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Liquid-liquid two-phase flow patterns in Y-junction microchannels Mayur Darekar, Krishna Kumar Singh, Sulekha Mukhopadhyay, and Kalsanka Trivikram Shenoy Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03164 • Publication Date (Web): 19 Sep 2017 Downloaded from http://pubs.acs.org on September 27, 2017

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Liquid-liquid two-phase flow patterns in Y-junction microchannels Mayur Darekar1,2, Krishna Kumar Singh1,2,*, Sulekha Mukhopadhyay1,2, Kalsanka Trivikram Shenoy1 1

Chemical Engineering Division, Bhabha Atomic Research Centre, Trombay, Mumbai, Maharashtra, India, 400085 2 Homi Bhabha National Institute, Anushaktinagar, Mumbai, INDIA-400094

*Corresponding author: [email protected]

Abstract Experiments are carried out to study liquid-liquid two-phase flow patterns in Y-junction microchannels etched in glass chips. The liquid-liquid test systems used in the experiments are the three standard test system recommended by the European Federation of Chemical Engineering (EFCE) for extraction studies. Four different types of flow patterns slug flow, slug and droplet flow, droplet flow and parallel flow – are observed. Effects of microchannel diameter, flow rate, physical properties of liquids and hydrophobicity of channel wall on two phase-flow patterns have been studied and flow regime maps are presented and discussed. Keyword: Y-junction, microchannel, flow pattern, slug flow, droplet flow, parallel flow

1. INTRODUCTION There are several advantages in using microchannels for carrying out unit operations and unit processes. High surface to volume ratios of microchannels ensure intensified heat transfer making microchannels very attractive for carrying out highly exothermic reactions.1-5 Smaller inventories in microchannels make them suitable for processes involving hazardous chemicals.611

Also scale-up of processes based on microchannels is easier due to numbering up approach

followed for scale-up.12-16 Due to the above advantages microchannels are being explored for diverse applications in chemical engineering including processes based on multiphase flow.17 One of such applications is solvent extraction.18-20 Small flow cross-section of a microchannel ensures a high specific interfacial area and high overall volumetric mass transfer coefficient which eventually lead to intensification of mass transfer.21-27 Solvent extraction in microchannels has some limitations also such as high pressure drop and possibility of clogging if the feed contains solid particles. Flow inside a microchannel is cocurrent flow, thus a microchannel represents a single theoretical stage. Also a single microchannel can handle only a limited flow rate. For achieving more flow rate, numbering-up 1 ACS Paragon Plus Environment

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of microchannels is needed which requires designing a header which should equally distribute the two liquid phases into numbered-up microchannels. However, such limitations are gradually being addressed. For example, Al-Rawashdeh et al.13 and Darekar et al.28 have proposed designs of distributor for parallel microchannels. Clogging inside microchannel can be avoided by using clean feed or pre-filtering of feed. Willersinn et al.29 have reported a device for metallic membrane mediated counter-current device for solvent extraction in microchannels which does away with the limitation of only one theoretical stage in one microchannel. Performance of a microchannel for solvent extraction depends on the specific interfacial area available for mass transfer which, in turn, depends on the flow pattern generated at the junction of the microchannel. Flow pattern generated at a microfluidic junction, in turn, depends on several parameters such as physical properties of the liquids (density, viscosity, interfacial tension, and wall wettability), flow rates of the liquids, diameter and geometry of the junction of the microchannel. While flow patterns should be such that it provides a large specific interfacial area, it should also be such that the phases disengage quickly after solvent extraction. For example, parallel flow in which no dispersion is generated is very good for quick phase separation but at the same time specific interfacial area in parallel flow is less which is not good for mass transfer. Similarly, finely dispersed flow can provide very high specific interfacial area but settling of dispersion becomes slow.30 Thus, knowledge of the flow pattern expected at a microfluidic junction is important to ascertain whether for a given design of the junction and flow rates of the two liquid phases, the expected flow pattern is the desired one or not. Several studies have been reported on liquid-liquid two-phase flow patterns in microchannels. While there are studies which focus only on flow patterns 30-35, studies which go a step beyond to understand the influence of flow pattern on mass transfer have also been reported.23-25, 36, 37 This study is focused to understand the effects of various parameters that affect the flow pattern generated at Y-type microfluidic junctions. These parameters are physical properties of the liquids (interfacial tension, wall wettability), diameter of the microchannel and operating parameters (flow rates of the aqueous and the organic phases). There are several studies reported in literature on liquid-liquid two-phase flow patterns. Some of these studies are summarized in Table 1. It can be observed that experiments with different test systems in different types of microchannels have been reported. Since flow pattern strongly depends on physical properties of the liquids involved as well as on the geometry of the 2 ACS Paragon Plus Environment


