchannel.

Controlling the fate of particles and cells in microfluidic devices is critical in many biomedical applications, such as particle and cell alignment and separation. Recently, viscoelastic polymer solutions have been successfully used to promote transversal migration of particles and cells toward fixed positions in straight microchannels. When inertia is negligible, numerical simulations have shown that strongly shear-thinning polymer solutions (fluids with a shear viscosity that decreases with increasing flow rates) promote transversal migration of particles and cells toward the corners or toward the centerline in a straight microchannel with a square cross section, as a function of particle size, cell deformability, and channel height. However, no experimental evidence of such shifting in the positions for particles or cells suspended in strongly shear-thinning liquids has been presented so far. In this work, we demonstrate that particle positions over the channel cross section can be shifted "from the edge to the center" in a strongly shear-thinning liquid. We investigate the viscoelasticity-induced migration of both rigid particles and living cells (Jurkat cells and NIH 3T3 fibroblasts) in an aqueous 0.8 wt % hyaluronic acid solution. The combined effect of fluid elasticity, shear-thinning, geometric confinement, and cell deformability on the distribution of the particle/cell positions over the channel cross section is presented and discussed. In the same shear-thinning liquid, separation of 10 and 20 μm particles is also achieved in a straight microchannel with an abrupt expansion. Our results envisage further applications in viscoelasticity-based microfluidics, such as deformability-based cell separation and viscoelastic spacing of particles/cells.

Control of two-phase flow in microfluidics using out-of-phase electroconvective streaming

We propose herein to make use of rotating electric fields for achieving flexible control on the hydrodynamic behavior of two miscible co-flowing water solutions in straight microchannels, in the context of a new manipulation tool for stratified liquid contents of microfluidic systems. Our theoretical analysis indicates that, while fluids of distinct electrical conductivities and identical permittivity are parallel pumped into the mainchannel, a circularly traveling field, as emitted from a four-phase electrode array surrounding the channel sidewalls, can direct the deflection of diffusing phase interface between the side-by-side miscible electrolyte streams asynchronously at half of the interfacial relaxation frequency, where the co-field electrorotational torque becomes most appreciable. An immediate application of the out-of-phase electroconvective streaming is that time required for electrolyte mixing is reduced because the area of two-phase contact interface is dramatically enlarged, and a serial combination of several rotating electrode arrays of alternate propagating directions can further rectify the mixing of microfluidics by inducing chaotic advection.

Microfluidic blood-plasma separation chip using channel size filtration effect

Blood separation is an essential first step when performing blood tests for clinical diagnosis purposes. Such tests are increasingly performed using microfluidic systems due to their rapid response time, low cost and potential for automated operation. This study proposes a simple microfluidic chip comprising a straight microchannel and a grating-type plasma filtration microchannel for the automatic separation of plasma from whole human blood for further biochemical detection. In the proposed device, the whole blood sample is introduced into the microfluidic chip by means of capillary forces and the plasma is then extracted from the whole blood via a size exclusion effect induced in the plasma filtration channel. The experimental results show that the plasma is extracted with an average flow rate of 0.01 μL/s. Moreover, the plasma collected in the outlet reservoirs has a residual cell concentration of less than 0.1%. The feasibility of the proposed device for clinical applications is demonstrated by means of creatinine detection tests with sample concentrations ranging from 0.11 to 1.78 mg/dL (10---160 μM) and prothrombin time (PT) tests with blood samples drawn from 20 healthy individuals. The experimental results show that the device has a detection sensitivity and degree of linearity of 12.68 count/μM and 95.38%, respectively. Moreover, the mean coagulation time in the PT tests is equal to approximately 13.3 s.

Comparison of Various Pressure Based Boundary Conditions for Three-Dimensional Subsonic DSMC Simulation

