Design for Efficient Micro-mixer Design Based on“Split-Recombination”

Anil Kumar RamRakhyani,Department of Electrical and Computer Engineering, University of Utah, Salt Lake City, USA

Abstract—Micromixer is an integrated part of the microfluidicsystem design and traditionally used for analytical chemistry andlife science. There has been active research to design an efficientmicromixer that requires low pressure to sustain the input flowrate and has a small foot print on the microfluidic chip. Inthis design, the performance of the micromixer is studied fordifferent design parameters and a novel “split-recombination”based micromixer is presented that can achieve more than 90% mixing efficiency and 150 Pa pressure drop for flow rateof 1 ml/hr. The designed micromixer is fabricated using softlithography and measured data is compared with the simulationresults.

I. INTRODUCTION

Micromixer is one of the key components of microfluidicchip design. In the microscale regime, where the feature sizeis below 500µm, laminar flow of the liquids reduces themixing efficiency between two liquids [?]. Thus, micromixinghas become a crucial process for micrototal analysis system(µTAS). Based on different mixing techniques, micromixersare classified into two categories named passive and activemixer. Despite the good mixing efficiency of the active mi-cromixers, passive micromixers have become more populardue to their low complexity [?].

Diffusion and chaotic advection are two popular mecha-nisms that can achieve efficient mixing in the microchannels.There are multiple approaches taken that includes T- or Y-shaped mixers, parallel and serial lamination and focusedenhanced mixers [?], [?]. The mixer’s performance is char-acterized by their mixing efficiency, pressure drop, mixingtime, and fabrication complexity. Thus, the optimum designof a passive micromixer, that can be fabricated reliably withfewer fabrication steps, is an active research in microfluidicchip design [?].

In this current design, we designed a passive micromixerbased on parallel lamination in which the input liquids are splitand recombine into multiple streams. In the parallel laminationtechnique, the diffusion length can be reduced significantly tofacilitate efficient mixing. In this design, a novel inlet designis proposed to split the input streams of both input liquids andrecombine them in an alternative order to create the parallellamination.

This work targets the design and optimization of an efficientmicromixer. Section II provides the FEM based numericalmodeling of the micromixer to analyze the effect of differentdesign parameters on the efficiency. Section III provides thefabrication process for the SU-8 based mold and fabrication ofPDMS based micromixer. Measurement results are discussed

in Section IV and compared with the simulations. Section Vdiscusses the performance analysis of the mixer along withfuture modifications.

II. MIXER DESIGN

We aim to achieve high mixing efficiency between twofluids for a wide range of flow rates. To ensure a lowpressure drop from the inlet to the outlet, cross section areaof the channel should be high enough to reduce the flowresistance. In this design, the channel height of 100µm isaimed. Channel width is one of the design parameter that needsto be optimized to achieve a trade-off between the pressuredrop and mixing efficiency. The following section providesthe design methodology for the current micromixer.

A. Design Approach

The designed mixer needs to be tested with the output flowrates of 0.1 ml/hr, 1 ml/hr and 10 ml/hr. For inlets widthof 300µm, input flow velocity is computed to calculate theReynolds number and the mixing time. In general, the mi-cromixer has Reynolds number (Re) below 100 which is a verysmall value as compared to the turbulent threshold (Re> 2300)[?]. In current design, Reynolds number is calculated based onEquation 1, which ranges from 0.069 to 6.9 for the flow rateof 0.1 ml/hr to 10 ml/hr, respectively.

Re =uDHρ

µ(1)

DH =4×Area

Perimeter(2)

where u, ρ , µ are the velocity, density, and viscosity of theinput fluids, respectively.

For laminar flow of the fluids, diffusion is one of the dom-inant mechanisms for the fluid mixing. For the same mixingtime, mixing efficiency increases with the reduction of diffu-sion length [?], [?]. To reduce the diffusion length between thestreams of two input fluids, the split-recombination techniqueis used in which each inlet is subdivided into multiple streamsand recombined with the other fluid alternatively. This tech-nique enhances the mixing efficiency significantly without therequirement of multiple inlets or a thinner channel. For higherflow rate, chaotic advection can improve the mixing efficiency,which can be generated by different techniques. One type ofmethodology is called “Dean’s Effect” and requires curvedchannels.

