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Droplet demulsification using ultra-low voltage based electrocoalescence ABHISHEK SRIVASTAVA, S. Karthick, K.S. Jayaprakash, and Ashis K. Sen Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03323 • Publication Date (Web): 13 Dec 2017 Downloaded from http://pubs.acs.org on January 8, 2018
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Droplet demulsification using ultra-low voltage based electrocoalescence A. Srivastava, S. Karthick, K. S. Jayaprakash, A. K. Sen* Department of Mechanical Engineering, Indian Institute of Technology Madras, Chennai-600036, India *
Author to whom correspondence should be addressed. Email:
[email protected] ABSTRACT Demulsification of droplets stabilized with surfactant is very challenging due to their low surface energy. Here, we report ultra-low voltage based electrocoalescence phenomenon for the demulsification of aqueous droplets with an aqueous stream. In the absence of electric field, due to the disjoining pressure resulting from the tail-tail interaction between the surfactant molecules present on the aqueous droplets and interface, coalescence of aqueous droplets with the aqueous stream is prevented. However, above a critical electric field, the electrical stress overcomes the disjoining pressure thus leading to the droplet coalescence. The influence of surfactant concentration, droplet diameter and velocity on the electrocoalescence phenomena is studied. The macroscopic contact between the aqueous droplet with the aqueous stream enables droplet coalescence at much lower voltage (10 to 90 V), which is at least two orders of magnitude smaller than voltages used in prior works (1.0 to 3.0 kV). The electrocoalescence phenomena is used for the extraction of microparticles encapsulated in aqueous droplets into the aqueous stream and size-based selective demulsification. A new paradigm of droplet electrocoalescence and content extraction is presented that would find significant applications in chemistry and biology. INTRODUCTION Droplets are indispensable for various applications such as chemical and biological research1–3, single cell analysis4,5 and fabrication of novel microstructures6. In most cases, the aqueous droplets are stabilized using surfactants in the oil phase7. When two droplets of the same liquid are brought in contact with each other, the general assumption is that the droplets would coalesce since the surface energy of the merged droplet would be less than the sum of the surface energies of individual droplets8. However, droplets stabilized by surfactants avoid microscopic contact with each other due to the presence of a surfactant monolayer around the surface of each droplet9. Such stabilized droplets present in a secondary immiscible phase are known as emulsion10. The breaking of emulsion into individual phases i.e. demulsification, is an important phenomenon which finds varied applications including inkjet printing11, water separation from crude oil12, and droplet content extraction in microfluidics13. There are several demulsification techniques available such as chemical demulsification13, centrifugal settling14, heat treatment15, and electrocoalescence16. Among these, electrocoalescence phenomenon is widely used because of its higher efficiency in the breaking of the waterin-oil emulsions17. The efficiency is proportional to the electric field strength at relatively lower magnitudes of the electric fields and varies as the square of the electric field strength at higher magnitudes of the electric fields. The characterization of the stability of a pair of drop in an AC (alternating current) electric field as a function of the separation distance between the drops showed that there exists a critical separation distance beyond which the coalescence is independent of the AC electric field16. In various biochemical applications4, water–in–oil emulsions (with surfactant for emulsion stabilization7) are generated to encapsulate microparticles inside the droplets. Subsequently, the micro particles18 encapsulated in droplets need to be extracted into an aqueous phase for downstream analytical study19. Most of the methods currently available for the extraction of microparticles from emulsion make use of chemicals (e.g. perfluorooctanol) to break the emulsion stability. As a suitable alternative, droplets can be rapidly manipulated using electrical energy in which external electric field can be employed to coalesce and sort droplets16,20–23. Extraction of the microparticles initially present inside an aqueous droplet into an aqueous phase using electric field perpendicular to the flow direction have been reported21,24. However, a very high voltage (~ 1.5 kV) and consequently electric field in the range of 106 to 107 V/m was used. Surfactant mediated extraction