LiquidâLiquid Two-Phase Flow Patterns in a Serpentine Microchannel

Article pubs.acs.org/IECR

Liquid−Liquid Two-Phase Flow Patterns in a Serpentine Microchannel P. S. Sarkar,‡ K. K. Singh,*,† K. T. Shenoy,† A. Sinha,‡ H. Rao,† and S. K. Ghosh† †

Chemical Engineering Division, Bhabha Atomic Research Centre, Trombay, Mumbai, India 400085 Neutron and X-ray Physics Facilities, Bhabha Atomic Research Centre, Trombay, Mumbai, India 400085

‡

ABSTRACT: Liquid−liquid two-phase flow patterns in a serpentine glass microchannel have been studied using a camera with very high shutter speed. The phase system is equilibrated water−succinic acid−n-butanol system. Observed flow patterns have been classified into seven different types: slug flow, slug and droplet flow, droplet flow, unstable annular flow, annular flow, annular dispersed flow, and fully dispersed flow. Two different ways of introducing the aqueous and organic phases into the microchannel have been studied. Flow regime maps are presented and discussed. Voronoi diagrams of the flow regime maps are also presented.

1. INTRODUCTION Driven by the unique advantages offered by microchannels, they have been subject of many studies reported in the recent past. Microchannels are characterized by high surface to volume ratio and hence are found very useful for carrying out highly exothermic reactions where temperature control of the reaction mixture is very important to avoid thermal degradation of the products.1 The low inventory requirement of the microchannels makes them attractive for carrying out reactions that have a tendency of explosion and involve hazardous and toxic chemicals so that in case of any eventuality the potential harm to the operator and environment is limited.2 Microchannels are very useful for two-phase reactions, gas−liquid, and liquid− liquid mass transfer operations as the transfer paths are minimal and specific interfacial areas are very high. Owing to this, twophase reactions in microchannels have been of considerable interest.3,4 Several studies on gas−liquid mass transfer5,6 and liquid−liquid mass transfer7 have been reported. In two-phase reactions and mass transfer operations in microchannels, the flow regime inside the microchannel will greatly affect its performance as it will determine the specific interfacial area available for reaction or mass transfer. Several studies have been carried out to obtain gas−liquid flow-regime maps in microchannels.8,9 For liquid−liquid flows in microchannels, there are several studies on generation of droplets in microfluidic devices10−13 and studies focused on a specific type of flow pattern.14−16 Studies covering a wide range of flow rates leading to observation of different possible flow patterns, and hence generation of complete flow regimes maps, however, are relatively fewer. Some of the studies focused on liquid−liquid two-phase flow regime maps in microchannels have been summarized in the Table 1 and discussed briefly.17−21 Dessimoz et al.17 found water−toluene system to give parallel flow for Y-junction microchannel and slug flow for T-junction microchannel. Addition of NaOH to the aqueous phase or CCl3COOH to the organic phase was found to affect the flow pattern. Salim et al.18 observed droplet flow, slug flow, and annular flow with water as the dispersed phase for channels initially filled with oil. The material of construction of the microchannel did influence the flow patterns. For initial © 2012 American Chemical Society

saturation with the aqueous phase, flow patterns showed dependence on the material of microchannel and oil was the dispersed phase. In the microchannel made of quartz slug flow, droplet flow and stratified flow were observed. For microchannel made of glass droplet, stratified and semi-stratified flow were observed. Cherlo et al.19 carried out experimental and computational studies on liquid−liquid flow in microchannels. Water was the dispersed phase in all the experiments. At low flow rates slug flow was observed and at high flow rates stratified flow was observed. The slug size was found to increase with reduction in viscosity of the continuous phase, increase in interfacial tension of the phase system, and increase in the diameter of the microchannel. Zhao et al.20 studied liquid− liquid two-phase flow patterns in a PMMA microchannel. To ensure that the organic phase is the dispersed phase surface modification of the microchannels was carried out. They provided flow regime maps separately for the T junction and at a point downstream of the junction. The flow patterns observed at the T junction were characterized as slug flow, monodispersed droplets, drop populations, parallel flow, parallel flow with wavy interface, and chaotic thin striation flow. On the downstream side of the junction slug flow, monodispersed droplet flow, droplet populations, parallel flow, and annular flow were observed. The parallel flow and parallel flow with wavy interface observed at the junction were found to evolve to parallel flow with smooth interface on the downstream side of the junction. Chaotic thin striation flow at the junction was found to give annular flow on the downstream side. Slug flow and monodispersed droplet flow were characterized as entrance phenomena. The droplet population flow was attributed to interfacial instabilities. The study reveals that flow pattern at the downstream side of a microfluidic junction can be different from that at the junction even for a straight channel. Kashid and Agar21 studied flow patterns in microchannels comprising Y-junctions followed by capillaries. Flow patterns were observed Received: Revised: Accepted: Published: 5056

