1 An integrated micro-fluidic device for continuous separation and pre

Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Integrated Microfluidic Device for Continuous Separation and Preconcentration of Surface Active Solutes Avinash Sahu and Pushpavanam Subramaniam* Chemical Engineering Department, Indian Institute of Technology Madras, Chennai 600036, India

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S Supporting Information *

ABSTRACT: Foam fractionation is extensively used for the separation and purification of surface active solutes from a low concentration mixture at room temperature. In this work we present the design of a novel microfluidic device to concentrate a solute by adsorbing it on a gas−liquid interface. The device is integrated with a downstream separator where the foam is separated from the residual liquid. Both gas and liquid streams flow continuously into the device. The device has a diverging and converging cross section to facilitate the formation of a foam-like structure. The device is demonstrated on the enrichment of bovine serum albumin (BSA) using cetyltrimethylammonium bromide (CTAB) as a surfactant. The effect of various parameters, i.e., gas flow rate, aqueous phase flow rate, and salt concentration, on the enrichment of the solute is investigated. We show how we can infer diffusiophoresis in the system at low gas flow rates by measuring macroscopic parameters like mean residence time.

1. INTRODUCTION Bovine serum albumin is a small, stable, moderately nonreactive protein used in biochemical assays and as a nutrient in microbial cell cultures. This biomolecule is sensitive to temperature and pH1 and is present in low concentrations in most solutions. A mild process available for the purification and concentration of biomolecules like bovine serum albumin (BSA) is foam fractionation in which the biomolecules are not subjected to any harsh treatment and hence do not denature. Foam fractionation relies on the formation of a gas−liquid interface and separates molecules based on their differences in surface affinity.2 Foam fractionation has been used extensively in wastewater treatment,3 recycling of paper,4 mineral processing,5 and biomolecules separation directly from broth media.6 Surfactin is a biosurfactant and a secondary metabolite of Bacillus subtilis. Its separation was studied using foam fractionation.7 Subsequently, many researchers have worked on developing systems for separation of nonsurface active solutes using a surfactant as a collector. Separation of creatine and phenol was studied by forming a creatine−SDS complex8 and a phenol−cetyltrimethylammonium bromide (CTAB) complex.9 The surfactants SDS and CTAB are amphiphiles and hence adsorb at the interface. They act as a collector which at a favorable pH forms a complex with the desired solute. This complex formation facilitates separation and purification of the © XXXX American Chemical Society

solute. The above studies demonstrate that biomolecules can be separated selectively from a culture broth. The culture broth can hence be used directly for foam fractionation in downstream processing to selectively remove molecules. Separation of nisin from a fermentation broth has been adopted at the industrial scale.10 Here the foam fractionation results in a low volume of a highly concentrated solution of the desired solute. Consequently, the logistics involved for further purification is drastically reduced, lowering the manufacturing costs. Novel foam fractionation column designs have been employed to enhance the enrichment of biomolecules11 and nanoparticles.12 These columns utilize drainage as a mode to concentrate the foamate. But it has also been reported that the liquid drainage leads to aggregation of biomolecules resulting in their functionality loss.13 Drainage from the film further can reduce the efficacy of adsorption as it removes the solute from the foam back to the bulk phase. This accelerates bubble coalescence which reduces the surface area available per unit volume for adsorption. To overcome these challenges, Received: Revised: Accepted: Published: A

April 20, 2018 July 24, 2018 July 30, 2018 July 30, 2018 DOI: 10.1021/acs.iecr.8b01724 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

Figure 1. Schematic of cross-junction microchannel having a diverging and converging cross section with a downstream foam separator: (a) top view, (b) side view.

(Qaq), and presence of ionic salts, on the working and efficiency of the device.