of organic compounds such as phenol and p-nitrophenol from an aqueous phase to hexane is also reported25. The droplets (200 to 547 µm size) are coalesced to form slugs using electric voltage (6 to 12 V). But, the device does not provide complete phase separation on-chip. Further, the slugs (of discrete phase) along with the continuous phase are taken off the chip in a vial for gravity-based separation. Here, we report an electrocoalescence mechanism that is capable of complete demulsification, size based selective demulsification of droplets and extraction of encapsulated microparticles into an aqueous phase using much lower voltage (10 to 90 V) on-chip. In this approach, stable droplets flowing in an oil stream are
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in macroscopic contact with a parallel stream of aqueous phase and under the influence of electric field, the aqueous droplets coalesce with the aqueous stream. Moreover, in literature, complete demulsification and size-based selective demulsification of droplets has not been attempted yet which is demonstrated in this work. First, we present a description of the device and depict the electrocoalescence mechanism for demulsification. Then, materials and method used in the experiments are outlined. Finally, experimental results are presented and discussed. THEORY A schematic of the device for electrocoalescence based droplet demulsification is depicted in Fig. 1a. It comprises two inlet channels (inlet 1 and 2) leading to a wider main channel. Emulsions of aqueous droplets (as the discrete phase (DP)) present in an oil (containing surfactant for droplet stabilization) as primary continuous phase (CP1) is infused into inlet 1 and aqueous phase (DI water) is infused into inlet 2 as secondary continuous phase (CP2) thus forming a planar water–oil interface in the expanded channel26,27. A pair of electrodes is located at some location along the expanded channel. In the absence of electric field, the aqueous droplets, that are stabilized with a surfactant monolayer, do not coalesce with the aqueous phase. However, when electric field is turned on, the electric force can overcome the disjoining pressure thus coalescence of the aqueous droplet with the aqueous stream takes place. In case of aqueous droplets containing microparticles, when the droplets get coalesced with the CP2, the microparticles are extracted into the secondary aqueous phase (Fig. 1b). In case of an emulsion containing droplets of different sizes, the interface location can be adjusted by controlling the flow rates of the CP1 and CP2 such that only the larger droplets are in contact with the interface get coalesced thus enabling size based selective demulsification (Fig. 1c). The underlying mechanism of the coalescence phenomena is described as follows. Two droplets of the same phase that are not stabilized by surfactant would coalescence when brought in contact8. However, when the two droplets (of the same phase present in another immiscible phase) stabilized by surfactant come into contact with each other, coalescence is prevented due to the repulsive stress created by the surfactant molecules, which is governed by the disjoining pressure. In order to coalesce the stabilized droplets that are in physical contact and under the influence of electric field, the Maxwell stress (due to the external electric field) needs to overcome the disjoining pressure (Fig. 1d-i). On the other hand, in order to coalesce stabilized droplets that are some distance away from each other, the Maxwell stress needs to first deform the droplets against the interfacial tension (IFT) so that the droplets come in contact and then overcome the disjoining pressure for coalescence to occur (Fig. 1d-ii). The Maxwell stress tensor is given as = −
(1)
where is the electric field, is the permittivity of free space and
is the identity tensor. The electric field required to deform the droplets against interfacial tension is very high compared to the electric field required to overcome the disjoining pressure28. The above explanation is equally valid for droplet – planar interface (i.e. coalescence of an aqueous droplet with an aqueous stream). In literature24, coalescence of two droplets as well as coalescence of a droplet with a liquid stream that are not in physical (macroscopic) contact have been studied that require much higher electric fields (~107 V/m) and voltage ~kV. Here, the droplets that need to be coalesced with the liquid stream are brought in contact with the aqueous stream by adjusting the location of the interface by controlling the flow rate of the aqueous phase in channel 2. Thus, in this case, the droplet coalescence can be achieved with a much lower electric field (~104 to 105 V/m) and voltages ~tens of volts. In the present case (Fig. 1a), the surfactant molecules accumulate over the droplet surface and the oil – aqueous phase planar interface thus preventing the microscopic contact of the droplet with the interface due to the repulsive force caused by the surfactant molecules. This repulsive force can be due to the tail-tail repulsion in case of non-ionic surfactants or ion– ion repulsion in case of ionic surfactants. The ion–ion repulsive forces are long-ranged while the non-ionic (tail–tail) repulsive forces are short-ranged. In this work, we have used a non–ionic surfactant. The interactive force which counters the repulsive force is the van der wall stress, which is given by = −