In this current design, multiple features are targeted toensure high mixing efficiency and low pressure drop. Toachieve the same pressure drop between the split and recom-bination center of the fluids, a symmetric design with equalchannel length should be used. For channel width of W at therecombination region, N number of input splitting reduces thediffusion length to W/N between the two fluids. The curvedchannel is used after the recombination to generate “Dean’sEffect”. The current mixer is a two layer design and doesn’trequire a intermediate membrane layer.

B. Numerical Simulation

Mixing efficiency and pressure drop depends on the mixer’sstructure and dimensions. Thus, to design an optimum mixer,a detailed study of the mixer’s parameters needs to be done.Numerical simulation is one of the practical approaches tosimulate the effect of each parameter to identify the trade-off between different design parameters. In the current work,FEM (Finite Element Method) based numerical simulator(COMSOL MultiPhysics 4.3a) is used for the 3-dimensionalmicrofluidic simulation of the micromixer. To ensure theaccuracy of the simulation, finer mesh is used to representthe 3-D model of the mixer. A convergence test is performed,for which the number of mesh cells are increased from 300thousand elements to 1.3 million. It shows the variation in themixing efficiency is less than 0.1 % confirming the accuracyof the solution.

Effectiveness of the “split-recombination” based micromixerdepends on the structure and dimension of the split channels(Fingers) and interaction between the fingers. Two input inletsare split in multiple fingers and these fingers cross each otheron the upper and lower layer of the mixer. In this study,the effect of the finger count per inlet, angles of crossingbetween the fingers, and fingers width is discussed as a part ofefficient inlet design. Figure 1 shows the inlets configurationand mixing efficiency for 2 ml/hr outflow rate. Comparisonof Figure 1(a) and (b) shows that for angle of crossing of90o, mixing efficiency is higher however routing of the inputinlets is difficult for configuration (a). The angle of crossingis defined as the outer angle between the outmost finger tothe horizontal line parallel to the width of the recombinationregion. For the same number of fingers per inlet, increasing thewidth of the finger as compared to channel height significantlyreduces the mixing efficiency (Figure 1(b) and (c)).

By removing the left most finger for each inlet and placingcircular obstacles after the recombination region, mixing effi-ciency improve significantly as shown in Figure 2 (a) and (b).To improve the mixing efficiency above 90%, the design isextended with curved channels to incorporate “Dean’s flow”.Figure 2 (c) and (d) shows that mixing efficiency is above 90 %and doesn’t improve significantly with placement of obstacleholes in the curved channel.

C. Simulation Results

Based on the parametric optimization of the micromixer formaximum mixing efficiency and lowest pressure drop, design

Fig. 1. Parametric variation of design feature: (a) Inlet configuration with90o crossing angle (b) 4-finger inlet with channel width 150 µm and 120o

crossing angle (c) 4-finger inlet with channel width 74 µm (d) 4-finger inletwith gap of 300 µm between the combination point. All efficiency is for theoutput flow rate of 2ml/hr.

Fig. 2. (a) Inlet design with 3 fingers each inlets,(b) Extended recombinationchannel with circular obstacles (c) Curved channel extension of the mixer, (d)Curved channel with circular obstacles.

2 (c) is chosen as the final micromixer. The optimization isperformed using three key parameters that includes (1) numberof fingers per inlet, (2) width to the height ratio at the fingers’soverlapping region between two layers, and (3) number andposition of the circular obstacles after recombination of inputfluids as shown in Section II-B.

To characterize the mixer performance for different flowrates, numerical simulation is performed. Figure 3 shows theconcentration profile of the two liquids inside the mixer andrequired input pressure for output flow rates of 0.1ml/hr,1ml/hr, and 10 ml/hr.

As shown in Table I, the mixing efficiency improves withthe reduction of the flow rate, which is expected from the

Fig. 3. Concentration profile for the mixer with two input fluids at outputflow rate of (a) 0.1 ml/hr, (c) 1 ml/hr and (e) 10 ml/hr. Pressure drop alongthe mixer with two input fluids at output flow rate of (a) 0.1 ml/hr, (c) 1 ml/hrand (e) 10 ml/hr.

current diffusion dominant mixer. The pressure drop acrossthe mixer increases linearly with the flow rate. However, themixing time reduces linearly with the increase of flow rate.

