A novel passive micromixer: lamination in a planar channel system

A novel passive micromixer concept is presented. The working principle is to make a controlled 90° rotation of a flow cross-section followed by a split into several channels; the flow in each of these channels is rotated a further 90° before a recombination doubles the interfacial area between the two fluids. This process is repeated until achieving the desired degree of mixing. The rotation of the flow field is obtained by patterning the channel bed with grooves. The effect of the mixers has been studied using computational fluid mechanics and prototypes have been micromilled in poly(methyl methacrylate). Confocal microscopy has been used to study the mixing. Several micromixers working on the principle of lamination have been reported in recent years. However, they require three-dimensional channel designs which can be complicated to manufacture. The main advantage with the present design is that it is relatively easy to produce using standard microfabrication techniques while at the same time obtaining good lamination between two fluids.     

Thermocapillary actuation of droplet in a planar microchannel

A study on thermocapillary actuation of liquid droplet in a planar microchannel has been carried out by both theoretical modeling and experimental characterization. The driving temperature gradients are provided by four heaters at the channel boundaries. In the modeling, the temperature distributions corresponding to transient actuation are calculated, and are coupled to the droplet through the surface tensions which drive the droplet to move inside the channel. The droplet trajectories and final positions are predicted, and are compared with the experimental observations, in which a silicon oil droplet was actuated inside a 10 mm  ×  10 mm planar channel with four heater fabricated on the substrate plate. The results show that the droplet can be positioned anywhere in the channel, determined by a heating code related to the heating strengths. Qualitative agreement between the modeling results and experimental data, in terms of temperature distributions, droplet trajectories and positions, has been obtained.     

Mixing performance of split-and-recombine micromixer with offset inlets

Microsystem Technologies - This study proposes a novel 3D split-and-recombine passive micromixer with offset-inlets. The micromixer is composed of non-aligned inlets and spatially repeating mixing...     

A centrifugal force-based serpentine micromixer (CSM) on a plastic lab-on-a-disk for biochemical assays

In this paper, a centrifugal force-based serpentine micromixer (CSM) on a plastic lab-on-a-disk (LOD) for biochemical assay was designed, fabricated, and fully characterized with numerical and experimental methods. The CSM comprised two inlets, an outlet, and a serpentine microchannel composed of five circumferential channels with connecting radial channels in one layer. The centrifugal force induced in the rotating disk thoroughly mixed the sample and reagent together throughout the serpentine microchannel of the CSM. Despite its simple geometry, effective mixing performance was achieved inside the CSM because of transverse secondary flows and the three-dimensional stirring effect in the microchannel. Numerical simulation showed that the interfaces of the two streams inside the circumferential microchannel were efficiently stirred by the induced transversal velocity field. The plastic LOD was fabricated by CNC-micromilling on one layer of the thermoplastic substrate, followed by thermal bonding with a cover plastic substrate. Mixing performance of the CSM was also investigated experimentally by means of colorimetric analysis using phenolphthalein. High levels of distributive mixings were obtained within a short required mixing length. As a proof-of-concept example, a biochemical assay of albumin level was successfully determined with the help of the LOD containing the CSM. Owing to the mass-producible simple geometry, excellent mixing performance, and convenience, the CSM can be applied to biochemical assays based on the centrifugal microfluidics.     

Valve-based microfluidic droplet micromixer and mercury (II) ion detection

A valve-based microfluidic micromixer was developed for multiply component droplets generation, manipulation and active mixing. By integrating pneumatic valves in microfluidic device, droplets could be individually generated, merged and well mixed automatically. Moreover, droplet volume could be controlled precisely by tuning loading pressure or the flow rate of the oil phase, and certain droplets fusion conditions were also investigated by adjusting the droplet driving times and oil flow rates. In these optimized conditions, fluorescence enhancement of droplets was used to detect Hg (II) ions in droplet by mixing with probe droplets (Rhodamine B quenched by gold nanoparticle). This method would have powerful potential for tiny volume sample assay or real-time chemical reaction study.     

Performance analysis of a folding flow micromixer

The performance of a folding flow micromixer in the Stokes flow regime is investigated computationally and experimentally. Consistency with a previously derived general scaling relation is demonstrated and the geometric parameters in the scaling relation are determined for this mixer. Measured data from a second similar mixer are correctly predicted using the scaling relation, thus showing that the approach allows quantitative prediction of mixing. This paper focuses on the errors associated with such predictions. Basic errors, expressed as variations in the standard deviation of the concentration profile, were estimated to be −10% for the computation and −30% for the experiment at the highest values of Péclet number considered. It is shown that this experimental error was mostly due to depth averaging of the spectroscopic technique used for concentration measurement at the high Péclet numbers. However, extra uncertainty is associated with chip fabrication tolerances and this was investigated further. Measurements at the outlet of nine different mixer chips of notionally identical design revealed variations in mixing of ±26%. This variation was attributed to misalignment of the glass layers determining the geometry of the mixer in the chip. Thus, the combination of measurement error and misalignment means predictions of the concentration standard deviation for the mixer may get non-uniformity wrong by up to 50%. Ensuring a required uniformity, however, simply requires adding a few further elements to the mixer to allow for this uncertainty. Application of the scaling relation to mixer design is highlighted by a discussion of the options available for improving the performance of the experimental mixer.     