microchannels can be employed. They can be designed to operate with both phases flowing continuously while having a high gas−liquid interfacial area (mimicking foam flow) and low residence times. Gravitational effects are not significant here, and hence foam drainage can be avoided. Besides, if the adsorption of solute in a microchannel is integrated with a downstream separator, the continuous separation of the two phases can occur simultaneously. This system can also be employed to preconcentrate a dilute solution for sensors based detection. Given the ability of microchannels to process low fluid volumes, this study has a high potential for point of care diagnostics where preconcentration of the solute can be an important factor. The adsorption of methyl orange dye on CTAB as a collector in a cross-junction microchannel was studied recently.14 This experimental study shows that the adsorption of solute and separation of two phases can happen simultaneously. For high value, low volume products from the pharmaceutical and fine chemical industries foam fractionation in a microchannel can be used effectively for separation and purification. There have been extensive studies on hydrodynamics of gas−liquid flows in microchannels with varying cross-sectional areas.15 However, adsorption in these systems was not studied. There are several studies in the literature on semibatch foam fractionation of BSA. In these the focus was on improving the enrichment using a novel foam fractionation design11by varying pH,10 changing salt concentration, and using an antifoaming agent.16,17 In this work we discuss the design and performance of a microdevice where adsorption occurs on a gas−liquid interface followed by continuous separation of the foamate and residual phase. We demonstrate the performance of this device on foam fractionation of bovine serum albumin. This work is a first of its kind to focus on adsorption of solute in a microchannel. The focus of this work is on performing controlled experiments and characterizing the role of various parameters, i.e., gas flow rate (Qg), aqueous phase flow rate

2. MATERIALS AND METHODS Analytical grade cetyl triammonium bromide (CTAB) was procured from Spectro Chem. Sodium chloride (NaCl), phosphoric acid, and ethanol were purchased from Merck. Coomassie brilliant blue G-250 (CBBG) was obtained from LOBA Chemie. Molecular grade crystallized bovine serum albumin was purchased from Hi Media. DI water was used for preparing all solutions. The concentration of BSA was determined using a linearized Bradford method. The dye reagent required for this method was prepared by dissolving 100 mg of the dye CBBG in 50 mL of absolute ethanol. One hundred milliliters of phosphoric acid (88% w/v) was added to this, and the solution was diluted to 1 L with DI water. This reagent was stored at 4 °C and protected from light and used as such. Four milliliters of the dye reagent solution and 1 mL of the protein sample were mixed, and the absorbance of solution was measured using a UV−vis spectrophotometer (Jasco-V-630). The ratio of absorbance at 590 and 450 nm (A590 /A450) was used to determine the BSA concentration.18 The performance was analyzed using two indices. Recovery ratio (RR) is defined as the ratio of mass of solute in the foamate phase to that in the initial feed: i Mass of the solute in the foam yz zz × 100 = jjj k Mass of the solute in the initial feed {

Recovery ratio

(1)

Enrichment ratio (ER) is defined as the ratio of concentration of the solute in the foamate phase to that in the initial feed: B

DOI: 10.1021/acs.iecr.8b01724 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research Enrichment ratio i Concentration of solute in the foam yz zz = jjj k Concentration of solute in the initial feed {

straight channel, i.e., with no diverging and converging section, results in slug flow. The slugs collapse on exiting the channel, and consequently no separation can be achieved. The low shear rates at the gas−liquid interface ensure that the adsorbed biomolecules do not denature.23 The last section shows the separator attached to the exit of the microchannel. Here the foam is separated from the aqueous solution continuously. The separator consists of a U-shaped tube whose two limbs are connected to the collectors of the foamate F and residue R as shown in Figure 1b. The two phase mixture enters one of the vertical limbs of the foamate collector and gets separated into foamate and residue. The foam flows up the inlet limb to the F-collector, and the U is filled with the residual liquid which flows to the R-collector. The R-collector is connected to the other limb and is at the same level as the inlet. Further details of the microchannel device and the separator design can be found in the patents filed.24,25 CTAB is used as the surface active agent to stabilize the foam in all experiments. This choice is motivated by the higher enrichment observed for BSA using this surfactant in earlier studies.16 Several studies have reported an increase in enrichment in the presence of salt.16,26 To analyze the dependency of the ER and RR of BSA on operating conditions, three sets of experiments were carried out. 1. The feed solution contains BSA and CTAB, and the flow is through a cross junction, i.e., aqueous streams enter through I1 and I3 and gas stream through I2. 2. The feed solution contains BSA, CTAB, and salt, and the inlet streams flow through a cross junction. 3. The feed solution contains BSA, CTAB, and salt. The aqueous phase and gas stream flow through I1 and I2, respectively, and meet in T-junction. The inlet I3 is sealed.

