Straight Microchannel Research Articles

A generalized formula for inertial lift on a sphere in microchannels.

Inertial microfluidics has been widely used in high-throughput manipulation of particles and cells by hydrodynamic forces, without the aid of externally applied fields. The performance of inertial microfluidic devices largely relies on precise prediction of particle trajectories that are determined by inertial lift acting on particles. The only way to accurately obtain lift forces is by direct numerical simulation (DNS); however, it is burdensome when applied to practical microchannels with complex geometries. Here, we propose a fitting formula for inertial lift on a sphere drawn from DNS data obtained in straight channels. The formula consists of four terms that represent the shear-gradient-induced lift, the wall-induced lift, the slip-shear lift, and the correction of the shear-gradient-induced lift, respectively. Notably, as a function of the parameters of a local flow field, it possesses good adaptability to complex channel geometries. This generalized formula is further implemented in the Lagrangian particle tracking method to realize fast prediction of particle trajectories in two types of widely used microchannels: a long serpentine and a double spiral microchannel, demonstrating its ability to efficiently design and optimize inertial microfluidic devices.

Complex flow dynamics around 3D microbot prototypes.

A new experimental setup for the study of the complex flow dynamics around 3D microbot prototypes in a straight microchannel has been developed and assessed. The ultimate aim of this work is focused on the analysis of the morphology of different microbot prototypes to get a better insight into their efficiency when they swim through the main conduits of the human circulatory system. The setup consists of a fused silica straight microchannel with a 3D microbot prototype fastened in the center of the channel cross-section by an extremely thin support. Four different prototypes were considered: a cube, a sphere and two ellipsoids with aspect ratios of 1 : 2 and 1 : 4, respectively. Flow visualization and micro-particle image velocimetry (μPIV) measurements were performed using Newtonian and viscoelastic blood analogue fluids. An efficiency parameter, ℑ, to discriminate the prototypes in terms of flow disturbance has been proposed.

3D simulation and analytical model of chemical heating during silicon wet etching in microchannels

We investigate chemical heating of a Silicon-on-Glass (SOG) chip during a highly exothermic reaction of silicon etching in potassium hydroxide (KOH) solution in a microchannel of 100-micron width inside a 1x1 cm SOG chip. Two modeling approaches have been developed, implemented and compared. (1) A detailed 3D model is based on unsteady Navier-Stokes equations, heat and mass transfer equations of a laminar flow of viscous incompressible fluid in microchannel, coupled to the heat transfer equation in the solid chip. 3D simulation results predicted temperature distributions for different KOH flow rates and silicon etching areas. Microchannels of a small diameter do not heat the chip due to the insufficient chemical heating of the cold fluid, whereas large-area etching (large channel diameter and/or length) leads to local overheating that may have negative effects on the device performance and durability. (2) A simplified analytical model solves a thermal balance equation describing the heating by chemical reactions inside the microchannel and energy loss by free convection of air around the chip. Analytical results compare well with the 3D simulations of a single straight microchannel, therefore the analytical model is suitable for quick estimation of process parameters. For complex microstructures, this simplified approach may be used as the first approximation.

Particulate Blood Analogues Reproducing the Erythrocytes Cell-Free Layer in a Microfluidic Device Containing a Hyperbolic Contraction.

The interest in the development of blood analogues has been increasing recently as a consequence of the increment in the number of experimental hemodynamic studies and the difficulties associated with the manipulation of real blood in vitro because of ethical, economical or hazardous issues. Although one-phase Newtonian and non-Newtonian blood analogues can be found in the literature, there are very few studies related to the use of particulate solutions in which the particles mimic the behaviour of the red blood cells (RBCs) or erythrocytes. One of the most relevant effects related with the behaviour of the erythrocytes is a cell-free layer (CFL) formation, which consists in the migration of the RBCs towards the center of the vessel forming a cell depleted plasma region near the vessel walls, which is known to happen in in vitro microcirculatory environments. Recent studies have shown that the CFL enhancement is possible with an insertion of contraction and expansion region in a straight microchannel. These effects are useful for cell manipulation or sorting in lab-on-chip studies. In this experimental study we present particulate Newtonian and non-Newtonian solutions which resulted in a rheological blood analogue able to form a CFL, downstream of a microfluidic hyperbolic contraction, in a similar way of the one formed by healthy RBCs.