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microfluidic junction and microchannel, it is usually difficult to extend the results of a particular combination of the test system and microchannel geometry to a different combination of the test system and microchannel geometry. As a result, despite of several reported studies, one still needs to carry out the experiments afresh for the test system and microchannel geometry of interest. Since the standard tests systems for solvent extraction have already been identified by the EFCE38, it is recommended that more and more work should be carried out using the standard test systems. In this study we have worked with the standard test systems and Yjunction microchannels. In a previous study, we have reported flow patterns for one of the standard test systems in a serpentine microchannel.30 We hope that in the time to come more studies will be carried out on flow patterns in other types of microchannels using the standard test systems and all such studies with standard test systems, put together will lead to a comprehensive database that can be used to predict the flow pattern expected for a given combination of test system and microchannel without resorting to experiments provided the physical properties specially the interfacial tension of the new test system falls in the range covered by the standard test systems.

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Table 1: Summary of some previous studies on liquid−liquid two-phase flow patterns in microchannels Referen ce

MoC, Geometry and Layout of the microchannel

DH (µm)

23

Glass T, Y-junctions, straight microchannels

270, 400, 500

30

Glass, + junction, serpentine channel

31

PMMA T-junction, straight channel

270, 300

400

Flow rate/ flow velocities

Interfacial Tension and Wall wettability

1 – 18 mL/min

Test system

water-acetonetoluene

0.05 – 1.6 ml/min

1.7 mN/m

9.26 × 10-4 2.78 m/s

0.045 mN/m

water-succinic acid-n-butanol

Water-kerosene

Observed flow regimes Slug flow, slug-droplet flow, deformed interface flow, parallel/annular flow, slug-dispersed flow, dispersed flow Slug flow, slug and droplet flow, droplet flow, unstable annular flow, annular flow, annular dispersed flow and fully dispersed flow Slug flow, droplet flow, parallel flow, chaotic striation

5–200 ml/h 32

PTFE Y-junction, straight channel

33

Quartz and glass Tjunction, straight channel

2501000

1-42 cm/s

34



400, 480, 534

36

Glass T, Y-junctions, straight channel

400, 269

mineral oil-water

Slug flow, droplet flow, annular flow, stratified flow, semi-stratified flow

Water- kerosene 0.045 N/m

540, 1000

35

Slug flow, droplet flow, deformed Interface flow

30.1 mN/m

793, 667

5-350 ml/h

Watercyclohexane

kerosene-water; coconut oil-water

0.002-0.3 m/s

1-6 ml/hr

Slug flow and stratified flow

37 - 38.1 mN/m

Water-Toluene 0.0371 N/m Water Hexane

Parameters Varied Diameter, flow rate

Flow rate

Flow rate

Diameter of Yjunction and capillary, flow rate

MoC of microchannel (wettability), flow rate, diameter of microchannel Flow rates, diameter of microchannel, and physical properties Diameter, flow rate

cyclohexanecarboxyl methyl cellulose

Slug flow, droplet flow, parallel flow and jet flow

water – toluene; water & NaOH – toluene;

Slug flow, parallel flow


Diameter of Yjunction and capillary, flow rate,


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

0.0511 N/m

37

39

PMMA T-junction, straight channels

SU-8 T-junction, straight channel

400

0.09 – 23.7 ml/min

267

study

Glass Y-junctions, straight microchannels

260, 760

interfacial tension

0.045 N/m Water-kerosene kerosene – water (46.2 mN/m), paraffin oil – water (45.2 mN/m) and castor oil – paraffin oil (17 mN/m)

kerosene – water, paraffin oil – water and castor oil – paraffin oil

1.75 mN/m, 14.1 mN/m, 36 mN/m

Water-n Butanol, Water- n Butyl acetate, WaterToluene (test systems recommended by the EFCE)