(2)

Foamate collected after each run is subjected to centrifugation, and the resulting liquid volume is considered as foamate volume (FV). ij yz Foamate volume z = jjj j Time required to collect the foamate phase zzz k {

Volumetric flow rate of foamate phase

(3)

We use the foamate residence time (FRT) of bubbles in the microchannel to interpret the observed trends. This is defined as the ratio of volume of channel to the volumetric flow rate of foamate phase, i.e., bubbles collected at outlet. Foamate residence time Volume of the channel ji zyz = jjj j Volumetric flow of the foamate phase zzz k {

(4)

The volume between the cross junction and the separator is 2.84 mm3. 2.1. Design and Operation of the Microfluidic Device. A cross-junction microchannel with a rectangular cross section was used to produce foam in the microfluidic experiments. The device comprises a diverging section and a straight section followed by a converging section. The PMMA microchannel used was supplied by Achira Laboratories. The inlet and outlet ports of the microchannel have a square cross section of 200 μm × 200 μm. The diverging and converging section of the channel had an angle and length (along the axis) of 45° and 0.9 mm, respectively. The straight section connecting the diverging and converging section has a dimension of 5 × 2 × 0.2 mm. Nitrogen gas is sent through a moisture trap and a mass flow controller MFC (Bronkhorst) to the central inlet (I2). The aqueous phase which is a solution of CTAB, BSA, and NaCl in DI water is sent (through the side inlets (I1, I3) using syringe pumps (Elite 11, Harvard Pump). The in-house designed foam separator is connected to the channel outlet where the residue is separated from the foamate phase. The flow was visualized using bright field microscopy (Axioskop 2 MAT, Carl Zeiss) with a 1.5× objective lens. The flow is recorded at 2000 fps using a high speed camera (NX-4, IDT). The images were analyzed to quantify the surface area, gas fraction, number of bubbles, and bubble size using the Image processing toolbox of MATLAB R2016b. The mass balance of solute in all experiments was verified to be accurate to within ±10% accuracy. All the experiments were carried out at room temperature in triplicates to ensure reproducibility. Figure 1 represents a schematic of the cross-junction microchannel used in our experiments. The channel consists of three sections as shown in Figure 1a. The first section shows the cross junction which establishes contact between the gas and aqueous phases which is used for generating slugs/ bubbles.19 In the second section the channel has a diverging zone followed by a straight section and a converging zone. This geometry helps in the generation of a foam-like structure in the device.19 The diverging and converging section in the channel is necessary for production of foam-like structure. When a stable foam is formed, the two phase foamate and the residual can be separated elegantly at the channel exit.20−22 Having a

3. RESULTS AND DISCUSSION We first discuss the hydrodynamics, i.e., different flow regimes or patterns observed as a function of the flow rate of the two phases. We then discuss their influence on the adsorption of BSA. 3.1. Hydrodynamics. The hydrodynamics of the flow through a cross junction was experimentally studied first in the absence of salt. The aqueous solution consists of 40 mg/L CTAB and 100 mg/L BSA in DI water at pH 7.4. Qaq was kept constant at 0.1 mL/min, and Qg was varied from 0.4 to 1.1 mL/min. Three layer flow and four layer flow were obtained at various gas flow rates. This is shown in Figure 2. In an N-layer flow pattern there is a row of N bubbles which are arranged in an orderly manner in the diverging−converging section. Our experiments revealed that there is a critical value of the gasflow rate above which the system exhibits a four layer flow for a fixed liquid flow rate. Figure 3a shows the variation in bubble size (BS) and number of bubbles (NB) with Qg. BS increases with Qg initially, and then upon further increase in Qg, it decreases. A decrease in the size of bubbles results in an increase in the number of bubbles produced in the microchannel. Therefore, when BS increases with Qg, NB decreases and upon further increase in Qg, BS decreases and hence NB increases. The variations in BS and NB affect gas fraction (GF) and total interfacial area (TSA) in the microchannel. Figure 3b depicts the variations of these quantities with Qg. The gas fraction is high (low) whenever the bubble size is high (low). C


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Figure 4. Flow patterns obtained at different Qg with salt at Qaq = 0.1 mL/min in a cross-flow junction. (a) Bubbly flow with coalescence at Qg = 0.46 mL/min (Supporting Information video S1). (b) Bamboo flow with coalescence at Qg = 4 mL/min (Supporting Information video S2).