Electrowetting droplet microfluidics on a single planar surface

Electrowetting refers to an electrostatically induced reduction in the contact angle of an electrically conductive liquid droplet on a surface. Most designs ground the droplet by either sandwiching the droplet with a grounding plate on top or by inserting a wire into the droplet. Washizu and others have developed systems capable of generating droplet motion without a top plate while allowing the droplet potential to float. In contrast to these designs, we demonstrate an electrowetting system in which the droplet can be electrically grounded from below using thin conductive lines on top of the dielectric layer. This alternative method of electrically grounding the droplet, which we refer to as grounding-from-below, enables more robust droplet translation without requiring a top plate or wire. We present a concise electrical-energy analysis that accurately describes the distinction between grounded and non-grounded designs, the improvements in droplet motion, and the simplified control strategy associated with grounding-from-below designs. Electrowetting on a single planar surface offers flexibility for interfacing to liquid-handling instruments, utilizing droplet inertial dynamics to achieve enhanced mixing of two droplets upon coalescence, and increasing droplet translation speeds. In this paper, we present experimental results and a number of design issues associated with the grounding-from-below approach.     

Evaluation of the mixing performance in a planar passive micromixer with circular and square mixing chambers

Microsystem Technologies - In this paper, passive planar micromixers based on circular and square mixing chambers spaced at equidistant along the length of micromixer are proposed to operate in the...     

A high-efficiency planar micromixer with convection and diffusion mixing over a wide Reynolds number range

Over a wide Reynolds number range (0.1 ≤ Re ≤ 40), the new planar obstacle micromixer has been demonstrated over 85% mixing efficiency covering the mixing improvement in both convection-enhanced (higher Re flow) and diffusion-enhanced (lower Re flow) mechanisms. Mixing behavior between two operation windows was investigated by numerical simulations and experiments. For the adaptive design, numerical simulations and Taguchi method were used to study the effect of four geometrical factors on sensitivity of mixing. The factors are gap ratio (H/W), number of mixing units, baffle width (W _b) and chamber ratio (W _m /W). The degree of sensitivity using the Taguchi method can be ranked as: Gap ratio > Number of mixing units > Baffle width > Chamber ratio. Micromixer performance is greatly influenced by the gap ratio and Reynolds number. Beside the wide Reynolds number range, good mixing efficiency can be obtained at short distance of a mixing channel and relatively low-pressure drop. This micromixer had improved both complex fabrication process of multi-layer or 3D micromixers and low mixing efficiency of planar micromixer at Re < 100. The trend of the verified experimental results is in agreement with the simulate results.     

An efficient passive planar micromixer with ellipse-like micropillars for continuous mixing of human blood

In this paper, a passive planar micromixer with ellipse-like micropillars is proposed to operate in the laminar flow regime for high mixing efficiency. With a splitting and recombination (SAR) concept, the diffusion distance of the fluids in a micromixer with ellipse-like micropillars was decreased. Thus, space usage for micromixer of an automatic sample collection system is also minimized. Numerical simulation was conducted to evaluate the performance of proposed micromixer by solving the governing Navier–Stokes equation and convection–diffusion equation. With software (COMSOL 4.3) for computational fluid dynamics (CFD) we simulated the mixing of fluids in a micromixer with ellipse-like micropillars and basic T-type mixer in a laminar flow regime. The efficiency of the proposed micromixer is shown in numerical results and is verified by measurement results.     

Flow field analysis of a passive wavy micromixer with CSAR and ESAR elements

Microsystem Technologies - The performance analysis of wavy micromixers with split and recombine (SAR) elements has been carried out. The two types of SAR elements viz. circular split and recombine...     

Generation of uniform-size droplets by multistep hydrodynamic droplet division in microfluidic circuits

A microfluidic system is presented to generate multiple daughter droplets from a mother droplet, by the multistep hydrodynamic division of the mother droplet at multiple branch points in a microchannel. A microchannel network designed based on the resistive circuit model enables us to control the distribution ratio of the flow rate, which dominates the division ratios of the mother droplets. We successfully generated up to 15 daughter droplets from a mother droplet with a variation in diameter of less than 2%. In addition, we examined factors affecting the division ratio, including the average fluid velocity, interfacial tension, fluid viscosity, and the distribution ratio of volumetric flow rates at a branch point. Additionally, we actively controlled the volume of the mother droplets and examined its influence on the size of the daughter droplets, demonstrating that the size of the daughter droplets was not significantly influenced by the volume of the mother droplet when the distribution ratio was properly controlled. The presented system for controlling droplet division would be available as an innovative means for preparing monodisperse emulsions from polydisperse emulsions, as well as a technique for making a microfluidic dispenser for digital microfluidics to analyze the droplet compositions.     