TABLE ISIMULATION RESULTS

Flow Rate Efficiency Pressure Drop Mixing Time(ml/h) (%) (Pa) (sec)

0.1 96.01 14.75 4.41.0 92.23 151.45 0.4410.0 87.66 1527.4 0.044

Due to a wide channel width of 200µm, the current mixerrequires low pressure to drive the input fluids, even for thehighest flow rate (10 ml/hr). This pressure can be easilyachieved by using on-chip micropumps [?].

D. Pre-fabrication

To fabricate the micromixer, a negative mask needs tobe created to pattern the SU-8 based mold. The currentmicromixer is a two-layer design. Thus, to create the mask,the upper and lower layers of the design is exported as dxffiles. Figure 4 shows the dxf files of the layers. During maskcreation, both layers are patterned on the same mask to reducethe number of masks required for the fabrication.

Fig. 4. (a) Upper and (b) lower layer of the micromixer.

III. FABRICATION AND EXPERIMENTAL SETUP

A. Mold Creation

To create the PDMS based micromixer, the SU-8 basedmold is chosen due to its stability at curing temperature,creation of high aspect ratio channel, and well-defined processparameters. SU-8 is a negative photoresist requiring a darkmask of the design features. Before spreading the SU-8 overthe 4” diameter Si-wafer, the wafer is cleaned and dehydratedat 100o C for 5 minutes to improve the adhesion of the SU-8over wafer. In our design, we aim to achieve the channel heightof 100µm. To achieved that, the SU-8 is spun coated over thewafer at 1500 rpm for 30 seconds followed by a high speeduniformity step for 45 seconds. The pattern is transferred tothe SU-8 coated wafer using a dark mask in the suss alignerwith UV exposure. Repeated UV exposure of 5 seconds ONand 10 seconds OFF is done for 25 times at 7mJ/cm2/secexposure power density. The wafer is cleaned using IPA anda N2 gun. The wafer is post-exposure baked on a hot plate for5 minutes at 70o C followed by 10 minutes baking at 100o C.To remove any dust particles, the wafer is cleaned using theIPA solution. Due to UV exposure, the location where mixer’sfeatures are exposed have hardened. To develop the features,the wafer is agitated in a SU-8 developer for 10 minutes andrinsed by IPA to clean the wafer. To remove the microcracksin the SU-8 surface the wafer is annealed for 5 minutes at250o C. Figure 5 shows the features of micro-mixer in theSU-8 mold under the microscope. The current micromixer isa two layer design and both the layers are patterned on thesame SU-8 mold.

B. Characterization

To characterize the surface, the wafer is aligned in thesurface profilometer and a feature height of 58µm is recorded,as shown in Figure 6. In the original design, the channel heightof 100µm was desired, which requires some change in thethe process parameter of the “SU-8 coating step” over thewafer. For soft lithography of the PDMS based micro-mixer,the current SU-8 mold with channel height of 58µm is used.However, in future, higher coating height will be achieved forthe mold by reducing the spinner speed [?].

Fig. 5. Perspective view of the (a) upper layer’s and (b) lower layer’s featuresin the SU-8 mold. Top view of the (c) upper layer’s and (d) lower layer’sfeatures in the SU-8 mold.

Fig. 6. Height of the SU-8 features under the surface profilometer.

C. Soft-lithography

PDMS based microfluidic components are well knowndesigns to perform biological studies [?], [?]. To transfer themixer’s features in the PDMS substrate, silicone is mixedwith the platinum based hardener at 10:1 volumetric ratio.The mixer is degassed for 15 minutes to remove the trappedbubbles. The silicone mixer is poured over the SU-8 basedmold. To ensure good bonding between the layers, the PDMSis partially cured inside a covered aluminium box and placedon a hot plate for 7 minutes at 100o C. The cured PDMS(Figure 7 (a)) is peeled off from the mold and cut in rectan-gular shapes featuring the mixer’s layers. To create the portconnection, inlets and outlet are drilled using the coring tool.To remove the adsorbed dust particles, the upper and lowerlayers are washed using soap water and dehydrated with the N2gun. The layers are aligned under the microscope followed byfull curing on a hot plate for 15 minutes at 130o C. Figure 7(b)shows that the micromixer achieves good alignment accuracybetween layers.