July 22, 2011 February 21, 2012 February 24, 2012 February 24, 2012 dx.doi.org/10.1021/ie201590f | Ind. Eng. Chem. Res. 2012, 51, 5056−5066

Industrial & Engineering Chemistry Research

Article

Table 1. Summary of Studies on Liquid−Liquid Two-Phase Flow Reported in Literature reference 17

channel type

Dh (μm)

L (mm)

Vmax (cm/s)

phase system

56, 40

18

T, Y junction, straight 400, 269 channel T junction, straight channel 793, 667

120

water−toluene; water + NaOH−toluene; water− toluene + trichloroacetic acid mineral oil−water

19 20

T junction, straight channel 540, 1000 T junction, straight channel 400

10 60 mm

kerosene−water; coconut oil−water water−kerosene

57 278

21

Y junction, straight capillaries

>500

water− cyclohexane

113

250−1000

4.6 42

observed flow regimes slug, parallel (stratified) droplet, slug, annular, stratified, semistratified slug, stratified slug, droplet flow, parallel, chatoic striation slug, drop, deformed interface

Figure 1. Schematic diagram of the experimental setup.

in the capillaries. The flow patterns observed were classified as slug flow, drop flow, and deformed interface flow. The above studies highlight that flow regime in a microchannel may depend on material of construction, physical properties of the phase system, geometry of the junction and channel, as well as the way the flow is started. The flow pattern down the length of the channel may be different from the flow pattern at the junction even for straight channels. Dependence of flow pattern on so many parameters makes it difficult to present a generalized flow regime map which can be used to predict the flow pattern for a microchannel and phase system of interest. Though numerical simulations can help one predict the flow regime they are yet to be validated extensively for all flow regimes.22 In view of this, experimental investigation of flow patterns for a microchannel and phase system of interest assumes importance. As can be observed, the above-mentioned studies have been carried out using straight channels having T or Y junctions. The objective of the present study is to investigate liquid−liquid two-phase flow patterns in a serpentine microchannel having a junction at which one phase comes from two sides and the other phase comes from the center. Compared to a straight microchannel a serpentine microchannel offers a very large channel length (hence residence time) on a compact chip and can be used for carrying out reactions and solvent extraction at a relatively large throughput. Also due to repeated change in the flow directions at the bends in the microchannel, the mixing and

shearing is likely to be much different from a straight channel and different kinds of flow patterns may result.

2. EXPERIMENTAL SETUP A schematic diagram of the experimental setup is shown in Figure 1. Two positive displacement pumps (0−10 mL/min flow rate range) pump the organic and the aqueous phases at the desired flow rates which can be set at the digital panel of the pumps. The microchannel is mounted vertically on a stand. Light emanating from a fiber optical illuminator passes through the microchannel and a magnifying lens and is reflected by a mirror to reach the aperture of the camera which is connected to a computer. The magnified microchannel can be seen on the computer screen in real time. The phase system is equilibrated water−succinic acid− n-butanol system. This is one of the systems recommended for characterization of solvent extraction equipment.23 The physical properties of this phase-system are given in Table 2.24 Table 2. Physical Properties of Water−Succinic Acid− Butanol System phase

density (kg/m3)

viscosity (Pa·s)

interfacial tension (mN/m)

aqueous organic

981.69 837.01

0.00144 0.00334

1.7

In all the experiments, first aqueous phase is started at the desired flow rate and then the organic phase is intro5057