(2)
where and ℎ denote the Hamaker constant and thickness of the oil film between the aqueous droplet and the aqueous stream, respectively. The summation of the repulsive and the attractive stresses is known as the disjoining pressure ℎ which is function of the thickness ℎ of the thin film between the droplets (or droplet and planar aqueous phase) as
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ℎ =
!"#$% +
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Previous studies28 suggests that two droplets or a droplet and a liquid stream in contact would coalesce when the applied voltage is higher than a critical voltage such that the electrical stress overcomes the disjoining pressure. Moreover, when two droplets or a droplet and a liquid stream approach each other and are in contact macroscopically, the thin film of oil between the two needs to drain completely before the tail–tail or ion–ion interaction due to surfactant molecules could take place28. Thus, in addition to the disjoining pressure, the electrical stress also need to overcome the viscous dissipation due to the drainage of the thin film in between the two aqueous phases (i.e. aqueous droplet and aqueous stream).
Fig. 1 (a) Schematic showing microfluidic device for electrocoalescence based droplet demulsification (b) Extraction of microparticles from aqueous droplets into an aqueous stream (c) Size based selective demulsification of aqueous droplets (d-i) Droplet and interface in contact (d-ii) droplet and interface located at some distance from each other.
RESULTS AND DISCUSSIONS The details of the device fabrication, materials and methods are reported in the Supporting Information. Experiments were performed to demonstrate electrocoalescence of DI water droplets generated in mineral oil containing 4% Span 85 with a DI water stream. The droplets were generated using a T-junction upstream of the inlet 1. The flow rates of the mineral oil as the primary continuous phase (CP1), DI water as the discrete phase (DP) and DI water stream as the secondary continuous phase (CP2) were &'() ⁄&'(* = 0.37, &0( = 1.17 μl/min, respectively. Sinusoidal AC voltage of 5 V was applied (at 80 kHz) to give rise to an electric field of 2×104 V/m. Fig. 2a shows the dynamics of the electrocoalescence of a DI water droplet of size 145 µm at various time instants (also see Video S1 in the Supporting Information). As observed, the droplet is spherical in shape before it enters the expanded channel region. In the expanded region, since the width of the CP1 (mineral oil) is ~115 µm (which is less than the diameter of the droplet), the water droplet comes in macroscopic contact with the water-oil interface. However, due to the tail-tail repulsion between the surfactant molecules, the resulting disjoining pressure prevents coalescence of the droplet with the aqueous stream. As observed, the disjoining pressure leads to the stretching of the droplet and deformation of the interface. When the droplet enters the high electric field region around the electrodes (simulation results in Fig. 2b), the electrical stress competes with the disjoining pressure and if the electric field is above a critical value, the droplet coalesces. As soon as the droplet arrives at the high electric field region, the droplet coalesces almost instantaneously within @
This explains why the critical electric field required for droplet coalescence is higher at higher surfactant concentrations. To study the effect of droplet size on the critical voltage required for coalescence, initially, the flow rates of CP1 and CP2 are adjusted such that the smallest droplet taken for the study (76.8 µm) is in macroscopic contact with the planar CP1-CP2 interface. Then, the flow rate ratio of the continuous phases are kept fixed (&'( ⁄&'( = 0.22) and the droplet size is varied by changing the discrete phase flow rates (in the range &0( = 1.66 CD 2.45 μG/HIJ). For a fixed surfactant concentration (3%, 4% or 5%) and flow rate ratio of CP1 and CP2, with increase in the droplet diameter the critical voltage required for coalescence decreases, as shown in Fig. 3a. In case of a larger droplet, the thickness of the thin film of oil between