Fig. 7. (a) Soft lithography for micromixer using PDMS substrate, (b) alignedlayers of the micromixer.

D. Measurement Setup

To control the input flow rates in the mixer’s inlets, anexternal syringe pump is used. The syringe pump can beconfigured based on the syringe’s diameter and desired flowrate. Figure 8(a) shows the syringe pump setup. To connectthe syringe pump to the mixer, port connections are made tothe inlets and outlet using plastic tubes (Figure 8(b), (c)). Tocapture the concentration profile of the input and output liquidflow, the mixer is placed under the microscope’s focal plane.To measure the pressure drop across the mixer, a pressuregauge is used between the syringe pump output and mixer’sinlet 1.

Fig. 8. (a) Syringe pump setup. Micromixer with port connection (b), (c).

IV. MEASUREMENT RESULTS

To characterize the mixing efficiency of the mixer, glasssyringes are filled with two different colored fluids (water andwater with green dye). At the steady state flow of the inputliquids, the concentration profile of the mixer are capturedusing camera attached to the microscope. To estimate themixing efficiency from the captured images, an image analysissoftware “ImageJ” [?] is used that can import the image filedirectly and shows the Gray scale profile of the selected region.

To calculate the mixing efficiency, the Gray scale profile atthe outlet is captured which is represented by 52 pixels onthe image. The Gray scale value of any point on the capturedimage depends on the lighting condition, angle of the mixerfrom the microscope’s focus plane, and focus depth. Assuminguniform lighting condition over the mixer, the Gray scale valueneeds to be normalized with respect to the Gray scale valueat the inlets. To calculate the mixing efficiency, Equation 3 is

used based on the Gray scale profile at the input and outputof the micromixer.

Grayinlet1 = mean(Grayscale− liquid1)Grayinlet2 = mean(Grayscale− liquid2)

Mingray = min(Grayinlet1,Grayinlet2)

Maxgray = max(Grayinlet1,Grayinlet2)

NormConcentration =Grayoutlet −Mingray

Maxgray −Mingray

StdevConcentration = stdev(NormConcentration)

E f f iciency = 100× [1−2×StdevConcentration] (3)

The image data of steady state fluid flow is captured forall three flow rates. Figure 9 shows the captured image for0.1ml/hr flow rate. The concentration profile at the outlet isselected to plot the Gray scale profile and shown in Figure 9.Based on Equation 3, the computed efficiency is 71.72 %. Forthis flow rate, the measured pressure drop is below 600 Pa.

Fig. 9. (a) Steady state concentration profile in micromixer at 0.1 ml/hr flowrate. (b) concentration profile at the outlet. (c) Gray scale profile across outlet.

Similar measurements are done for the flow rate of 1 ml/hr.For this setup, mixing efficiency of 74.38 % and pressure dropof 9.65 kPa is measured. Figure 10 shows the captured imageof mixer’s concentration profile and Gray scale profile at theoutput.

For the flow rate of 10 ml/hr, the computed mixing effi-ciency is 74.41 %. Pressure drop for this setup is measure as27.57 kPa. Figure 11 shows the concentration profile acrossthe channel.

Comparison between the simulated and measured mixingefficiency and pressure drop is done in Table II. It can beseen that for all the flow rates, simulated and measured valuesare considerably different from each other.

V. DISCUSSION

As seen in Table II, the measured efficiency and pressuredrop does not change linearly with the flow rate, whichwas not expected from the numerical simulations. There arefew possible reasons which reduces the mixing efficiencyconsiderably and can cause the non-linear behaviour of the

Fig. 10. (a) Steady state concentration profile in micromixer at 1 ml/hr flowrate. (b) concentration profile at the outlet. (c) Gray scale profile across outlet.

Fig. 11. (a) Steady state concentration profile in micromixer at 10 ml/hrflow rate. (b) concentration profile at the outlet. (c) Gray scale profile acrossoutlet.