0.05 – 10 ml/min Present

water - toluene & CCl3COOH

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Droplet flow, parallel flow

Surface modification, flow rate Velocity ratios


Parallel flow, slug flow, plug flow, dispersed (droplet) flow, and rivulet flow

Slug flow, Slug and droplet flow and parallel flow

Diameter of Yjunction and microchannel, flow rate, interfacial tension, wall wettability

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2. EXPERIMENTAL SETUP The schematic diagram of the experimental setup is shown in Figure 1. The aqueous and organic phases are pumped to the microchannel etched in a glass chip at the required flow rates by a twochannel syringe pump. The microchannel is placed horizontally on a stand with a high speed camera and microscope mounted on top of it. The magnified microchannel can be seen on the computer screen in real time. The camera is set to capture video at the rate of 100 to 200 fps. The images are stored in the computer memory for further analysis. The liquids coming at the inlets of the microchannel meet at the Y-junction and dispersion is generated. The acquired images are used to determine the flow pattern prevailing inside the microchannel. The observed flow patterns are plotted in the form of flow regime maps. The glass chip having microchannels etched in it is shown in Figure 2. The chips are procured from M/s. Dolomite Microfluidics Ltd. UK. Each chip has two microchannels, one with hydrophobic coating and one without hydrophobic coating. To make the channels hydrophobic silane coating is used. The procured microchannels are used as such without any further characterization. A chip holder holds the glass chip. This chip header ensures connection of the microchannel with the pump with the help of PTFE tubings of 1/16” OD. The other components of the setup are high speed imaging and illumination system.

Figure 1. Schematic diagram of the experimental setup 6 ACS Paragon Plus Environment


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Microchannels of two different diameters (260 µm and 760 µm) are used to study the effect of diameter on flow pattern. For each diameter, microchannels with and without hydrophobic coating are used to study the effect of wall wettability on the flow pattern. Silane coating is used to make the microchannel hydrophobic. The test systems used in the experiments are the standard test systems recommended by EFCE.18, 19, 24, 37 Water-butanol test system has a very low interfacial tension. Water-butyl acetate test has a medium interfacial tension while water-toluene has a high interfacial tension. The physical properties of the three test systems are given in Table 2.40 Aqueous and organic phases are mutually equilibrated. Table 2: Physical properties of the test systems used in the experiments at 200 C Test System Aqueous Organic Aqueous Organic Aqueous Organic

Water n-Butanol Water n-Butyl acetate Water Toluene

ρ (kg/m3)

µ (mPa·s)

985.6 846 997.6 880.9 998.2 865.2

1.426 3.364 1.0274 0.734 0.963 0.584

σ

(mN/m) 1.75 14.1 36

(a)

(b)

(c) 250 µm

750 µm

270 µm

770 µm

Figure 2: (a) Microchannel chip with holder for microbore tubing (b) Cross-section of the microchannel having DH = 260 µm (c) Cross-section of the microchannel having DH = 760 µm


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3. RESULTS AND DISCUSSION 3.1 Typical Flow Patterns Four different types of flow patterns such as Slug flow (SF), Slug and droplet flow (SDF), Droplet flow (DF) and Parallel flow (PF) are observed in the experiments. Typical images of these flow patterns are shown in Figure 3.

Slug flow (SF)

Slug and droplet flow (SDF)

Droplet flow (DF)

Parallel flow (PF)

Figure 3. Different liquid-liquid two-phase flow patterns observed in the experiments (images from uncoated microchannel, DH = 760 µm)

3.1.1 Slug Flow Slug flow regime is observed at low flow rates of the aqueous and organic phases. The flow regime responsible for generation of slugs is the squeezing flow regime.30, 41 In this regime, shear stress is not important and breakage of dispersed phase into slugs is governed by the interfacial tension and pressure gradient due to the obstruction to the flow of the continuous phase caused by the growing slug. The slug occupies almost the whole cross-section of the microchannel with a very thin layer of the continuous phase between the slug and the wall of the microchannel. In all the experiments, the aqueous phase was the dispersed phase and the organic phase was the continuous phase. At constant dispersed phase flow rate increase in the flow rate of the continuous phase was observed to cause reduction in slug size, as shown in Figure 4. Further increase in continuous phase flow rate led to transition of flow pattern from slug flow to slug and 8 ACS Paragon Plus Environment