Figure 2. Flow patterns obtained at different Qg without salt at Qaq= 0.1 mL/min in a cross-flow junction. (a) Three layer flow at Qg = 0.46 mL/min. (b) Four layer flow at Qg = 1.08 mL/min.

In the next set of experiments we consider the effect of salt on the hydrodynamics. The aqueous phase composition used is 40 mg/L of CTAB, 100 mg/L of BSA, and 0.1 mol/L of NaCl in DI water at pH 7.4. We observed different bubble flow patterns as a function of Qg for two Qaq (0.1 and 0.05 mL/ min). Qg (nitrogen) is varied in the range 0.4−5 mL/min for each liquid flow rate. Here the presence of salt results in two different flow patternsbubbly flow and bamboo flow at low and high Qg as shown in Figure 4 and Figure 5. These represent snapshots of the flow regimes in the videos in the Supporting Information. The nonuniform size of the bubbles is attributed to the coalescence in the channel which is induced by the presence of salt (refer to Table 1). According to the literature, for CTAB, common black film (CBF) is observed.27,28 CBFs follow DLVO theory which is based on electrostatic and van der Waals forces. The electrostatic repulsive forces act as stabilizers, and van der Waal attractive forces act as a destabilizer. The surfaces of the bubble are covered by positively charged surfactant. Hence the charges on these surfaces are similar, and the bubbles experience a repulsive force. The chloride ion from the salt screens the electrostatic repulsion between the bubble interfaces. A bond is formed between the chloride ion and CTAB present at the interface. The presence of counterions decreases the Debye screening length (κ−1) which reduces the repulsion between the bubbles. This decreases the film thickness and the disjoining pressure, enhancing the rate of coalescence.29 When the bubbles come close, the van der Waals attraction

Figure 5. Flow patterns obtained at different Qg with salt at Qaq = 0.05 mL/min in a cross-flow junction. (a) Bubbly flow with coalescence at Qg = 0.46 mL/min. (b) Bamboo flow with coalescence at Qg = 2.02 mL/min.

dominates and coalescence occurs. The significant deviation observed in the size of bubbles is attributed to the large fluctuations in bubble sizes due to coalescence. The flow patterns or hydrodynamics in the device were also analyzed by stopping the flow of aqueous phase from one of the symmetric inlets in the cross junction. This was analyzed for the case when salt is present in the aqueous phase. Nitrogen sent through inlet I2 and the aqueous phase was

Figure 3. Effect of Qg at Qaq = 0.1 mL/min on (a) BS and NB; (b) GF and TSA. D


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Table 1. Hydrodynamic Parameters for Different Operating Conditions When Qg Is Varied from 0.4 to 1.1 mL/min for Each Qaq [CTAB] = 40 mg/L, [BSA] = 100 mg/L parameters

[NaCl] = 0 mol/L

aqueous phase flow rate (Qaq) (mL/min) bubble size (μm) number of bubbles total surface area of bubble/volume of channel (*10−9 m−1) volume of bubble/volume of channel (*10−13)

0.1 200−350 20−50 0.1−9.89 4.99−5.51

pumped through inlet I1. The aqueous phase flow from inlet I3 was stopped. This reconfigures the channel junction from a cross junction to a T-junction. This mode of contacting two phases was found to drastically reduce bubble coalescence although the effective flow rates of the two phases were the same as in the cross junction. In this T-junction contacting, the pinching of gas phase occurs from one direction. We attribute this reduction in coalescence to the change in the inlet contact configuration: from cross junction to T-junction. Pinching of bubbles at the T-junction configuration results in a reduced screening effect of the chloride ion on the repulsion experienced by the gas−liquid interface. This causes electrostatic repulsion at the bubble interface to dominate, reducing the rate of coalescence. We next discuss the flow patterns observed at different gas flow rates for this case. Here the Qaq was kept constant at 0.1 mL/min, and Qg was varied from 0.4 to 1.1 mL/min. The flow pattern observed for two different Qg values is bubbly flow as portrayed in Figure 6. More details of the hydrodynamic parameters are listed in Table 1.