Continuous sheath-free separation of particles by shape in viscoelastic fluids

Shape is an important indicator of cell type, cycle, and state, etc., and can thus serve as a specific marker for label-free bioparticle separation. We demonstrate in this work a shape-based separation of equal-volumed spherical and peanut particles in viscoelastic fluids through straight rectangular microchannels. This continuous sheath-free separation arises from the shape-dependent equilibrium particle position(s) as a result of the flow-induced elasto-inertial lift and shear thinning effects. A continuous transition from single to dual and to triple equilibrium positions is observed for both types of particles with the increase in flow rate. However, the flow rate at which the transition takes place differs with the particle shape. This phenomenon occurs only in microchannels with a large aspect ratio (width/height) and has not been reported before. It is speculated to correlate with the dissimilar dependences of elastic and inertial lift forces on particle size and flow rate as well as the rotational effects of non-spherical particles.

New designs of ceramic hollow fibres toward broadened applications

Three structural designs for ceramic membranes have been achieved for the first time through the co-extrusion of polymeric and ceramic layers. During co-extrusion, micro-channels are initiated due to the Rayleigh–Taylor instability and they propagate through the different layers. The polymeric layer(s) is then calcined off during the heat treatment step, which opens the micro-channels and following sintering a ceramic membrane with open micro-channels ranging from a few to a few tens of micrometres in diameter can be formed. These long, straight and non-tortuous micro-channels can be controlled to be open at any or all of the surfaces. Design 1 has open micro-channels passing through the entire membrane wall, Design 2 has a separation layer at the lumen and open micro-channels at the shell side, and Design 3 has open micro-channels from both lumen and shell sides sandwiching a separation layer of sponge-like structure. Aside from having much improved mass transfer property due to the reduced effective membrane thickness, they can be easily incorporated into hybrid systems with anticipated improvements in unit compactness and performance. The pure water permeation of Design 2 reached up to 159,000L/m2hbar with pore sizes in the micro-filtration range. The micro-channels are easily accessible from the shell/lumen side; therefore catalysts or adsorbents can be easily deposited into the micro-channels. Examples of possible applications include a high-efficiency dispersing device realised with Design 1; a gas chromatography column for gas separation with very low pressure drop realised with Design 2 and a highly compact membrane micro-reactor for consecutive reactions proposed with Design 3.

Theoretical study of time-dependent, ultrasound-induced acoustic streaming in microchannels.

Based on first- and second-order perturbation theory, we present a numerical study of the temporal buildup and decay of unsteady acoustic fields and acoustic streaming flows actuated by vibrating walls in the transverse cross-sectional plane of a long straight microchannel under adiabatic conditions and assuming temperature-independent material parameters. The unsteady streaming flow is obtained by averaging the time-dependent velocity field over one oscillation period, and as time increases, it is shown to converge towards the well-known steady time-averaged solution calculated in the frequency domain. Scaling analysis reveals that the acoustic resonance builds up much faster than the acoustic streaming, implying that the radiation force may dominate over the drag force from streaming even for small particles. However, our numerical time-dependent analysis indicates that pulsed actuation does not reduce streaming significantly due to its slow decay. Our analysis also shows that for an acoustic resonance with a quality factor Q, the amplitude of the oscillating second-order velocity component is Q times larger than the usual second-order steady time-averaged velocity component. Consequently, the well-known criterion v(1)≪c(s) for the validity of the perturbation expansion is replaced by the more restrictive criterion v(1)≪c(s)/Q. Our numerical model is available as supplemental material in the form of comsol model files and matlab scripts.