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droplet flow, droplet flow and parallel flow depending upon the dispersed phase flow rate. Increase in dispersed phase flow rate at constant continuous phase flow rate caused transition from slug flow to parallel flow. There is no significant change in slug size with change in continuous phase flow rate at a constant dispersed phase flow rate as shown in Figure 5.

Q A = 2 ml/min Qo =1 ml/min



Figure 4. Slugs formed for diffrent continuous phase (organic phase) flow rates at a constant dispersed phase (aqueous phase) flow rate (phase system: water– butyl acetate, DH = 760 µm, uncoated microchannel)


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Q A = 0.2 ml/min Qo =2 ml/min




Figure 5. Slugs formed for different dispersed phase (aqueous phase) flow ratee at a constant continuous phase (organic phase) flow rate (phase system: water– butyl acetate, DH = 760 µm, uncoated microchannel)

3.1.2 Slug and Droplet Flow This type of flow regime is observed at flow rates which are just higher than the flow rates at which slug flow is observed. This flow pattern marks the transition of squeezing flow regime to dripping flow regime. A typical image of slug and droplet flow pattern is shown in Figure 3.

3.1.3 Droplet Flow A typical image of the droplet flow pattern is shown in Figure 3. This flow pattern is observed after complete transition from squeezing regime to dripping regime.30, 41 It is characterized by the drops having diameters less than the diameter of the microchannel. This flow pattern typically occurs at relatively high continuous phase flow rates and low dispersed phase flow rates i.e. the condition when the inertial force associated with higher flow rate of the continuous phase is high enough to knock down the dispersed phase into small droplets and the resistance offered by the dispersed phase is small due to its lower flow rate. As the flow rate of the continuous phase increases for a constant dispersed phase flow rate, the size of the droplets reduces, as shown in Figure 6.



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Q A = 0.05 ml/min Qo = 0.8 ml/min

Q A = 0.05 ml/min Qo =1.4 ml/min

Q A = 0.05 ml/min Qo = 2 ml/min

Figure 6. Effect of the continuous phase (organic phase) flow rate on the size of the droplet in droplet flow regime. (phase system: water – butanol, DH =760 µm, coated microchannel)

3.1.4 Parrallel Flow: This flow pattern is the result of jetting flow regime in microchannel. At higher flow rates of both the phases, the inertial force dominates over the interfacial tension force and the two phases flow past each other without generation of dispersion. Typical images of parallel flow are shown in Figure 3 and Figure 7. Figure 7 shows that the position of the interface shifts inside the microchannel as the relative flow rates of the continuous and dispersed phase change.


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Q A = 8 ml/min Qo = 4 ml/min

Figure 7. Effect of the continuous phase (organic phase) flow rate on the position of the interface between the two liquids in parallel flow. (phase system: water – butyl acetate, DH = 760 µm, uncoated microchannel).

3.2 Effect of flow rate on the flow regime map To illustrate the typical effect of flow rate on the flow pattern, the flow regime maps for the intermediate interfacial tension system (water - butyl acetate) and high interfacial tension system (water - toluene) are shown in Figure 8. These flow patterns are completely repeatable. Figure 8 shows that slug flow is obtained for very small values of the flow rates of the aqueous phase and the organic phase. When the dispersed phase (aqueous phase) flow rate is kept constant and the continuous phase flow rate is increased, slug flow changes to slug and droplet flow or droplet flow. This is because, for a constant flow rate of the dispersed phase, an increase in the flow rate of the continuous phase increases its inertial force leading to enhancement in its tendency to break down the dispersed phase in smaller fragments causing transition from slug flow to slug and droplet flow or to droplet flow. Flow regime for high flow rate of the continuous phase and low flow rate of the dispersed phase is the droplet flow. On continued increase in continuous phase flow rate, flow regime does not change further but size of the droplets keeps reducing.