dx.doi.org/10.1021/ie201590f | Ind. Eng. Chem. Res. 2012, 51, 5056−5066


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duced at the desired flow rate. The microchannel chip used in the experiment was procured from M/s. Syrris Ltd., UK. The schematic layout of the microchannel chip is shown in Figure 2. There are three inputs. For the first set of experiments aqueous phase is introduced at Input-3 (I-3) and organic phase is introduced at Input-1 (I-1). Input-2 (I-2) is kept closed. In the second set of experiments aqueous phase is introduced at I-1 and organic phase is introduced at I-3. I-2 is once again kept blocked. The liquid coming at I-1 is split into two streams. These two streams meet the liquid coming at I-3 from both the sides at the second junction on the chip. The first few serpentine channels following the junction are called mixing channels and have smaller cross-section and straight length between the bends. The channels following the mixing channels are called reaction channels. The cross-sectional area of the reaction channels is more than the cross-sectional area of the mixing channels, as shown in Figure 3. The role of the mixing channels is to ensure intimate contact of the liquids meeting at the junction. The role of the subsequent reaction channels is to provide adequate time for reaction/mass transfer to take place. Total volume of the chip is 250 μL. The chip shown in Figure 2 is housed in a chip holder and a metallic header is used to feed the liquids to the inlets and withdraw the two phases from the outlet. In the complete assembly due to chip header there is no optical access to the junction and mixing channels and hence it is not possible to observe the flow patterns there. Flow patterns, therefore, have been observed only in the reaction channels. The flow rates of the aqueous and organic phase during the experiments were varied between 0.05 and 1.6 mL/min. We have used a highly sensitive intensified charge coupled device (ICCD) based analog camera system having internal gating time as low as 2 ns. The white light (halogen 150 W) after transmission through the microchannel assembly is focused by the lens to the ICCD camera. To minimize the overall length of the imaging setup a front coated mirror is placed between the lens and ICCD to bend the beam toward the ICCD. Neutral density filters are placed before the ICCD aperture for the safety of the camera system in case of accidental increase of the intensity of incident light beam. The CCD pixel size is 8.6 μm × 8.3 μm and in terms of pixels, an image size is 736 × 572. The analog images are digitized by a PC frame grabber and realtime images at 25 frames per second with the set acquisition parameters can be continuously grabbed and archived in the PC. A Nikon 105 mm, f 1:1.8

Figure 3. Cross-sections of mixing channel (left) and reaction channels (right).

lens was coupled to the ICCD system for imaging purposes. The first few experiments were done to finalize the order of the exposure time of the camera to capture the flow pattern. This is important in view of the velocities being very high. For a given set of flow rates of aqueous and organic phases, images were taken at different exposure times ranging from hundred nanoseconds to hundred microseconds. The results are shown in Figure 4. The

Figure 4. Effect of exposure time on the quality of the image. Qo = 0.6 mL/min, Qa = 0.4 mL/min.

walls of the microchannel are quite thick and dark in the acquired images. This is due to the curved walls of the channel cross-section shown in Figure 3. As can be seen, for exposure times of the order of hundred microseconds the edges of the droplets in the images are blurred. Hence in all the experiments the exposure time of the camera was kept at 100 ns.

Figure 2. Schematic layout of the microchannel in the glass chip. 5058



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Figure 5. Slug flow.

Figure 6. Slug and droplet flow.

3. FLOW PATTERNS Flow patterns observed in the experiments have been classified into seven different types: (a) slug flow (SF) (b) slug and droplet flow (SDF) (c) droplet flow (DF) (d) unstable annular flow (UAF) (e) annular flow (AF) (f) annular dispersed flow (ADF) (g) fully dispersed flow (FDF). Typical images for slug flow are shown in Figure 5. Flow in this figure and the following figures is from top to bottom in the left half of the bend and bottom to top in the right half of the bend. For the smallest flow rate, i.e., 0.05 mL/min, of both phases the slugs occupied almost complete cross-section of the channel. For higher flow rates slugs did not occupy the full cross-section of the channel. Slug and droplet flow regime is a transition flow pattern between the slug flow and droplet flow and is observed frequently. Typical images for this are shown in Figure 6. Size of slugs and droplets was found to reduce with increase in flow rate of the continuous phase as evident from the first and second images of Figure 6 in which organic phase

is the continuous phase. Typical images for droplet flow pattern are shown in Figure 7. The drops were found to reduce in size with increase in flow rate of the continuous phase as can be seen in the third and fourth images of Figure 7 in which organic phase is the continuous phase. Typical images of unstable annular flow are shown in Figure 8. In this flow pattern the inner layer breaks down and recovers periodically. This flow pattern was not observed when the organic phase was applied at I-1 and aqueous phase was applied at I-3. In the annular flow pattern the inner layer remains stable. Typical images of this flow pattern are shown in Figure 9. In annular flow most of the times the inner interface is hidden in the shadow in the left half of the bend. Due to centrifugal action at the bend, this layer is thrown outward and emerges outside the shadow region. The outer interface clearly visible in the left half of the bend is also pushed outward closer to the outer wall and is hidden in the shadow in the right half of the bend. The location of the interfaces keeps changing with time. Most of the time the outer interface is visible in only the downward flow in the left half of the bend and inner interface is visible only in the upward flow in the right half of the bend. In some instance both the interfaces can be clearly seen as in case of the third image in 5059



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Figure 7. Droplet flow.

for fully dispersed flow. This flow pattern may result right at the junction of the microchannel or be due to excessive interfacial instabilities in annular flow leading to thorough dispersion of one phase into the other. The droplets are so small that they appear as a haze. Drop sizes in fully dispersed flow reduce with increase in the flow rate of the continuous phase as can be seen by comparing first image of Figure 11 with second image of Figure 11 and third image of Figure 11 with fourth image of Figure 11.