A three-dimensional quantitative study on the hydrodynamic focusing of particles with the immersed boundary – Lattice Boltzmann method

Hydrodynamic focusing of particles is numerically studied by the immersed boundary – lattice Boltzmann method. Particle focusing entropy is proposed to quantitatively characterize processing performance and final results of hydrodynamic focusing. Simulations of hydrodynamic focusing in several straight microchannels are carried out to evaluate versatility of the focusing entropy. Time evolutions of focusing entropies and particle trajectories are analyzed contrastively. The results demonstrate that the focusing entropy is an effective scale to measure particles ordering degree and hydrodynamic focusing performance. Higher ordering degree determines lower focusing entropy, which indicates better focusing performance. Channel cross section, particle rigidness and channel Reynolds number are three major factors influencing focusing dynamics and final results. Rectangular microchannel is more advantageous than circular and square ones in hydrodynamic focusing. Particles of different rigidness in rectangular microchannel can be separated significantly with the flow mediation. Increasing channel Reynolds numbers can lead to higher efficiency and better focusing performance.

COMPUTATIONAL MODEL OF PLATELET FLOW IN CAROTID ARTERY BIFURCATION

Atherosclerotic plaque tends to occur and develop in carotid artery sinus, where stenoses and other lesions can cause cerebral disturbances. The hemodynamics and the platelet profiles in the carotid artery are simultaneously simulated in the present study to reveal the mechanism of the atherosclerotic plaque formation. Firstly, an unsteady PISO solver on unstructured tetrahedral grids is developed to simulate the hemodynamics of carotid artery bifurcation. And small platelets are treated as spherical solid particle described by Newtonian motion equation in the simulation, including inertial force, drag force, shear lift force, virtual mass force and pressure gradient force. Secondly, the visualization experiment for the platelet flow in the straight micro-channel is setup to validate the numerical model. Using the present method, the platelet flow in a carotid artery bifurcation is analyzed. The results show that the outer vascular wall near the bifurcation is more vulnerable in carotid artery sinus, which is mainly due to the lower wall shear stress (WSS) and platelets backflow in the region.

Fabrication of alumina micromixer with two‐dimensional serpentine microchannels by centrifuge‐assisted micromoulding

A two-dimensional (2D) serpentine mixer was made of alumina using centrifuge-assisted micromoulding. Concentrated aqueous alumina slurry was filled into microstructured polydimethylsiloxane micromoulds with the aid of centrifugation to form green micromixers, which were then sintered to obtain the final products. In contrast to the micromixer fabricated by micromoulding (filling without the aid of centrifugation), it is found that using centrifugation significantly improved the structural integrity of the green micromixer. Straight and serpentine microchannels with rectangular cross-sections were precisely fabricated. After sintering at 1600(o)C, defect-free alumina micromixer with a relative density of 98.7%, linear shrinkage of about 18%, hardness of 20.9 GPa, flexural strength of 328.2 MPa and roughness of 143.9 nm was attained. The results show that ceramic microdevices with complex-shaped 2D microchannels (with diameters 100 m and aspect ratios 2) can be reliably fabricated by centrifuge-assisted micromoulding.

Preconcentration of lipid vesicles using concentration polarization in a microfluidic chip

This paper presents the first lipid vesicle preconcentrator using concentration polarization in a microfluidic chip. Concentration polarization is a well-known electrokinetic phenomenon, occurring near a micro/nanochannel interface. The preconcentrator is composed of a straight microchannel and a nanoporous membrane (Nafion strip) in it, which serves as a preconcentrating zone. We visualized the concentrated lipid vesicles near the Nafion strip and characterized the concentrating performance for different applied voltages. The maximum performance is the concentration ratio of 160 with 100V in 10min. This performance is at least comparable with the previous work and we believe that it has the advantages of lab-on-a-chip system compatibility and a relatively easy fabrication. Our concentration polarization platform on a PDMS chip is versatile so that the same device can be applied to other biomolecule preconcentration.