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(a)

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(b)

Figure 8. Effect of flow rate on flow pattern in coated microchannel. DH = 260µm, QA, max = 3 ml/min, QO, max=3

ml/min (a) water – butyl acetate (b) water – toluene

When the dispersed phase (aqueous phase) flow rate is increased keeping the continuous phase (organic phase) flow rate constant, dispersion tends to become coarser with shorter slugs changing to longer slugs or droplet flow changing to slug and droplet flow or slug flow. On continued increase in the dispersed phase flow rate, continuous phase is eventually not able to break down the dispersed phase and the two phases flow side by side in parallel flow.

3.3 Effect of diameter on flow pattern Figure 9 illustrates the effect of microchannel diameter on the flow pattern for two test systems (water- butyl acetate and water - toluene). Figure 9(a) is compared with Figure 9(b) and Figure 9(c) is compared with Figure 9(d). The comparison of the flow regime map for the larger diameter and smaller diameter microchannels suggests that all the flow regime transitions occur at lower flow rates in 260 µm diameter microchannel compared to 760 µm diameter microchannel. For example, for water-butyl acetate system the organic phase flow rate at which transition from slug flow to droplet flow occurs at the smallest flow rate of the aqueous phase is about 3.3 ml/min in 760 µm compared to 0.75 ml/min in 260 µm. Dominance of the parallel flow regime over other flow regimes can also be observed in smaller diameter microchannel. In previous studies on liquid-liquid two-phase flow patterns in microchannels, different researchers have used different coordinates numbers to present flow regime maps.35 For example, Dessimoz et al.36 used Capillary number (Ca = µU/σ) and Reynolds number (Re = DH ρ U/µ). Kashid et al.23, Sarkar et al.30, Kashid et al.32 and Salim et al.33 used velocities or flow 13 ACS Paragon Plus Environment

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rates. Zhao et al.31 and Cherlo et al.34 used Weber number (We = ρ U2 DH/σ). The data of Figure 9 can be used to identify which dimensionless number is most appropriate to present the generalized flow regime maps. The relations between the flow rates in the microchannels of two different diameter for keeping Ca or Re or We same for a given test system (constant physical properties) are expressed by eq 1:

 D  2  H 2  Q1 ∀ Ca 2 = Ca 1  D H1   1  D H 2   Q1 ∀ Re 2 = Re1 Q 2 =  D  H 1   1.5   D H 2  Q ∀ We = We 1 2 1  D H1  

(1)

Using Eq. 1, the flow rate (Q2) of the organic phase in 260 µm diameter (DH2) microchannel at which transition from slug flow to the droplet flow occurs at the minimum flow rate of the aqueous phase is evaluated using the experimental values of the organic phase flow rate (Q1) at which this transition occurs in 760 µm diameter (DH1) microchannel. The evaluated value of Q2 can be compared with the experimental value of Q2. The evaluation is done both for water – butyl acetate and water – toluene system. The results are shown in Table 3. For water-butyl acetate as well as water-toluene system, equivalence of We gives the best estimate of Q2 followed by equivalence of capillary number. This suggests that among the three dimensionless numbers mentioned above, We is the most appropriate to present the flow regime maps using dimensionless numbers. Table 3: Comparison of the experimental and estimated values of the organic phase flow rate (Q2) at which transition from slug flow to droplet flow occurs at the lowest flow rate of the aqueous phase in 260 µm (DH2) microchannel (Q1 = 3.5 ml/min for water – butyl acetate system, Q1 = 6.5 ml/min for water – toluene system, DH1 = 760 µm) Water – butyl acetate Water - toluene

Experimental 0.69 1.05

Q2 (ml/min) Ca2 = Ca1 Re2 = Re1 0.41 1.20 0.76 2.22


We2 = We1 0.70 1.30


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(a)

(b)

(c)

(d)

Figure 9. Effect of microchannel diameter on flow regime map. (a) phase system: water – butyl acetate, DH = 760 µm, uncoated microchannel, QA, max = 10 ml/min, QO, max = 10 ml/min (b) phase system: water – butyl acetate, DH = 260 µm, uncoated microchannel, QA, max = 3 ml/min, QO, max = 3 ml/min (c) Phase system: water – toluene, DH = 760µm, uncoated microchannel, QA, max = 10 ml/min, QO, max = 10 ml/min (d) phase system: water – toluene, DH = 260µm, uncoated microchannel, QA, max = 3 ml/min, QO, max = 3 ml/min.