4. FLOW REGIME MAPS 4.1. Organic Phase at Inlet-1, Aqueous Phase at Inlet3. Experiments were conducted in two ranges of flow rates. In the lower range, flow rates were varied between 0.05 and 0.20 mL/min. In the higher range, flow rates were varied between 0.2 and 1.6 mL/min. The flow regime map for the lower range is shown in Figure 12. As can be seen, only three flow regimes (SF, SDF, and DF) were observed. Here a new concept of phase continuity map is proposed. In liquid−liquid two-phase flow, in addition to knowing the flow pattern, it is equally important to know which phase is the dispersed one and which is the continuous one. A flow regime map alone does not give information on phase continuity and a separate phase continuity map is required. Figure 13 shows the phase continuity map for the lower range of flow rates. In Figure 13, y = x line is the equal flow rate line and partitions the phase continuity map into two regions. The region below this line is the region in which the flow rate of the phase on the abscissa is more and the region above this line is the region where the flow rate of the phase on the ordinate is more. If the physical properties of the two phases are similar then phase continuity will be decided mainly by the flow rates alone. For such a situation, in the region below the equal flow rate line the phase on the abscissa is likely to be the continuous phase whereas in the region above the equal flow rate line the phase on the ordinate is likely to be the continuous phase. Because the physical properties seldom are the same, they will also have a role to play in determining the phase continuity, and the phase on abscissa may be continuous in the region above the equal flow line and/or the phase on the ordinate may be continuous in the region below the equal flow rate line. The phase continuity map will, therefore, show the tendency of a phase to be the dispersed phase or continuous phase. In the present

Figure 8. Unstable annular flow.

Figure 9. Typical images of annular dispersed flow pattern are shown in Figure 10. This flow pattern appears to emerge from interfacial instabilities in otherwise annular flow. Similar kinds of flow pattern have been observed in oil−water stratified flow in horizontal or inclined pipes and have been the subject of several experimental and theoretical studies.25−28 In these studies, such flow patterns have been referred to as dual continuous flow as dispersion is observed in both the phases. In the present study, however, dispersion is observed only in one phase. When the aqueous phase is applied at I-1 and the organic phase comes in the center from I-3, the fines are observed in the inner layer as in the case of the first two images in Figure 10. When the aqueous phase is applied at I-3 and organic phase is applied at I-1, the droplets are observed in the outer layer as in case of third and fourth images of Figure 10. This shows that in both cases fines generated are of the aqueous phase. This may be attributed to lesser viscosity of the aqueous phase which causes more velocity gradients for a given shear stress at the interface making it more susceptible to dispersion and entrainment. In some cases, instead of fines, large droplets were observed in the inner layer indicating that at times instability may grow to a bigger size before getting detached from the bulk phase. Figure 11 shows typical images 5060



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Figure 9. Annular flow.

Figure 10. Annular dispersed flow.

Figure 11. Fully dispersed flow.

study the phase continuity was determined by observing the settling pattern of the dispersion in the waste collection bottle. In Figure 13 all the points indicate organic phase continuity

(OC) and there is no point indicating the aqueous phase continuity (AC). This shows that for the lower range of the flow rates the aqueous phase has a tendency to get dispersed. 5061



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the flow rate of the organic phase at 0.20 mLpm, flow pattern changes from slug to slug and droplet flow to droplet flow indicating that increase in flow rate of the aqueous phase which happens to be the continuous phase leads to finer dispersion. The same happens when the organic phase flow rate is increased keeping the aqueous phase flow rate at 0.20 mLpm. Here the flow pattern changes from slug flow to slug and droplet flow to droplet flow and then fully dispersed flow. When the flow rates of both the phases are more than 0.20 mLpm annular flow or its variants such as annular dispersed flow or fully dispersed flow are observed. Unstable annular flow is not observed for these inlet conditions. When the flow rate of the aqueous phase, i.e., the phase coming at center, is higher than the flow rate of the organic phase, i.e., the phase coming from sides, fully dispersed flow or annular dispersed flow are often observed. When flow rate of the organic phase, i.e., the phase coming from sides, is more than the flow rate of the aqueous phase, i.e., the phase coming from the center, stable annular flow is observed. This indicates that the interfacial instabilities are more when the flow rate of the phase coming at the center is more than the flow rate of the phase coming from sides. Figure 15 shows the phase continuity map for higher range of flow rates. For annular flow none of the phases is dispersed.