Conjugate heat transfer in a wavy microchannel using nanofluid by two-phase Eulerian–Lagrangian method

In this paper, conjugate heat transfer in wavy microchannels using nanofluid has been studied numerically. The nanofluid has been simulated as two-phase model using Eulerian–Lagrangian approach. Nanofluid is water–Al2O3 with 120nm diameter and volume fraction from 0% to 2%. Three-dimensional governing equations including continuity, Navier-Stokes and energy equations have been solved by well-known SIMPLE method. The governing equations for particles have been solved by four order of Runge–Kutta. Two-ways coupling between the fluid and particles phases is employed and appears in the momentum and energy equations of fluid as source terms. The results show a good agreement with experimental results. The Nusselt number increases because of wavy walls of the microchannel in compare with straight walls microchannels and because of using nanofluid instead of water as working fluid. Also, the increase in particles volume fraction result in Nusselt number increment without sensitive increase in pressure drop.

Investigation of nanofluids heat transfer in a ribbed microchannel heat sink using single-phase and multiphase CFD models

In this paper, laminar forced convective heat transfer of water/alumina nanofluids in a straight microchannel is studied numerically using CFD modeling. In the first part, single-phase and two-phase mixture models have been used for prediction of hydrodynamics and heat transfer parameters of nanofluids in a simple microchannel heat sink. The CFD predictions were compared to the experimental data and it was concluded that the two-phase approach gives better predictions compared to the single-phase model. The effects of ribs turbulators on the fluid flow and heat transfer performance of microchannel were investigated in the second part. The effects of geometrical characteristics of the ribs were studied, and the results showed that the Nusselt number and friction coefficient of nanofluids in the ribbed microchannel are higher than those of simple microchannel, and this enhancement increased with increasing the width of the ribs.

Peclet Number Dependence of Mass Transfer in Microscale Segmented Gas–Liquid Flow

A detailed understanding of the scaling behavior associated with the fluid flow and the transport of gas molecules from a train of elongated gas plugs into neighboring liquid segments is of great importance for a broad range of microscale applications. The indirect dependence of the parameters affecting the Capillary and Peclet numbers and thereby scaling behavior (i.e., the velocity and length of the gas plugs, and the length of the liquid segments) on the directly adjustable experimental inputs (i.e., flow rate or pressure of each phase) has hindered the systematic investigation of scaling behavior in microscale gas–liquid flows. Here, we take advantage of an image-based feedback strategy that allows us to directly impose Capillary and Peclet numbers. We custom fabricated a long, straight microchannel (width 300 μm, length-to-width ratio 700) in a gas impermeable silicon–glass substrate. We automatically determined the length reduction of initially uniformly sized gas plugs at different positions along ...

Numerical analysis of mixing performance in sinusoidal microchannels based on particle motion in droplets

This numerical study was conducted to analyze and understand the parameters that affect the mixing per- formance of droplet-based flow in sinusoidal microfluidic channels. Finite element analysis was used for modeling fluid flow and droplet formation inside the microchannels via tracking interface between the two heterogeneous flu - ids along with multiple particle trajectories inside a droplet. The solutions of multiphase fluid flow and particle trajec - tories were coupled with each other so that drag on every single particle changed in every time step. To solve fluid motion in multiphase flow, level set method was used. Para - metric study was repeated for different channel dimensions and different sinusoidal channel profiles. These results were compared with mixing in droplets inside a straight microchannel. Additionally, tracking of multiple particles inside a droplet was performed to simulate the circulating flow profile inside the droplets. Based on the calculation of the dispersion length, particle trajectories, and velocities inside droplets, it is concluded that having smaller chan- nel geometries increases the mixing performance inside the droplet. This also shows that droplet-based fluid flow in microchannels is very suitable for performing chemical reactions inside droplets as it will occur faster. Moreover, narrower and sinusoidal microchannels showed better dis- persion length difference compared to straight and wider microchannels.

On mechanisms of choked gas flows in microchannels

Choked gas flows in microchannels have been reported before based solely on experimental measurements, but the underlining physical mechanism has yet to be clarified. In this work, we are to explore the process via numerical modeling of choked gas flows through a straight microchannel that connects two gas reservoirs. The major theoretical consideration lies in that, since the gas in microchannels may not be necessarily rarefied even at a high Knudsen number, a generalized Monte Carlo method based on the Enskog theory, GEMC, was thus used instead of direct simulation Monte Carlo (DSMC). Our results indicate that the choked gas flows in microchannels can be divided into two types: sonic choking and subsonic choking, because the sonic point does not always exist even though the gas flows appear choked, depending on the inlet–outlet pressure ratio and the length–height ratio of the channel. Even if the gas flow does not reach a sonic point at the outlet region, the effective pressure ratio (pi/po) acting on the channel becomes asymptotically changeless when the pressure ratio on the buffer regions (pi′/po′) is higher than a certain value. The subsonic choking may caused by the expansion wave or the strong non-equilibrium effect at the outlet.