3.5 Effect of interfacial tension The effect of interfacial tension on the flow regime map is illustrated through Figure 10 which shows flow regime maps of the three test systems in 760 µm diameter microchannel. Figure 10 shows that for water-toluene system which has the maximum interfacial tension, slug flow dominates the flow regime map. As interfacial tension reduces, prominence of slug flow in flow regime map diminishes and for the water – butanol system which has a low interfacial tension, slug flow regime is observed in only a few experiments. On the contrary, with an increase in interfacial tension, prominence of parallel flow in flow regime map reduces. While parallel flow is observed for many experimental points in the flow regime map for water-butanol system,


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parallel flow is obtained for less number of points in the flow regime map for water – toluene system. The data of Figure 10 can be once again used to identify which of the important dimensionless numbers is most appropriate to present the flow regime maps. For the same microchannel, the equivalence of the three dimensionless numbers requires the following relations between the flow rates of two different test systems.

 σ  µ   2  1 Q1 ∀ Ca 2 = Ca 1  σ1  µ 2    µ  ρ  Q 2 =  2  1 Q1 ∀ Re 2 = Re1  µ1  ρ 2  0.5 0.5   σ 2   ρ1  Q ∀ We = We 1 2 1  σ1   ρ 2   (2) Using Eq. 2, the flow rate (Q2) of the organic phase for water-butanol and water – acetone test systems at which transition from slug flow to the droplet flow occurs at the minimum flow rate of the aqueous phase can be estimated using the experimental value of the organic flow rate (Q1) at which this transition occurs for water – butyl acetate system. The experimental value of Q2 and its estimate based on equivalence of the three relevant dimensionless numbers are listed in Table 4. For water-butanol system, the equivalence of Ca gives the best estimate followed by equivalence of We. For water- toluene system equivalence of We gives the best prediction followed by equivalence of Ca. Thus, the data listed in Table 3 and Table 4 suggest that in majority of the cases We is the best dimensionless number to represent flow regime maps. Table 4: Comparison of estimated and experimental value of organic flow rate (Q2) at which transition from slug flow to droplet flow occurs for the lowest flow rate of the aqueous phase (Q1 = 3.5 ml/min for water – butyl acetate system, DH = 760 µm)

Water - butanol Water - toluene

Experimental 0.30 6.50

Q2 (ml/min) Ca2 = Ca1 Re2 = Re1 0.09 16.61 11.23 2.83


We2 = We1 1.26 5.64


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(a)

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(b)

(c)

Figure 10. Effect of interfacial tension between the two liquids on the flow regime maps in uncoated microchannels of 760 µm diameter (a) phase system: water – butanol, QA, max = 2 ml/min, QO, max = 2 ml/min (b) phase system: water – butyl acetate, QA, max = 10 ml/min, QO, max = 10 ml/min) (c) phase system: water – toluene system, QA, max = 10 ml/min, QO, max = 10 ml/min).

3.6 Effect of hydrophobicity of channel Figure 11 shows the effect of hydrophobicity of channel wall on the flow regime map. Flow regime maps for water – toluene system in 760 µm diameter microchannel show that dispersed flow (slug flow and slug and droplet flow) is more prominent than parallel flow in the microchannel having hydrophobic coating. As the water is dispersed phase, the hydrophobic coating promotes dispersed flow than parallel flow in which the aqueous phase must wet the wall of the microchannel. Similar observation can be made from the flow regime maps of water – butanol and water – butyl acetate, as shown in Figure 11.