Figure 12. Flow regime map for lower flow rates: aqueous at Inlet-3, organic at Inlet-1.

Figure 13. Phase continuity map for lower flow rates: aqueous at Inlet-3, organic at Inlet-1.

This may be attributed to lesser viscosity of the aqueous phase making it more prone to dispersion. The findings here match with the observation reported earlier for similar microfluidic junctions in which one phase enters from two sides and the other enters from the center. In one of these studies the aqueous phase (water) enters from sides and the organic phase (olive oil) comes from the center and the aqueous phase is the dispersed phase.29 In the other study aqueous phase (water) enters from the center and organic phase (silicone oil) enters from the sides and once again aqueous phase is the dispersed phase.10 Figure 14 shows the flow regime map for the higher range of flow rates. As the aqueous phase flow rate is increased keeping

Figure 15. Phase continuity map for higher flow rates: aqueous at Inlet-3, organic at Inlet-1.

It was difficult to observe the phase continuity for annular dispersed flow. Such flow patterns are marked as the variants of annular flow (VAF) on the phase continuity map. As can be seen from phase continuity map, when the flow is neither annular flow nor annular dispersed flow, the aqueous phase is the continuous phase in the region above the equal flow rate line and the organic phase is the continuous phase in the region below the equal flow rate line. This indicates that at higher flow rates the inertial forces also play an important role in determining the phase continuity. Observation of dispersed flows at lower flow rates and annular flow or its variants at higher flow rates in the present study is similar to observations made in earlier studies wherein at high flow rates annular or stratified or semi-stratified flows were observed and at lower flow rates dispersed flows were observed.18,19 A unified picture of dispersion at T microfluidic junction has been reported in literature and three distinct regimessqueezing, dripping, and jettingat the junctions are identified.22 At very low flow rates the squeezing regime prevails, in which slug in the making at the junction grows in

Figure 14. Flow regime map for higher flow rates: aqueous at inlet-3, organic at inlet-1. 5062



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channel before getting detached from its bulk phase. As the flow rate increases, slug will become smaller. Dripping regime in which point of breakage will go further inside the channel will give slug and droplet flow and droplet flow. The jetting regime will give annular flow and unstable annular flow. Attrition at the interfaces of the annular flow down the channel may give rise to annular dispersed flow and fully dispersed flow. Presence of serpentine mixing channels immediately after the junction, wherein flow undergoes change in direction repeatedly, may cause redispersion or coalescence, and what has been observed in the reaction channels may be a combined effect of the microfluidic junction and the mixing channels. Few experiments were repeated to ensure that observed flow patterns are reproducible. Good reproducibility was observed. In all these experiments aqueous phase was started first and then organic phase was introduced. Few experiments were repeated with organic phase starting first. No change in flow patterns was observed. Figure 17 gives the Voronoi diagram of

size leading to reduction in flow area available for the continuous phase. Due to constriction in the flow area the pressure starts building up on the upstream side of the slug and it is gradually squeezed in downstream direction. Gradually the neck connecting the slug in the making to its bulk phase becomes thin and finally it is broken and a slug is formed. On further increasing the flow rates, dripping regime is observed. In this, breakup appears to occurs when the interfacial force is nearly balanced by the shear force though pressure effect as in squeezing regimes might also be contributing to the breakup. Compared to the squeezing regime, point of breakup of slug/ droplet in the making from its bulk phase moves in downstream direction. On further increase in flow rate inertial forces completely dominate over the interfacial forces resulting in parallel flow or stratified flow or annular flow. This is called jetting. To explain the transition from dispersed flow patterns at smaller flow rates to stratified or annular or parallel flow patterns at higher flow rates a very simple analogy can be drawn using flow of water from a faucet. When the flow rate is very small, the water does not flow as a continuous stream but trickles down in the form of small droplets. This is due to surface tension forces playing an important role while inertial forces are small. At high flow rates, water flows in the form of a continuous stream. This is due to inertial forces completely overwhelming the surface tension forces. Though the above regimes are reported for T junction channel, similar regimes are likely to occur for the microfluidic junction used in the present study. Though there is no optical access to the junction in the present setup, Figure 16 gives an idea of what

Figure 17. Voronoi diagram of the flow regime map: aqueous at inlet3, organic at inlet-1.