Live-streaming: Time-lapse video evidence of novel streamer formation mechanism and varying viscosity.

Time-lapse videos of growing biofilms were analyzed using a background subtraction method, which removed camouflaging effects from the heterogeneous field of view to reveal evidence of streamer formation from optically dense biofilm segments. In addition, quantitative measurements of biofilm velocity and optical density, combined with mathematical modeling, demonstrated that streamer formation occurred from mature, high-viscosity biofilms. We propose a streamer formation mechanism by sudden partial detachment, as opposed to continuous elongation as observed in other microfluidic studies. Additionally, streamer formation occurred in straight microchannels, as opposed to serpentine or pseudo-porous channels, as previously reported.

Shear-induced tail breakup of droplets (bubbles) flowing in a straight microfluidic channel

Dispersed droplets and bubbles were generated in a microfluidic flow-focusing device to investigate the morphological developments of the droplets (bubbles) flowing in the downstream straight micro-channel by a high speed camera. The present study is mainly aimed at the influences of both the dispersed droplet (bubble) size and two-phase average flow velocity on the formation of tip stream in the rear of a droplet (bubble). It was found that the particle deformation increased with the droplet (bubble) length or the capillary number. There exists a critical droplet (bubble) length dependent on capillary number, beyond which the tip stream ejecting tiny daughter droplets will take place. The generated tiny droplets are usually three orders of magnitude smaller than primary droplet. This study provides the possibility to avoid a high polydispersity index in microchannels.

Versatile liquid-core optofluidic waveguides fabricated in hydrophobic silica aerogels by femtosecond-laser ablation

We report on the fabrication and characterization of versatile light waveguides exploiting filaments of a polar liquid confined within hydrophobic silica aerogels. Aerogels are highly porous materials with extremely low refractive index which makes them suitable as rigid cladding of liquid-core optofluidic waveguides based on total internal reflection of light. In this article, we introduce a new microfabrication technique that allows direct and precise processing of monolithic silica aerogels by ablation with femtosecond laser pulses. Using fast scanning of the focused laser ablation beam synchronized with the motion of the processed aerogel sample, we created high-quality straight microchannels of ∼5mm length with controlled cross-sections inside monolithic aerogels. After the ablation, we filled the channels with high-refractive index ethylene glycol, forming multimode liquid core – solid cladding optofluidic waveguides. Subsequently, we carried out light-guiding experiments to measure overall optical attenuation of these waveguides. The characterization of waveguide transmission yielded values of propagation losses lower than 10dBcm-1, demonstrating that the liquid-core waveguides with laser-ablated aerogel cladding represent an attractive alternative in optofluidic applications targeting controlled routing of light along arbitrary three-dimensional paths.

Size-Based Separation of Particles and Cells Utilizing Viscoelastic Effects in Straight Microchannels.

Viscoelasticity-induced particle migration has recently received increasing attention due to its ability to obtain high-quality focusing over a wide range of flow rates. However, its application is limited to low throughput regime since the particles can defocus as flow rate increases. Using an engineered carrier medium with constant and low viscosity and strong elasticity, the sample flow rates are improved to be 1 order of magnitude higher than those in existing studies. Utilizing differential focusing of particles of different sizes, here, we present sheathless particle/cell separation in simple straight microchannels that possess excellent parallelizability for further throughput enhancement. The present method can be implemented over a wide range of particle/cell sizes and flow rates. We successfully separate small particles from larger particles, MCF-7 cells from red blood cells (RBCs), and Escherichia coli (E. coli) bacteria from RBCs in different straight microchannels. The proposed method could broaden the applications of viscoelastic microfluidic devices to particle/cell separation due to the enhanced sample throughput and simple channel design.