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(a)

(b)

(c)

(d)

(e)

(f)

Figure 11. Flow regime map for uncoated microchannel and coated microchannel of 760 µm diameter (a) phase system: water – butanol, uncoated microchannel, QA, max = 2 ml/min, QO, max = 2 ml/min. (b) phase system: water – butanol, coated microchannel, QA, max = 2 ml/min, QO, max = 2 ml/min (c) phase system: water – butyl acetate, uncoated microchannel, QA, max = 10 ml/min, QO, max=10 ml/min. (d) phase system: water – butyl acetate, coated microchannel, QA, max = 10 ml/min, QO, max = 10 ml/min. (e) phase system: water – toluene, uncoated microchannel, QA, max = 10 ml/min, QO, max = 10 ml/min. (f) phase system: water – toluene, coated microchannel, QA, max = 10 ml/min, QO, max = 10 ml/min.



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3.7 Generalized flow regime map In this study, diameter of microchannel, physical properties as well as flow rates have been varied. The resulting experimental data offer an opportunity to check if the data is plotted using appropriate dimensionless numbers a generalized flow regime map emerges or not. In the previous sections, suitability of various dimensionless numbers in prescribing flow regime map was evaluated and Weber number was found to be the most appropriate dimensionless number. Weber number (We = ρ U2 DH/ σ) is also recommended to present flow regime map by Zhao et al.31 and Cherlo et al.34 As mentioned previously, different researchers have used different combinations of dimensionless numbers as coordinates to present flow regime maps for liquidliquid two-phase flow in microchannel. Therefore, in Figure 12 we have plotted our experimental data in different ways to identify the best combination of dimensionless numbers as coordinates for presenting generalized flow regime map for liquid-liquid two-phase flow in Y-junction microchannels. Flow regime map given in Figure 12(a) is based on Weber number as the coordinates. Following Kashid et al.23, Sarkar et al.30, Kashid et al.32 and Salim et al.33, flow regime map given in Figure 12(b) is based on velocity as the coordinates. It shows that points corresponding to slug flow are clustered in two distinct groups separated by points corresponding to other flow patterns. The same is true for slug and droplet flow regime. Similar observation holds when flow regime map is presented using capillary number (Ca = µU/σ) and Reynolds number (Re = DH ρ U/µ) as seen in Figure 12(c). Capillary number and Reynolds number were used as the coordinates by Dessimoz et al.36 Figure 12(d) uses capillary numbers as the coordinates. Figure 12(e) uses We·Oh as the coordinates as prescribed by Yagodnitsyna et al.39 for generalized flow regime maps for a T-junction microchannel. Here Oh represents Ohnesorge number (Oh = µ/(ρσd)1/2). Figure 12(d) which is based on capillary number as the coordinates shows that points corresponding to a particular flow pattern do cluster together. Thus there are separate zone on flow regime map for each flow pattern. But the size of the zone in which overlapping of points corresponding to different flow patterns occurs is minimum when We·Oh is used as the coordinates, as shown in Figure 12(e). Thus, as was reported for T-junction microchannels, for Y-junction microchannels also, We·Oh is found to be good for presenting generalized liquid-liquid flow regime map.


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The type of flow pattern generated in liquid-liquid two-phase flow in microchannels is dictated by the interfacial tension force, inertial force and viscous force as elucidated by De Menech et al.41 who proposed squeezing, dripping and jetting regimes of dispersion at T-type microfluidic junction. Weber number takes into account inertial force and interfacial tension force but does not account for the viscous force. The combination We·Oh accounts for viscous force also and, therefore, is more suitable than We alone for presenting the generalize flow regime map which has flow patterns belonging to squeezing, dripping as well as jetting regime. Whether the generalized flow regime map proposed for liquid-liquid dispersion at Y-junction microchannel as given by Figure 12(e) of this study holds good for the data reported previously is also checked. For this, data of Kashid et al.23 have been plotted in Figure 12(f) which has the same demarcation lines or curves as used in Figure 12(e). The data of Kashid et al.23 are for a Yjunction microchannel having a square cross-section and 269 µm hydraulic diameter. The phase system used by Kashid et al.23 is water-acetone-toluene system. Figure 12(f) shows that slug flow and parallel flow data of Kashid et al.23 fit well in our generalized flow regime map. Kashid et al.23 did not differentiate between slug and droplet flow and droplet flow. Their data points for slug and droplet flow when put in our generalized flow regime map are spread in the regions of slug flow, droplet flow, parallel flow and overlapping region. Like the data points of slug and droplet flow of Kashid et al.23 some of our data points of slug and droplet flow are present in the overlapping region of the generalized flow regime map. Thus, only 5 data points of Kashid et al.23 out of 69 violate our generalized flow regime map. The violation is not significant as the data points violating the generalize flow regime map are not deep inside the regions representing different flow patterns but close to the demarcation lines or curves. A good match with the data of Kashid et al.23 suggests the utility of the generalized flow regime maps for liquid-liquid dispersion at Y-junction microchannels presented in this study.