the flow regime map. The region surrounding each experimental point is the region in which any point is closer to this experimental point than any other experimental point in the diagram. This diagram may be used to predict the flow pattern for any intermediate combination of the aqueous and inorganic flow rates. 4.2. Aqueous Phase at Inlet-1, Organic Phase at Inlet-3. A second round of experiments was conducted after reversing the inlets, i.e., organic phase now applied at inlet-3 and aqueous phase applied at inlet-1. Figure 18 gives the flow regime map for this case for lower flow rates up to 0.20 mL/min. For lower flow rates, once again only dispersed flow patterns (SF, SDF, DF) are observed. Figure 19 gives the phase continuity map for the lower flow rates. As can be seen in most cases organic phase is the continuous phase even when aqueous phase flow rate is more than the organic phase flow rate. Aqueous phase is continuous when its flow rate is high (0.20 mLpm) and flow rate of organic phase is small (0.05, 0.10 mLpm). This indicates that viscosity still dictates the phase continuity though inertial forces do matter. Figure 20 gives the flow regime map for higher range of flow rates in which flow rates were varied between 0.2 and 1.6 mL/min. Figure 21 shows the phase continuity map for higher flow

Figure 16. Flow visualization for the case when the phase getting dispersed comes from the center.

these different regimes should look like for the microfluidic junction used in this study. The flow patterns shown in Figure 16 are based on the results obtained from numerical simulations of liquid−liquid two-phase flow at a microfluidic junction similar to the one used in the present study. These simulations, part of a separate study in our lab, were aimed at estimating specific interfacial area for the reaction of sunflower oil with KOH-laden methanol to produce biodiesel in a serpentine microchannel. Simulations were carried out for different flow rates of the oil and methanol phases. Phase-field method of interface tracking available in COMSOL Multiphysics 3.5 was used to carry out these numerical simulations. The simulations reveal the transition from squeezing to dripping to jetting regime as the flow rate increases. The blue and red colors indicate methanol phase and oil phase, respectively. Though these observations are for a different phase system, the transition from one regime to another will be observed for all phase systems and probably in all kinds of microfluidic junctions. The squeezing regime will give slug flow. When the flow rates are small, a slug will grow to occupy almost full cross-section of the 5063



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Figure 18. Flow regime map for lower flow rates: aqueous at Inlet-1, organic at Inlet-3.

Figure 21. Phase continuity map for higher flow rates: aqueous at Inlet-1, organic at Inlet-3.

be more stable compared to the case when the less viscous phase surrounds the more viscous phase. This conclusion needs to be verified with other phase systems and reasons for the same need to be thought of. As can be seen from Figure 20, when the aqueous phase flow rate is more than the organic phase flow rate annular flow or unstable annular flow is observed most of the time. When organic phase flow rate is more than the aqueous phase flow rate, annular dispersed flow or fully dispersed flow is observed. This observation once again shows that interfacial instabilities are more when the flow rate of the phase coming at the center is more than the flow rate of the phase coming from the sides. Figure 22 shows the visualization

Figure 19. Phase continuity map for lower flow rates: aqueous at Inlet-1, organic at Inlet-3.

Figure 22. Flow visualization for the case when the phase getting dispersed comes from the sides.

Figure 20. Flow regime map for higher flow rates: aqueous at Inlet-1, organic at Inlet-3.

rates. For higher flow rates the phase having higher flow rate is the continuous phase indicating that inertial forces are dictating the phase continuity. The main difference in the two cases (aqueous at inlet-3, organic at inlet-1 and aqueous at inlet-1, organic at inlet-3) is that in the former the annular flow was very stable. This can be seen from the comparison of Figure 14 and Figure 20. Whereas in Figure 14 in most cases the flow is annular flow, in Figure 20 the flow patterns are unstable variants of annular flow. This leads us to conclude that when the more viscous phase surrounds the less viscous phase the annular flow is found to

Figure 23. Voronoi diagram of the flow regime map: aqueous at inlet-1, organic at inlet-3. 5064



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of the flow at the junction for this way of applying aqueous and organic phases at the inlets. In the squeezing regime slugs will form alternatively from two sides. In dripping regime the point of breakage of droplets from its bulk phase will move downward. There will be two threads sticking to the walls on either side. Slugs and/or droplets will be ruptured off from the tips of these threads. Finally at higher flow rates the jetting regime will prevail. Figure 23 shows the Voronoi diagram of the flow regime map for this case.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; phone:+91-22-25592601; fax: +91-22-25505151. Notes

The authors declare no competing financial interest.