[NaCl] = 0.1 mol/L 0.1 200−450 10−30 8.10−9.06 4.86−5.93

0.05 200−1000 5−20 6.82−8.93 3.74−5.32

0.1 (T-junction configuration) 150−260 40−75 0.1−8.78 3.73−5.3

from 0.46 to 0.56 mL/min. It then increases upon increasing the Qg to 0.76 mL/min. Beyond Qg = 0.76 mL/min a sudden drop in ER was observed as shown in Figure 7a. Figure 7b shows the variation of FRT with respect to Qg. A high (low) residence time leads to high (low) ER. The ER and FRT follow the same trend of slight decrease followed by an increase and a sudden drop. On the other hand, the variation of FV and ER on gas flow rate shows an opposite trend. RR initially decreases when Qg changes from 0.46 to 0 .56 mL/min and then shows a continuous increase. At low Qg, RR decreases as ER also drops. However, RR keeps increasing with a further increase in Qg since FV increases. The maximum ER and RR obtained at Qg = 0.76 mL/min and Qaq = 0.1 mL/min are 12.74 and 91.27%, respectively. The average bubble size and number of bubbles produced are 264 ± 68 μm and 34 ± 2. 3.2.2. Foam Fractionation in Cross Flow in the Presence of Salt. The adsorption of BSA was studied in the presence of salt in aqueous phase for a fixed Qaq with varying Qg. The aqueous phase used had a composition of 40 mg/L of CTAB, 100 mg/L of BSA, and 0.1 mol/L of NaCl in DI water at pH 7.4. In the first set of experiments Qaq was maintained at 0.1 mL/ min, and Qg was varied from 0.4 to 1.1 mL/min. Figure 8a shows the effect of Qg on ER and RR under these conditions. ER initially increases when Qg increases from 0.46 to 0.56 mL/ min. It then decreases upon a further increase of Qg to 1.08 mL/min. In contrast to Figure 7a where in the absence of salt there is an initial dip in ER, we see that in the presence of salt there is a maximum in ER at Qg = 0.56 mL/min. This trend in ER closely follows the dependency of FRT on Qg (Figure 8b) as discussed before. Figure 8b shows the effect of Qg on FV and FRT. FV initially decreases when Qg changes from 0.46 to 0.56 mL/min and then increases upon a further increase in Qg to 1.08 mL/min. Whenever FV is low, i.e., at Qg = 0.56 mL/ min, the corresponding ER is high. The maximum ER obtained at Qg = 0.56 mL/min and Qaq= 0.1 mL/min is 5.74 and 24.36%, respectively. The average bubble size and number of bubbles in the microchannel for this condition are 329 ± 105 μm and 19 ± 5. Comparing Figure 7a and Figure 8a, we see that ER shows a local maximum at low Qg in the present of salt. This is attributed to the large increase in FRT seen in Figure 8b. To conclude, the addition of salt results in a maximum in the ER at low Qg. This is attributed to the sharp increase in FRT caused by diffusiophoresis. We discuss the physicochemical cause of this effect later in detail. Another set of experiments were carried out by varying Qg in the range from 0.4 to 1.1 mL/min with Qaq fixed at 0.05 mL/ min. Figure 9 shows the effect of Qg on ER, RR, FRT, and FV. We again see that the trends in residence time of bubbles and ER are similar. The maximum in FRT and ER is more pronounced for these operating conditions, and this shows that in the presence of salt there is a physicochemical mechanism in

Figure 6. Flow patterns obtained at different Qg with salt at Qaq = 0.1 mL/min in a T-junction configuration. Bubbly flow at (a) Qg = 0.46 mL/min, (b) Qg = 0.56 mL/min.

3.2. Foam Fractionation in Microchannel. The performance of the device developed for enrichment of solute is discussed in this section. The device was tested at different operating conditions, and the results are interpreted based on ER and RR (eqs 1 −2). 3.2.1. Foam Fractionation in Cross Flow in the Absence of Salt. The aqueous solution composition used is 40 mg/L CTAB and 100 mg/L BSA in DI water at pH 7.4. Qg was varied from 0.4 to 1.1 mL/min. Figure 7a shows the effect of Qg on ER and percentage RR in the absence of salt. The ER initially decreases from 10.85 to 9.27 when the Qg changes E


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Figure 7. Effect of Qg on (a) ER and percentage RR, (b) FRT and FV for the case of cross flow without salt at Qaq = 0.1 mL/min.

Figure 8. Effect of Qg on (a) ER and percentage RR, (b) FRT and FV in the presence of salt for a cross-flow inlet at Qaq = 0.1 mL/min.

