Mixing In Microfluidic Channels Research Articles

Experimental Investigations on the Cavitation Bubble Dynamics near the Boundary of a Narrow Gap

Cavitation bubbles near narrow gaps widely exist within microfluidic control devices. In the present paper, a laser-induced cavitation bubble is arranged in a narrow gap composed of two parallel plates. The inception position of the bubble is set to be at the same distance from the two plates so that the dynamic behaviors of the bubble are symmetrical. The collapse and rebound dynamics of the bubble near the boundary of a narrow gap are investigated through high-speed photography. The bubble behaviors (e.g., shape deformation, translational movement, and jet characteristics) are analyzed while considering the influence of the dimensionless distance between the bubble and the boundary and the dimensionless gap width. The principal findings include the following: (1) When the dimensionless distance is small, a violent jet towards the gap is generated during the bubble collapse stage, along with a weak counter-jet towards the boundary appearing during the rebound stage. (2) As the dimensionless distance increases, the translational distance of the bubble during the collapse stage initially decreases, then increases, and finally decreases to zero. (3) Within the parameter range considered in this paper, the dimensionless width mainly affects the expansion degree and movement direction of the bubble cloud during its rebound and subsequent stages. The above research findings can provide experimental support for bubble-driven flow control, pumping, and liquid mixing in microfluidic channels.

Quercetin nanocrystals prepared using a microfluidic chip with improved in vitro dissolution

Objective In order to improve the dissolution property of quercetin (QCT), the quercetin nanocrystals (QNCs) were prepared in this study. Methods QNCs were prepared by a 100 μm diameter Y-shape microfluidic channel. Some impact factors affecting the generation of QNCs such as concentration and flow rate were investigated. Furthermore, the fluid mixing in the microfluidic channel was simulated by fluid software. Results XRPD and DSC analyses indicated that the prepared QNCs were amorphous. Stable QNCs with a particle size of 77.9 ± 3.63 nm and polydispersity index of 0.26 ± 0.02 were obtained. TEM showed that the as-prepared QNCs had a uniform spherical shape with an average particle size of about 100–300 nm. In the dissolution medium without cosolvent Tween −80, the dissolution of QCT was poor, its final accumulated dissolution was only 3.95%, while that of QNCs was 66%. Conclusion When QCT was changed to QNCs by microfluidic technology, its dissolution property could be obviously improved. Therefore, microfluidic technology as a new method to prepare nanocrystals has a good applying prospect in improving dissolution property for poorly water-soluble drugs.

Magnetic Non-Spherical Particles Inducing Vortices in Microchannel for Effective Mixing.

Mixing in microfluidic channels is dominated by diffusion owing to the absence of chaotic flow. However, high-efficiency microscale mixing over short distances is desired for the development of lab-on-chip systems. Here, enhanced mixing in microchannels achieved using magnetic nonspherical particles (MNSPs), is reported. Benefiting from the nonspherical shape of the MNSPs, secondary vortices exhibiting cyclical characteristics appear in microchannels when the MNSPs rotate under an external magnetic field. Increasing the rotation rate enlarges the secondary vortices, expanding the mixing zone and enhancing the mixing, resulting in a mixing efficiency exceeding 0.9 at Re of 0.069-0.69. Complementary micro-particle image velocimetry (µPIV) for flow field analysis clarifies the mixing mechanism. In addition, a chaotic vortex area is generated in the presence of two MNSPs, which shortens the distance required for achieving an appropriate mixing efficiency. This study demonstrates the potential of employing MNSPs as efficient mixers in lab-on-chip devices.

Flow and mixing processes in a passive mixing microfluidic chip: Parameters’ estimation and colorimetric analysis