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5. CONCLUSIONS Liquid−liquid two-phase flow patterns have been studied in a serpentine microchannel using water−succinic acid−n-butanol phase system. The junction of the channel is such that one of the phases comes from two sides and the other comes from the center. Flow patterns for two different ways of applying the phases have been studied. In the first the organic phase comes from the sides and the aqueous phase comes from the center at the microfluidic junction. In the second way of applying phases, the aqueous phase comes from the sides and the organic phase comes at the center. Seven different flow patternsslug flow, slug and droplet flow, droplet flow, unstable annular flow, annular flow, annular dispersed flow, and fully dispersed flow are observed. Flow regime maps, phase continuity maps, and Voronoi diagrams for both cases are presented. Following are the important conclusions: (a) At low flow rates dispersed flow patterns, i.e., slug flow, slug and droplet flow, and droplet flow, are observed. (b) At higher flow rates annular flow or its variants such as unstable annular flow, annular dispersed flow, and fully dispersed flow are observed. (c) The sequence of starting the flow is found to have no effect on the flow patterns. (d) At low flow rates viscosity appears to determine the phase continuity while at higher flow rates inertial forces appear to dictate the phase continuity. (e) The annular flow is found to be more stable when the less viscous phase is surrounded by the more viscous phase. (f) Interfacial instabilities of the annular flow appear to be more when the flow rate of the phase coming at the center is more than the flow rate of the phase coming from the sides. Further experiments are planned to carry out the experiments with water−acetone−toluene (a high interfacial tension system) and water−acetone−n-butyl acetate (a medium interfacial tension system) to generate the flow regime maps of wider utility. These flow regime maps will help one identify the probable two-phase flow pattern for given flow rates of two immiscible liquid phases. Or the flow regime maps can be used to decide the flow rates if a particular flow pattern is desirable. For example if the chip studied in the present case is to be used for solvent extraction, flow rates may be chosen such that annular flow or unstable annular flow prevails. This will give higher throughputs as well as reduce to a great extent the settler size required for phase separation after solvent extraction. Combinations of flow rates that may lead to annular dispersed flow or fully dispersed flow should be avoided as they will generate very fine droplets which will be difficult to settle and may cause entrainment losses.

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NOTATIONS Dh = hydraulic diameter (μm) L = length of the channel (mm) Qa = Flow rate of the aqueous phase (mL/min) QMeOH = Flow rate of the methanol (mL/min) Qo = Flow rate of the organic phase (mL/min) Qoil = Flow rate of the oil phase (mL/min) Vmax = Maximum superficial velocity (cm/sec) ABBREVIATIONS AC = Aqueous phase continuous AF = Annular flow ADF = Annular dispersed flow DF = Droplet flow FDF = Fully dispersed flow OC = Organic phase continuous SDF = Slug and droplet flow SF = Slug flow UAF = Unstable annular flow VAF = Variant of annular flow REFERENCES

(1) Waterkemp, D. A.; Heiland, M.; Schluter, M.; Sauvageau, J. C.; Beyersdorff, T.; Thoming, J. Synthesis of ionic-liquids in microreactors − A process intensification study. Green Chem. 2007, 9, 1084−1090. (2) Brandt, J. C.; Wirth, T. Controlling hazardous chemical in microreactors: Synthesis with iodine azide. Beilstein J. Org. Chem. 2009, 5, Art. No. 30. (3) Sun, J.; Ju, J.; Ji, L.; Zhang, L.; Xu, N. Synthesis of biodiesel in capillary microreactor. Ind. Eng. Chem. Res. 2008, 47, 1398−1403. (4) Guan, G.; Kusakabe, K.; Moriyama, K.; Sakurai, N. Transesterification of sunflower oil with methanol in a microtube reactor. Ind. Eng. Chem. Res. 2009, 48, 1357−1363. (5) Niu, H.; Pan, L.; Su, H.; Wang, S. Flow pattern, pressure drop, and mass transfer in a gas-liquid concurrent two-phase flow microchannel reactor. Ind. Eng. Chem. Res. 2009, 48, 1621−1628. (6) Liu, D.; Wang, S. Gas-liquid mass transfer in Taylor flow through circular capillaries. Ind. Eng. Chem. Res. 2011, 50 (4), 2323−2330. (7) Kashid, M. N.; Renken, A.; Kiwi-Minsker, L. Influence of flow regime on mass transfer in different types of microchannels. Ind. Eng. Chem. Res. 2011, 50 (11), 6906−6914. (8) Triplett, K. A.; Ghiaasiaan, S. M.; Abdel-Khalik, S. I.; Sadowski, D. L. Gas-liquid two-phase flow in microchannels. Part-I: Two-phase flow patterns. Int. J. Multiphase Flow 1999, 25, 377−394. (9) Kawahara, A.; Chung, P. M. Y.; Kawaji, M. Investigation of two phase flow pattern, void fraction and pressure drop in a microchannel. Int. J. Multiphase Flow 2002, 28, 1411−1435. (10) Anna, S. L.; Bontoux, N.; Stone, H. A. Formations of dispersions using “flow focusing” in microchannels. Appl. Phys. Lett. 2003, 82 (3), 364−366. (11) Cubaud, T. Deformation and breakup of high-viscosity droplets with symmetric microfluidic cross flows. Phys. Rev. E 2009, 80, 026307. (12) Hsiung, S. K; Chen, C. T.; Lee, G. B. Micro-droplet formation utilizing microfluidic flow focusing and controllable moving-wall chopping techniques. J. Micromech. Microeng. 2006, 16, 2403−2410. (13) Nisisako, T.; Torii, T.; Higuchi, T. Droplet formation in microchannel network. Lab Chip 2002, 2, 24−26. 5065