Figure 9. Effect of Qg on (a) ER and percentage RR, (b) FRT and FV in the presence of salt for a cross-flow inlet at Qaq = 0.05 mL/min.

Figure 10. Effect of Qg on (a) ER and percentage RR, (b) FRT and FV in a T-junction inlet in the presence of salt at Qaq = 0.1 mL/min.

342 ± 122 μm and 19 ± 3. Here again at low Qg the FRT

place which is responsible for this behavior. RR does not show a dip here at low Qg as FV is almost constant at low gas flow rates. The maximum ER and RR obtained at Qg = 0.52 mL/ min and Qaq = 0.05 mL/min are 9.43 and 47.31%, respectively. The average bubble size and number of bubbles produced are

shows a strong peak resulting in a high ER and corresponding higher RR. This sharp peak is attributed to the presence of salt and BSA in the aqueous solution. F


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Industrial & Engineering Chemistry Research Table 2. Maximum ER Obtained for Different Channel Configuration flow configuration

salt

Qaq (mL/min)

Qg (mL/min)

ER

RR (%)

cross flow

absent present present present

0.1 0.1 0.05 0.1

0.76 0.54 0.52 0.56

12.74 5.74 9.43 10.78

91.27 24.36 47.31 31.84

T-junction

BS 264 329 342 204

± ± ± ±

NB 68 105 122 29

34 19 19 54

± ± ± ±

2 5 3 3

to be high for low gas flow rates. For case (a) the FRT decreases monotonically with Qg as expected. Here the aqueous phase contains only CTAB which is adsorbed on the bubble surface, rendering it positively charged. In case (b), at low Qg the bubbles spend significantly less time compared to case (a). However, for cases (c) and (d) the FRT exhibits maxima at a Qg of 0.54 and 0.76 mL/min, respectively. The aqueous solution now contains CTAB, BSA, and salt. This nonintuitive behavior in FRT indicates the existence of an additional force on the bubbles acting in a direction along or opposite to the flow. This force is dominant at low gas flow rates and arises due to diffusiophoresis.30 Diffusiophoresis is defined as transport of colloidal particles under the influence of a concentration gradient of an ionic salt even when there is no externally imposed electric field.30 This concentration gradient spontaneously induces an electric field in macroscale to ensure electroneutrality, and under its influence the particles or bubbles moves up or down the concentration gradient. The diffusiophoretic velocity comprises two components, i.e., electrophoresis and chemiphoresis (eqs 5−6).30 The chemiphoretic velocity (Uch) always causes particles to move from a high concentration to a low concentration region. The direction of movement due to electrophoresis (Uele) depends on the sign of zeta potential and the difference in diffusivity of ions making up the solute. In this work we consider the diffusiophoresis of the bubble and discuss its movement in the microchannel in the presence of surfactant and salt in an adsorption experiment. The diffusiophoretic velocity is given as

3.2.3. Foam Fractionation in T-Junction Configuration in the Presence of Salt. To achieve T-junction contacting between the two phases, the gas phase is pumped from inlet I2, and the aqueous phase is only pumped through inlet I1. Qg is varied from 0.4 to 1.1 mL/min and Qaq is kept constant at 0.1 mL/min. In Figure 10a, the effect of Qg on ER and RR is shown. In Figure 10b, the effect of Qg on FV and FRT is depicted. ER shows a sharp peak at low Qg. This peak corresponds to the Qg where the residence time of the foamate shows a maximum. For low Qg, percentage RR is constant as FV decreases and ER increases (Figure 10). For high Qg, the FV and ER show a decreasing trend, and hence the RR decreases. The maximum ER and RR obtained at Qg = 0.56 mL/min and Qaq = 0.1 mL/min are 10.78 and 31.84%, respectively. The average bubble size and number of bubbles produced are 204 ± 29 μm and 54 ± 3. We again observe that for this salt composition and this contacting there is a local peak in the FRT at low Qg. The maximum ER and corresponding RR obtained for different channel configurations are summarized in Table 2. 3.3. Foamate Residence Time (FRT). To better understand the different trends observed in the ER for different solute compositions, we depict the dependency of FRT on Qg in Figure 11 for the four different solute compositions: (a) 200