Objectives. The development of microfluidic systems is one of the promising areas of science and technology. In most procedures performed using microfluidic systems, effective mixing in microfluidic channels of microreactors (chips) is of particular importance, because it has an effect on the sensitivity and speed of analytical procedures. The aim of this study is to describe and evaluate the major parameters of the flow and mixing processes in a passive microfluidic micromixer, and to develop an information-measuring system to monitor the dynamics of flow (mixing) of liquids.Methods. This article provides an overview of the concept of microfluidic mixing chips (micromixers) and their classification, and analyzes the kinds of points of mixing and microfluidic channels for mixing. The article presents the description and calculations of the hydrodynamic similarity criteria (Reynolds, Dean and Peclet numbers), which are the critical parameters for creating and optimizing micromixers (for example, straight and curved channels in the flow rate range between 100 and 1000 µl/min). We have developed an information-measuring system to monitor the dynamics of flow (mixing) of liquids in a microfluidic channel, which consists of a microscope with a digital eyepiece (LOMO MIB, Russia), an Atlas syringe pump (Syrris Ltd., UK) and a passive mixing microfluidic chip of interest (made of clear glass). This system was designed to quickly illustrate the principles of mixing in microfluidic channels of different configurations.Results. The developed system has allowed carrying out a colorimetric analysis of the modes and dynamics of mixing two liquids (5% aqueous solution of azorubine dye and water) at the T-shaped mixing point, at the straight and curved (double-bend shaped) sections of the microfluidic channel of the passive-type micromixer with flow rates varying from 100 to 400 µl/min.Conclusions. According to the obtained calculations, the share of the advective mixing processes (formation of vortex flows and increase in the contact area of the mixed substances) in flowing liquids is significantly higher in curved microchannels. The developed information-measuring system to monitor the dynamics of flow (mixing) of liquids in a microfluidic channel is a convenient tool for optimizing the mixing modes in the channels of micromixers, and for designing new configurations of channels in microchips. It would allow intensifying processes and increasing the performance of microfluidic systems.

Rotational electrohydrodynamics of a non-Newtonian fluid under electrical double-layer phenomenon: the role of lateral confinement

We investigate the transient analysis of the transport features of a non-Newtonian fluid in a rotating microfluidic channel as modulated by the electrical double-layer effect. We use the power-law model to describe rheology of the non-Newtonian fluid in this study. We bring out the rotational force-induced development of the secondary flows inside the channel, taking the effects of the lateral confinement into account. We show that the consideration of lateral confinement into the analysis gives rise to a complex flow dynamics, allowing the formation of double-vortex structures as well as the sister vortexes in the flow field. In particular, we show that the sister vortexes formed in the flow field exhibit different senses of rotations under the influence of the electrical forcing, leading to a potential enhancement in mixing in microfluidic channel. Also, we show the variation of the volume flow rate through the channel for different cases and unveil the secondary flow-induced alteration in the device throughput. We believe that the inferences obtained from this analysis may improve the design of miniaturized systems/devices, typically used for the transportation of bio-fluids, which are largely non-Newtonian in nature, in a rotating platform.

Symmetry-breaking Bifurcations in T-channel Flows: Effects of Fluid Viscoelasticity

It is well known that, beyond a critical aspect-ratio-dependent Reynolds number, the flow in a three-dimensional T-channel junction, i.e. two-opposing planar channel streams joining and turning through 90 degrees, can break symmetry. For the case of two square cross-section inlets and an outlet arm of equal area (i.e. aspect ratio of two) this bifurcation is to a steady asymmetric flow. This flow bifurcation has been proposed as a method of enhancing mixing in microfluidic channels where significant increases in mixing quality are observed beyond the bifurcation. In the current work we investigate numerically the effects of viscoelasticity on this supercritical pitchfork bifurcation using a numerical finite-volume method. Results from both the upper-convected Maxwell and Oldroyd-B models show that the instability occurs at lower Reynolds numbers for viscoelastic fluids in comparison to the Newtonian base case. At higher Deborah numbers the transition leads directly to an unsteady flow.

Bifurcation in a T-channel junction: Effects of aspect ratio and shear-thinning

It is well known that, beyond a critical aspect-ratio-dependent Reynolds number, the flow in a three-dimensional T-channel junction, i.e. two-opposing planar channel streams joining and turning through 90°, can break symmetry. This flow bifurcation has been proposed as a method of enhancing mixing in microfluidic channels where significant increases in mixing quality are observed beyond the bifurcation. In the current paper we investigate the critical conditions for the onset of this steady symmetry-breaking phenomenon in terms of both the inlet and outlet channel aspect ratios across a wider parameter range than has been probed hitherto. In so-doing we show that the change in flow conditions from symmetric to asymmetric flow is, in most cases, a steady supercritical pitchfork bifurcation and we confirm that an existing correlation (Soleymani et al., 2008b) for predicting the critical conditions can fail under certain aspect ratio combinations. In addition we find bifurcation occurs for many cases in which a recent study claimed it was absent (Cherlo and Pushpavanam, 2010) and in one of these cases the bifurcation is to an unsteady asymmetric state. Additionally, we investigate the effect of non-Newtonian fluid characteristics on the critical Reynolds number. In particular we use the power-law model to investigate the effect of shear-thinning in the power-law index range 0.5≤n≤1.