Article

(14) Ghaini, A.; Mescher, A.; Agar, D. W. Hydrodynamic studies of liquid-liquid slug flows in circular microchannels. Chem. Eng. Sci. 2011, 66, 1168−1178. (15) Kashid, M. N.; Renken, A.; Minsker, L. K. CFD modelling of liquid-liquid multiphase microstructured reactor: Slug flow generation. Chem. Eng. Res. Des. 2010, 88 (3), 362−368. (16) Garstecki, P.; Fuerstman, M. J; Stone, H. A.; Whiteside, G. M. Formation of droplets and bubbles in a microfluidic T-junctionScaling and mechanism of break-up. Lab Chip 2006, 6, 437−446. (17) Dessimoz, A. L.; Cavin, L.; Renken, A.; Kiwi-Minsker, L. Liquidliquid two-phase flow patterns and mass transfer characteristics in rectangular glass microreactors. Chem. Eng. Sci. 2008, 63, 4035−4044. (18) Salim, A.; Fourar, M.; Pironon, J.; Sausse, J. Oil-water two-phase flow in microchannels: Flow patterns and pressure drop measurements. Can. J. Chem. Eng. 2008, 86, 978−988. (19) Cherlo, S. K. R.; Kariveti, S.; Pushpavanam, S. Experimental and numerical investigations of two-phase (liquid-liquid) flow behavior in rectangular microchannels. Ind. Eng. Chem. Res. 2010, 49, 893−899. (20) Zhao, Y.; Chen, G.; Yuan, Q. Liquid-liquid two-phase flow patterns in a rectangular microchannel. AIChE J. 2006, 52 (12), 4052− 4060. (21) Kashid, M. N.; Agar, D. W. Hydrodynamics of liquid-liquid slug flow capillary microreactor: flow regimes, slug size and pressure drop. Chem. Eng. J. 2007, 131, 1−13. (22) Menech, M. D.; Garstecki, P.; Jousse, F.; Stone, H. A. Transition from squeezing to dripping in a microfluidic T-shaped junction. J. Fluid Mech. 2008, 595, 141−161. (23) Benz, K.; Jackel, K. P.; Regenauer, K. J.; Schiewe, J.; Drese, K.; Ehrfeld, W.; Hessel, V.; Lowe, H. Utilization of micromixers for extraction processes. Chem. Eng. Technol. 2001, 24 (1), 11−17. (24) Zhao, Y.; Su, Y.; Chen, G.; Yuan, Q. Effect of surface properties on the flow characteristics and mass transfer performance in microchannels. Chem. Eng. Sci. 2010, 65, 1563−1565. (25) Boomkamp, P. A. M.; Miesen, R. H. M. Classification of instabilities in parallel two-phase flow. Int. J. Multiphase Flow 1996, 22, 67−88. (26) Lovick, J.; Angeli, P. Experimental studies on the dual continuous flow pattern in oil-water flows. Int. J. Multiphase Flow 2004, 30, 139−157. (27) Wahaibi, T. A.; Angeli, P. Transition between stratified and nonstratified horizontal oil-water flows. Part I: Stability analysis. Chem. Eng. Sci. 2007, 62, 2915−2928. (28) Wahaibi, T. A.; Smith, M.; Angeli, P. Transition between stratified and non-stratified horizontal oil-water flows. Part II: Mechanism of drop formation. Chem. Eng. Sci. 2007, 62, 2929−2940. (29) Mehrotra, R.; Jing, N.; Kameoka, J. Monodispersed polygonal water droplets in microchannel. Appl. Phys. Lett. 2008, 92, 213109.

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