Uele =

εkT ∇C (βζp) ηZe C

2ε i kT y Uch = − jjj zzz η k Ze {

Udp = Uele + Uch β=

D+ − D− D+ + D−

ÄÅ É i Zeζp zyÑÑÑÑyzz ∇C jj ÅÅÅ zzÑÑzz jjlnÅÅ1 − tanh2jjjj jj ÅÅ j 4kT zzÑÑÑzz C k {ÑÖ{ k ÅÅÇ

2i

(5)

(6) (7)

(8)

where D+ and D− is diffusion coefficient of cation and anion, respectively. We now explain how the local maxima in the FRT can be explained using diffusiophoresis acting on the bubbles. Diffusiophoresis is induced by three factors: (i) a zeta potential on the bubble surface, (ii) difference in diffusivity of the ions in a salt, and (iii) a concentration gradient of salt and BSA. In case (b), the zeta potential (ζp) in bubbles is positive as CTAB molecules are preferentially adsorbed on the interface. The diffusivities of the sodium and chloride ions are different, i.e., NaCl, β = −0.27. The sign of ζpand β is such that the diffusiophoretic velocity (Udp) is in the same direction as that of bulk flow due to convection. This results in a lower FRT in the presence of salt; i.e., FRT for case (b) is lower than that of

Figure 11. Effect of Qg on FRT for different solute concentrations: (a) CTAB (200 mg/L), (b) CTAB (200 mg/L), NaCl (0.1 mol/L), (c) CTAB (40 mg/L), BSA (100 mg/L), and (d) CTAB (40 mg/L), BSA (100 mg/L), NaCl (0.1 mol/L).

mg/L of CTAB, (b) 200 mg/L of CTAB, 0.1 mol/L of NaCl, (c) 40 mg/L of CTAB, 100 mg/L of BSA, and (d) 40 mg/L of CTAB, 100 mg/L of BSA, 0.1 mol/L of NaCl. The higher concentration of CTAB, i.e., 200 mg/L in case (a) and (b), is necessary to ensure the formation of a stable foamate phase in the collector. At low concentrations of 40 mg/L of CTAB, the foamate phase was not stable. The FRT represents the time spent by the foam in the microchannel. The FRT is expected G


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Figure 12. Schematics showing solute concentration distribution in the system responsible for diffusiophoresis acting on bubbles when aqueous phase consists of (a) CTAB and NaCl, (b) CTAB, BSA, and NaCl.

case (a) as depicted in Figure 11. A schematic of forces acting on bubbles for this case is shown in Figure 12a. Here the CTAB molecules are adsorbed on the surface. The salt concentration is lower downstream of the bubble. This results in an electric field along the direction of convective flow. This acts on the bubble increasing its velocity and lowers the FRT. For case (c), again the zeta potential in bubbles is positive. The diffusivity of the negatively charge BSA is D− = 0.059 × 10−9 m2/s. It is much lower than that of H+ (D+ = 9.311 × 10−9 m2/s) resulting in β = 0.9874.30 A concentration gradient is generated by the enrichment of BSA as the fluid flows along the microchannel. The direction of the diffusiophoretic velocity on bubbles now is opposite to the bulk convective flow as Uele is negative. This is dominant at low gas flow rates. This renders Udp becoming negative, as shown in Figure 13.

Figure 13. Schematic showing forces causing diffusiophoresis acting on bubble.

Figure 14. Various foam fractionation experiments carried out in microchannel.

Case (d) is similar to case (c), but now we have multi ion diffusiophoresis31 (Figure 12b) . The diffusion coefficient of the different ions follows the order DH+ > DCl− > DNa+ > DBSA. Due to their higher diffusion coefficient, H+ ions move faster, and negatively charged BSA lags behind them. This causes a spontaneous electric field to be generated to maintain electroneutrality. The direction of the electric field is such that the faster moving H+ ions have to slow down; i.e., it acts in a direction against the bulk flow. This causes the increase in FRT at low Qg which manifests as distinct maxima. The magnitude of diffusiophoretic velocity is high when the concentration gradient along the length of the channel is high. This high concentration gradient occurs for higher enrichment on bubbles. At higher Qg however the residence time decreases as the diffusophoretic effect is dominated by convection. These effects result in a local maximum in FRT at low gas flow rates and a corresponding high enrichment ratio (Figure 7, Figure 8, Figure 9, and Figure 10). Figure 14 shows the maximum ER and corresponding RR obtained for the four sets of adsorption experiments carried out in the microchannel. In the absence of salt the maximum ER and percentage RR obtained at Qaq = 0.1 mL/min and Qg = 0.76 mL/min are 12.74 and 91.27%, respectively. The bubble size and the number of bubbles produced varied from 200 to