TABLE IICOMPARISON

Flow Rate Efficiency Efficiency Pressure Pressure(ml/h) Simulated (%) Measured (%) Simulated Measured

0.1 96.01 71.72 14.75 < 600 Pa1.0 92.23 74.38 151.45 9.65 kPa10.0 87.66 74.41 1527.4 27..57 kPa

mixer. These effects include the reduction of channel heightduring fabrication, effect of layers’ alignment, trapped bubbles,fluid folding, and analysis mechanism of the image data.

A. Effect of Channel Height

During the inlet design, the effect of the channel widthat the fingers’ crossings are studied and found that a widerchannel compared to the channel height results in lowermixing efficiency (Figure 1(b),(c)). During the fabricationprocess of the SU-8 based mold, the measured height is 58µmcompared to an intended 100µm height (Figure 6). To estimatethe effect of channel height, the mixer is re-simulated withthe channel height of 58µm and compared with the originaldesign. Figure 12 shows that due to reduction in the channelheight, the flow and mixing profile of the input liquids changessignificantly. As compared to 100µm high channel, simulation

of the fabricated mixer results in the mixing efficiency of 77 %at 1 ml/hr output flow rate, which is very close to the measuredefficiency for the same flow rate. Thus, it is expected thatusing new SU-8 mold with height 100µm, mixing efficiencywill improve considerably.

Fig. 12. (Simulation of the micromixer for channel height of (a) 100µm and(b) 58µm. Simulation is done for output flow rate of 1 ml/hr.

B. Effect of Layers’ Alignment and Bubbles

The current design is a two-layer design. Thus, it requiresalignment between the two layers. Slight mismatch in thealignment can create shape corners near the recombinationregion causing trapped bubbles. In the mixer design, trappedbubbles can cause flow profile variation and results in low mix-ing efficiency and non-linear performance. Figure 11 shows thetrapped bubbles in the mixer near one of the finger’s output.Due to these trapped bubbles, the non-linearity in mixingefficiency and pressure drop is achieved (Table II). For higherflow rate, the bubble can be pushed to a different positionresulting into a different channel profile compared to low flowrate. The simulation of a mixer, including the effect of bubblesand alignment, is hard to perform. Thus, to achieve linearityin mixing efficiency, trapped bubbles need to be removed inthe vacuum chamber. In general, the bubbles are trapped nearthe sharp corner. Thus, the design features can be modified toreduce the chances of air bubble trapping.

C. Fluid Folding and Image Analysis

In the current characterization of the concentration profile,the image is captured from the top of the channel, whichonly shows the 2-dimensional concentration profile. Figure13 shows the simulated concentration profile of the twoliquids and demonstrates that the micromixer achieves a highefficiency due to 3-dimensional mixing. During the liquidflow in the mixer, one liquid folds over the other one dueto edges and corners in the mixer. Thus, a 2-dimensionalanalysis of mixing efficiency is highly susceptible to errors. Toimprove the accuracy of the measurement, the profile shouldbe captured from both (upper and lower layer) side of theoutput channel.

VI. CONCLUSION

In this work, we designed and optimized a two-layer “split-recombination” micromixer that can achieve mixing efficiency

Fig. 13. Concentration profile of two liquids along the micromixer showing3-D mixing.

higher than 87 % for all flow rate of 0.1 ml/hr, 1 ml/hr, and10 ml/hr. The current design occupies a small foot print of2 mm × 3 mm and can be easily interfaced with the on-chip microfluidic components. The design is chosen basedon multiple optimization parameters that can improve themixing efficiency and can reduce the pressure drop. We havefabricated the mixer using a soft lithography technique, whichachieves a mixing efficiency more than 70 % for all the testedflow rates. The difference between the simulated and measuredefficiency is caused by reduction of channel height duringfabrication, layer misalignment, and trapped bubbles. Due tofluid folding in the mixer and image capturing conditions,concentration analysis at the outlet provides inaccurate results,which requires multiple efficiency analysis at different posi-tions along the outlet. In future designs, a SU-8 mold with100µm will be used for the mixer fabrication to improve themixing efficiency.

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