Milliseconds mixing in microfluidic channel using focused surface acoustic wave

A feasible approach of acoustic wave based ultra-fast homogeneous mixing in microfluidic channel is reported. After comparing simulation models of energy distribution between parallel interdigital transducers (IDTs) and concentric circular type focused interdigital transducers (F-IDTs), the F-IDTs were designed and built into microfluidic device to generate focused surface acoustic wave in a specific region of the microchannel. In the acoustic enhanced mixing region, continuous laminar flow was mixed efficiently in milliseconds by concentrated acoustic radiation. The active enhanced ultra-fast mixing was optimized and analyzed experimentally. This method could be developed for fast chemical or biochemical reactions and assays.

Stimulus-responsive fluidic dispersions of rod shaped liquid crystal polymer colloids

Electrically responsive liquid crystal polymer nanorods were fabricated by template synthesis. Liquid crystal monomers are templated by alumina membranes. Molecular ordering of the liquid crystal molecules resulted from the confinement in the sub-micron channels and this ordering can be captured permanently through photo-polymerization. Template removal and sonication result in individual rods that can be reoriented by applied electrical and magnetic fields. Such anisotropic particles have significant potential applications in electro-rheological fluids and in active mixing in microfluidic channels.

CFD modelling of liquid–liquid multiphase microstructured reactor: Slug flow generation

Microreactor technology, an important method of process intensification, offers numerous potential benefits for the process industries. Fluid–fluid reactions with mass transfer limitations have already been advantageously carried out in small-scale geometries. In liquid–liquid microstructured reactors (MSR), alternating uniform slugs of the two-phase reaction mixture exhibit well-defined interfacial mass transfer areas and flow patterns. The improved control of highly exothermic and hazardous reactions is also of technical relevance for large-scale production reactors. Two basic mass transfer mechanisms arise: convection within the individual liquid slugs and diffusion between adjacent slugs. The slug size in liquid–liquid MSR defines the interfacial area available for mass transfer and thus the performance of the reactor. There are two possibilities in a slug flow MSR depending on the interaction of the liquids with the solid wall material: a dispersed phase flow in the form of an enclosed slug in the continuous phase (with film—complete wetting of the continuous phase) and an alternate flow of two liquids (without film—partial wetting of the continuous phase). In the present work, a computational fluid dynamics (CFD) methodology is developed to simulate the slug flow in the MSR for both types of flow systems. The results were validated with the experimental results of Tice et al. (J.D. Tice, A.D. Lyon and R.F. Ismagilov, Effects of viscosity on droplet formation and mixing in microfluidic channels, Analytica Chimica Acta 507 (1) (2004), pp. 73–77.).

Assessment of three AC electroosmotic flow protocols for mixing in microfluidic channel

This study performs an experimental investigation into the micromixer capabilities of three different protocols of AC electroosmotic flow (AC EOF), namely capacitive charging (CC), Faradaic charging (FC) and asymmetric polarization (AP). The results reveal that the vortices generated by the FC protocol (the frequency is around 50-350 Hz) are stronger than those induced by the CC protocol (the frequency is higher than 350 Hz), and therefore provide an improved mixing effect. However, in the FC protocol, the frequency of the external AC voltage must be carefully controlled to avoid damaging electrodes as a result of Faradaic reactions. The experimental results indicate that the AP polarization effect (the applied voltage and frequency are V(1) = 1 V(pp) and V(2) = 20 V(pp)/5 kHz) induces more powerful vortices than either the CC protocol or the FC protocol, and therefore yields a better mixing performance. Two AP-based micromixers are fabricated with symmetric and asymmetric electrode configurations, respectively. The mixing indices achieved by the two devices after an elapsed time of 60 seconds are found to be 56.49 % and 71.77 %, respectively. This result shows that of the two devices, an asymmetric electrode configuration represents a more suitable choice for micromixer in microfluidic devices.

Focusing-enhanced mixing in microfluidic channels

A focusing-based microfluidic mixer was studied. The micromixer utilizes the focusing process required for cytometry to reduce the diffusion distance of molecules to be mixed in order to facilitate the passive diffusion-controlled mixing process. It was found that both the high flow rate ratio of the sheath flow to the flows to be mixed and the low flow rate of the mixing fluids resulted in the short mixing length required within the microfluidic channel. It was shown that a complete mixing was achieved within a distance of 4 mm in the micromixer for the focused mixing fluids at a flow rate of 2 mulmin and a flow rate ratio of the sheath flow to the flows to be mixed at 4:1. The mixer described here is simple and can be easily fabricated and controlled.