350 μm and 20 to 50. The coalescence was minimal when the aqueous phase had no salt. The second set of adsorption experiments were carried out at Qaq = 0.1 mL/min in the presence of salt. The bubble size and the number of bubbles produced varied from 200 to 450 μm and 10 to 30. The maximum ER and percentage RR obtained at a Qg = 0.54 mL/ min are 5.74 and 24.36%, respectively. ER decreases as the rate of coalescence observed is high here. When two bubbles coalesce, the gas−liquid interface available for adsorption decreases which reduces the surface area available for adsorption and hence the efficiency of adsorption. The coalescence resulted in a larger bubbles size and lower number of bubbles. This decreases the overall surface area available for adsorption and reduces ER drastically. To further enhance the efficiency, the FRT and the surface area have to be increased and coalescence has to be reduced. For this a third set of adsorption experiments at Qaq = 0.05 mL/min were performed which increases the FRT. The bubble size and the number of bubbles produced varied from 200 to 1000 μm and 5 to 20 respectively. The adsorption in the system increased because of a larger FRT. The maximum ER and RR obtained were 9.43 and 47.31% at Qg = 0.52 mL/min. In the last set of adsorption experiments, a T-junction configuration was employed to H



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optimize the performance. The bubble size and the number of bubbles produced varied from 150 to 260 μm and 40 to 75. This configuration reduces coalescence and increases the surface area drastically. The maximum ER and RR here obtained at Qaq = 0.1 mL/min and Qg = 0.56 mL/min are 10.78 and 31.84%. Our systematic analysis in a microfluidic device has given us insights into the role of salt on BSA adsorption when CTAB is used as the surfactant.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Pushpavanam Subramaniam: 0000-0002-5782-0670 Notes

The authors declare no competing financial interest.

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4. CONCLUSIONS This work describes the design of a novel microfluidic device for separation and concentration of a surface active solute. The geometry of the channel was modified to a diverging and converging cross section. Foam fractionation in this microchannel was demonstrated using BSA as model compound. The proposed design has been successfully demonstrated for the adsorption of solute and separation of two phases at the channel exit. The role of salt on BSA adsorption has been physically interpreted. For foam fractionation in cross flow without salt, the maximum enrichment was obtained. Here the rate of coalescence was minimal, and the surface area available for adsorption was reasonable compared to other adsorption experiments in microchannel. The variation of enrichment ratio with gas flow rate is always in phase with that of foamate residence time. The recovery ratio was found to be dependent on both foamate volume and enrichment ratio. In our experiments we have observed a local maximum in FRT and enrichment at an intermediate Qg, and this is attributed to diffusiophoresis. In applications where the objective is to increase the sensitivity of a sensor, it is desirable to increase the ER since we want to have a larger concentration in the foamate phase. If however the objective is to purify a wastewater stream, the importance is on removing the contaminant. Here the RR has to be optimized. The functioning of this device was demonstrated by carrying out adsorption of solute and separation of two phases simultaneously. The stability studies of biomolecules separated using the device developed has to be carried out. Cyclodextrin can be used to reduce the noncooperative interaction between CTAB and BSA in the foamate phase. This can then help recover the protein from the enriched foamate phase.32,33 The device can be used for separation of molecules which are surface active or can form a complex with surface active molecules and can also help in increasing the sensitivity of sensors. In this work we have shown how we can infer the possible existence of diffusiophoresis using a macroscopic measurement like foamate residence time in a microchannel. While earlier studies have focused on showing diffusiophoresis using carefully planned experiments, this work shows its implications on adsorption in a continuous operation.

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Article

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.8b01724. S1: Bubbly flow with coalescence at Qg = 0.46 mL/min and Qaq = 0.1 mL/min (MPG) S2: Bamboo flow with coalescence at Qg = 4 mL/min and Qaq = 0.1 mL/min (MPG) I


Article

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1 An integrated micro-fluidic device for continuous separation and pre

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