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(a)

(b)

(c)

(d)

(e)

(f)

Figure 12. Generalized flow regime map for (a) uncoated microchannels with Weber numbers as the coordinates (b) uncoated microchannels with velocities as the coordinates (c) uncoated microchannels with capillary number and Reynolds number as the coordinates (d) uncoated microchannels with capillary numbers as the coordinates (e) uncoated microchannels with We·Oh as the coordinates. (f) Data of Kashid et al.41 plotted with We·Oh as the coordinates.

4. CONCLUSIONS Experiments are performed to study the effects of diameter, flow rate, interfacial tension and wall wettability on the liquid-liquid two-phase flow patterns generated at Y-junction 21 ACS Paragon Plus Environment

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microchannels. The experiments are conducted using the standard test systems prescribed by the EFCE. Four different types of flow patterns - slug flow, slug and droplet flow, droplet flow and parallel flow – are observed. Aqueous phase is found to be the dispersed phase. Slug flow is observed for low flow rates of dispersed and continuous phases. Slug flow changes to slug and droplet flow and then to droplet flow when continuous phase flow rate is increased keeping dispersed phase flow rate constant at a low value. Slug flow changes to parallel flow when dispersed phase flow rate is increased keeping the continuous phase flow rate constant at a low value. Parallel flow is observed for high flow rates of both dispersed phase and continuous phase. As microchannel diameter is reduced, flow pattern transitions are observed to occur at lower flow rates. With increase in interfacial tension, slug flow and other dispersed flow patterns (slug and droplet flow and droplet flow) become more prominent. Similarly, with hydrophobic coating which reduces wetting of the wall by the dispersed phase, slug flow and other dispersed flow pattern become more prominent at the cost of parallel flow. Different dimensionless numbers are evaluated for their suitability for presenting the generalized flow regime map. For this purpose, We is found to be a better candidate than Ca and Re. Thus the generalize flow regime map are presented using We and We·Oh and the latter is found to be better. These generalized flow regime map will be useful for estimating flow pattern in a Yjunction microchannel. In a previous study generalized flow regime maps for liquid-liquid two-phase flow in T-junction microchannels are presented. The present study augments the previous study by presenting generalized flow regime maps for Y-junction microchannels. It is expected that future similar studies on different types of microfluidic junctions will eventually lead to a comprehensive data base of generalized liquid-liquid two-phase flow regime maps for different types of microfluidic junction. Such data base will be very useful for researchers and designers working on microscale liquid-liquid extraction.

NOTATIONS A

Aqueous phase

Ca

Capillary number (-)

DH

Equivalent diameter of microchannel (µm) 22 ACS Paragon Plus Environment


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O

Organic phase

Oh

Ohnesorge number (-)

OhA

Ohnesorge number for aqueous phase (-)

OhO

Ohnesorge number for organic phase (-)

QA

Aqueous phase flow rate (ml/min)

QA, max Qo

Maximum aqueous phase flow rate (ml/min) Organic phase flow rate (ml/min)

Qo, max

Maximum organic phase flow rate (ml/min)

Re

Reynolds number (-)

U

Velocity of the liquid phase (m/s)

We

Weber number (-)

WeA

Weber number of aqueous phase (-)

Weo

Weber number of organic phase (-)

Greek letters

µ ρ

σ

Viscosity of the liquid phase (Pa s) Density of liquid phase (Kg/m3) Interfacial tension (N/m)

ABBREVIATIONS DF EFCE PF

Droplet Flow European Federation of Chemical Engineering Parallel flow

PMMA Polymethylmethacrylate PTFE SDF SF

Polytetrafluoroethylene Slug and droplet flow Slug flow


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