Three-dimensional (3D) direct simulation Monte Carlo (DSMC) has been used to simulate flow in a straight microchannel using an in-house parallelized code. In the present work, a comparative study of seven boundary conditions is carried out with respect to time required for achieving steady-state, accuracy in predicting the specified pressure at the boundaries, and the total simulation time required for attaining a statistical error within one percent. The effect of changing the Knudsen number, pressure ratio (PR), and cross aspect ratio (CAR) on these parameters is also studied. The presence of a boundary is seen to affect the simulated pressure in a cell when compared to the specified pressure, the difference being highest for corner cells and least for cells away from walls. All boundary conditions tested work well at the inlet boundary; however, similar results are not obtained at the outlet boundary. For the same cell size, the schemes that employ first- and second-order corrections lead to a smaller pressure difference compared to schemes applying no corrections. The best predictions can be obtained by using first-order corrections with finer cell size close to the boundary. For most of the simulated cases, the boundary condition employing the characteristic scheme with nonequilibrium effect leads to the minimum simulation time. Considering the nonequilibrium effect, prediction of inlet and outlet pressures and the speed of simulation, the characteristic scheme with nonequilibrium effect performs better than all the other schemes, at least over the range of parameters investigated herein.

Surface-conduction enhanced dielectrophoretic-like particle migration in electric-field driven fluid flow through a straight rectangular microchannel

An electric field has been extensively used to manipulate fluids and particles via electrokinetic flow in microchannels and nanochannels for various lab-on-a-chip applications. Recent studies have demonstrated the action of a dielectrophoretic-like lift force on near-wall particles in an electrokinetic flow due to the particles’ modifications of the field-line structure adjacent to a planar wall. This work presents a fundamental investigation of the lateral migration of dielectric particles in the electrokinetic flow of buffer solutions of varying molar concentrations through a straight rectangular microchannel. We find that the particle migration-induced electrokinetic centerline focusing is significantly enhanced with the decrease of the buffer concentration. This observed phenomenon may be attributed to the increased surface conduction effect in a lower-concentration buffer that yields a larger Dukhin number, Du. It seems qualitatively consistent with a recent theoretical study that predicts a greater wall-induced electrical lift with the increasing value of the Dukhin number for Du ≥ 1.

직접모사법을 이용한 형상 변화에 따른 미소채널 유동 해석

In the present study, to investigate the characteristics of subsonic rarefied gas flows in micro-channels with shape change, flow simulations of micro-channels with different geometries were conducted using a direct simulation Monte-Carlo (DSMC) solver based on unstructured meshes. For simulating micro-channel flows like pressure driven flows, the pressure boundary conditions at the inlet and the exit were additionally applied. Verification of the boundary condition was conducted by comparing with other researchers’ DSMC simulations with the pressure boundary condition. After verification of the boundary condition, the straight micro-channel of 15μm × 1μm was considered for the baseline channel. The pressure conditions at the inlet and the exit were 5×105 Pa and 1.66×105 Pa, respectively. The inlet gas temperature and wall temperature were both 300K, and a fully diffuse reflecting wall was considered. Three types of change of shapes were considered. The change of the length, height and bend angles were treated for this study. Firstly, to investigate the effects of the change of length, half and twice of the baseline channel length were considered. It was shown that the pressure gradient was dominantly affected by the change of the lengths. As a result, the velocity was more accelerated for short length channel than for the baseline channel. On the other hand, when the heights were become higher, the changes of flow properties were greater than the baseline channel due to the area of boundary layers or the forces by the pressure gradient. Finally, the effects of the bend angles in micro-channels were investigated with several bend angles. The angles were considered from 0 to 135 degrees. When the angle became greater, the flow separation was generated at the concave type corner. For larger angles, the variation of flow properties at the corner became greater. Flow properties at the exit were different trend from those at the corner. Pressure and density at the exit were not dependent on the bend angles, whereas the velocity at the exit was increased according to the bend angles.

Numerical study of enhanced mixing in pressure-driven flows in microchannels using a spatially periodic electric field.

We propose an innovative mechanism for enhancing mixing in steady pressure driven flow of an electrolytic solution in a straight rectangular microchannel. A transverse electric field is used to generate an electroosmotic flow across the cross-section. The resulting flow field consists of a pair of helical vortices that transport fluid elements along the channel. We show, through numerical simulations, that chaotic advection may be induced by periodically varying the direction of the applied electric field along the channel length. This periodic electric field generates a longitudinally varying, three-dimensional steady flow, such that the streamlines in the first half of the repeating unit cell intersect those in the second half, when projected onto the cross-section. Mixing is qualitatively characterized by tracking passive particles and obtaining Poincaré maps. For quantification of the extent of mixing, Shannon entropy is calculated using particle advection of a binary mixture. The convection diffusion equation is also used to track the evolution of a scalar species and quantify the mixing efficiency as a function of the Péclet number.