Micromachining of passive planar micromixer on poly (methyl methacrylate) substrate with excimer laser ablation

Excimer laser ablation technique was introduced into this work to fabricate a passive planar micromixer on the PMMA substrate. T-junction shaped and width-changed S-shaped microchannels were both designed in this micromixer to enhance mixing effect. The mixing experiment of distilled water and Rhodamine B with injection flow rate of 500 and 1,500 μm/s validates the mixing effectivity of this micromixer, and indicates the feasibility of excimer laser ablation in the microfabrication of μ-TAS device.     

Evolution of mixing in a microfluidic reverse-staggered herringbone micromixer

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

Inverse analysis of a planar two-spring system

In this article, a novel closed-form solution to the inverse analysis of a planar two-spring system is presented which may be extendible to the spatial three-spring system. It involves finding the six equilibrium configurations of a system of two springs connected at one end to a common pivot and at the other to a base. This formulation involves a transformation into polar coordinates where a sixth degree polynomial is obtained in terms of tan-half-angle for the rise angle of one of the springs. The derivation and the coefficients of this polynomial are much simpler than those obtained by Pigoski and Duffy, “An inverse force analysis of a planar two-spring system,” presented at the First Austrian IFTOMM Symposium, Seggauberg, Austria, July 4–9, 1993, also in press Trans. ASME where a sixth degree polynomial in one of the spring lengths was obtained. © 1996 John Wiley & Sons, Inc.     

高回流被动式微混合器设计及数值模拟

为了提高混合强度设计了一种高回流被动式微混合器,其反馈通道中的回流延长了混合时间并促进形成涡流,增强了对流和扩散作用。通过优化反馈通道形状和混合腔入口尺寸,该微混合器提升了其回流率和混合强度。利用仿真软件Fluent分析该微混合器的性能,仿真表明对于给定的微混合器其回流率和混合强度仅仅决定于雷诺数,回流率和混合强度均随雷诺数增大而增大。特别地,当雷诺数低至8.3时,仍可在反馈通道中找到回流;当雷诺数为99.6时,回流率达到24%,混合强度则超过60%。通过对反馈通道倾斜角度参数的模拟,该微混合器的回流率和混合强度均有明显提升。     

Passive mixing in a three-dimensional serpentine microchannel

A three-dimensional serpentine microchannel design with a “C shaped” repeating unit is presented in this paper as a means of implementing chaotic advection to passively enhance fluid mixing. The device is fabricated in a silicon wafer using a double-sided KOH wet-etching technique to realize a three-dimensional channel geometry. Experiments using phenolphthalein and sodium hydroxide solutions demonstrate the ability of flow in this channel to mix faster and more uniformly than either pure molecular diffusion or flow in a “square-wave” channel for Reynolds numbers from 6 to 70. The mixing capability of the channel increases with increasing Reynolds number. At least 98% of the maximum intensity of reacted phenolphthalein is observed in the channel after five mixing segments for Reynolds numbers greater than 25. At a Reynolds number of 70, the serpentine channel produces 16 times more reacted phenolphthalein than a straight channel and 1.6 times more than the square-wave channel. Mixing rates in the serpentine channel at the higher Reynolds numbers are consistent with the occurrence of chaotic advection. Visualization of the interface formed in the channel between streams of water and ethyl alcohol indicates that the mixing is due to both diffusion and fluid stirring     

Chaotic mixing using periodic and aperiodic sequences of mixing protocols in a micromixer

We conducted a numerical study on mixing in a barrier embedded micromixer with an emphasis on the effect of periodic and aperiodic sequences of mixing protocols on mixing performance. A mapping method was employed to investigate mixing in various sequences, enabling us to qualitatively observe the progress of mixing and also to quantify both the rate and the final state of mixing. First, we introduce the design concept of the four mixing protocols and the route to achieve chaotic mixing of the mixer. Then, several periodic sequences consisting of the four mixing protocols are used to investigate the mixing performance depending on the sequence. Chaotic mixing was observed, but with different mixing rates and different final mixing states significantly influenced by the specific sequence of mixing protocols and inertia. As for the effect of inertia, the higher the Reynolds number the larger the rotational motion of the fluid leading to faster mixing. We found that a sequence showing the best mixing performance at a certain Reynolds number is not always superior to other sequences in a different Reynolds number regime. A properly chosen aperiodic sequence results in faster and more uniform mixing than periodic sequences.