Laser-Induced Mixing in Microfluidic Channels

We demonstrate a novel strategy for mixing solutions and initiating chemical reactions in microfluidic systems. This method utilizes highly focused nanosecond laser pulses from a Q-switched Nd:YAG laser at lambda = 532 nm to generate cavitation bubbles within 100- and 200-microm-wide microfluidic channels containing the parallel laminar flow of two fluids. The bubble expansion and subsequent collapse within the channel disrupts the laminar flow of the parallel fluid streams and produces a localized region of mixed fluid. We use time-resolved imaging and fluorescence detection methods to visualize the mixing process and to estimate both the volume of mixed fluid and the time scale for the re-establishment of laminar flow. The results show that mixing is initiated by liquid jets that form upon cavitation bubble collapse and occurs approximately 20 micros following the delivery of the laser pulse. The images also reveal that mixing occurs on the millisecond time scale and that laminar flow is re-established on a 50-ms time scale. This process results in a locally mixed fluid volume in the range of 0.5-1.5 nL that is convected downstream with the main flow in the microchannel. We demonstrate the use of this mixing technique by initiating the horseradish peroxidase-catalyzed reaction between hydrogen peroxide and nonfluorescent N-acetyl-3,7-dihydroxyphenoxazine (Amplex Red) to yield fluorescent resorufin. This approach to generate the mixing of adjacent fluids may prove advantageous in many microfluidic applications as it requires neither tailored channel geometries nor the fabrication of specialized on-chip instrumentation.

Deconvolution Microscopy for Flow Visualization in Microchannels

Quantitative visualization of microflows is often needed to evaluate the efficiency of fluid mixing, study flow properties, investigate unusual flow behavior, and verify computational fluid dynamic simulations. In this work, we explore the technique of coupling a conventional optical microscope with a computational deconvolution algorithm to produce images of three-dimensional flows in plastic microfluidic channels. The approach, called deconvolution microscopy, is achieved by (1) optically sectioning the flow in the microchannel by collecting a series of fluorescence images at different focal planes along the optical axis and (2) removing the out-of-focus fluorescence signal by a deconvolution method to reconstruct the corrected three-dimensional concentration image. We compare three different classes of deconvolution algorithms for a uniform concentration test case and then demonstrate how deconvolution microscopy is useful for flow visualization and analysis of mixing in microfluidic channels. In particular, we employ the method to confirm the presence of twisting flows in a microchannel containing microfabricated ridges.

Ultrasonic mixing in microfluidic channels using integrated transducers.

This paper presents a microfluidic mixer that uses acoustic stirring created by ultrasonic waves. The ultrasound is introduced into the channel by integrated piezoelectric transducers. The transducers are made of a zinc oxide thin film, which is deposited on the bottom surface of a quartz substrate. The poly(dimethylsiloxane) channel is aligned to the transducers on the top surface of the substrate. The transducers are designed for operation around 450 MHz. The main mechanism of the mixing is the acoustic stirring of the fluid perpendicular to the flow direction. The radiation pressure that is generated by the transducer causes the stirring inside the microfluidic channel. The performance of the mixer is characterized by mixing phenolphthalein solution and sodium hydroxide dissolved in ethyl alcohol. Flow rates on the order of 1-100 microL/min are used. The transducers are driven by 1.2 V(rms) sinusoidal voltages at 450 MHz.

Effects of viscosity on droplet formation and mixing in microfluidic channels

This paper characterizes the conditions required to form nanoliter-sized droplets (plugs) of viscous aqueous reagents in flows of immiscible carrier fluid within microfluidic channels. For both non-viscous (viscosity of 2.0 mPa s) and viscous (viscosity of 18 mPa s) aqueous solutions, plugs formed reliably in a flow of water-immiscible carrier fluid for Capillary number less than 0.01, although plugs were able to form at higher Capillary numbers at lower ratios of the aqueous phase flow rate to the flow rate of the carrier fluid (in all the experiments performed, the Reynolds number was less than 1). The paper also shows that combining viscous and non-viscous reagents can enhance mixing in droplets moving through straight microchannels by providing a nearly ideal initial distribution of reagents within each droplet. The study should facilitate the use of this droplet-based microfluidic platform for investigation of protein crystallization, kinetics, and assays.