the droplet and the interface would be much smaller as compared to that in case of a smaller droplet. Thus the electric field required for complete drainage of the thin film (before it overcomes the disjoining pressure due to surfactant molecules) would be smaller in case of a larger droplet, which explains why a smaller electric field is adequate for the coalescence of larger droplets. Effect of droplet velocity on droplet coalescence To study the effect of residence time (i.e. time spent by droplets within the electric field region) while keeping the droplet diameter fixed, the flow rates of the continuous phases (mineral oil CP1 and aqueous stream CP2) and discrete phase (aqueous phase DP) were simultaneously varied in order to only vary the velocity of the droplets. Experiments were performed with droplets of two different sizes of 30 µm and 40 µm and for each droplet size, the effect of droplet velocity on the critical electric field required for coalescence was studied and the results are depicted in Fig. 3b. It is observed that as the droplet velocity is increased, the critical electric field required for droplet coalescence is higher. The critical electric field which leads to the coalescence of slower droplets is not adequate to coalesce the faster droplets of the same size. This is because; at higher droplet velocity, the residence time of a droplet in the electric field region becomes smaller, and the droplet does not spend enough time for the complete drainage of the thin film between the droplet and interface. At a higher droplet velocity, the flow capillary number KL = MN OL/OP increases due to which the coalescence time also increases26. Here, µm is the viscosity of continuous phase CP1, O is the interfacial tension of droplet – CP1 interface, L is the droplet radius and OP is the shear rate. However, in order to coalesce the faster moving droplets within this short residence time available, the droplet electrocapillary number KLQ = RN L /O needs to be higher26. Here, εm is the permittivity of continuous phase CP1 and E0 is the applied electric field. The droplet electrocapillary number is directly proportional to the electric field, which explains why the critical electric field required for the coalescence of the droplets is higher in case of the faster moving droplets. For a fixed droplet velocity, the critical electric field required for droplet coalescence is higher for a larger droplet, as discussed in the previous section. (a)
(b) 4.5 30 µm droplet 40 µm droplet
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the range &0( = 1.66 CD 2.45 μG/HIJ. (b) The variation of electric field with droplet velocity for droplets of two different sizes (30 µm and 40 µm), surfactant concentration 4%. Extraction of microparticles from aqueous droplet to aqueous stream In this section, the extraction of cells from aqueous droplet (DP) into the aqueous liquid stream (CP2) is demonstrated (Fig. 1b). DMEM (Dulbecco's Modified Eagle's medium) suspended with HeLa cells was used as the discrete phase and mineral oil with 4% Span 85 was used as the CP1. Phosphate Buffered Saline (PBS) was used as the CP2. Droplets encapsulating HeLa cells were generated in CP1 at a T-junction upstream. In the absence of electric field, when the droplets encapsulating the cells come in macroscopic contact with the mineral oil-PBS interface, however, due to the disjoining pressure, the droplets do not coalesce. Now, if we apply an electric field above a critical value, which is a function of the droplet diameter and velocity and surfactant concentration, the droplets coalesce with the CP2. For a surfactant concentration 5%, droplet diameter 90 µm and velocity 2.14 mm/s and electric field 7.6×104 V/m, the experimental images showing coalescence of DMEM droplets containing HeLa cells at different time instants are depicted in Fig. 4. As observed, when a droplet coalesces with the CP2, the cell encapsulated inside the droplet enters into the CP2 that can be collected at the CP2 outlet thus enabling a technique for extraction of cells from oil phase into aqueous phase. Next, cell viability test was performed to determine the effect of electric field on the viability of cells. The cell suspensions (HeLa and DU145 cells at concentration 7.9×105 and 5×105 cells per ml respectively) were infused through the microchannel device at a flow rate of 5µl/min and the cells were subjected to electric field in the range 0 to 6×105 V/m by applying voltage in the range of 0 to 150 V. At each electric field, the cells were collected at the device outlet and the cell viability was measured using Trypan blue test and a haemocytometer using the procedure detailed in the Supporting Information. The cell viability without application of electric field was also determined to account for the other effects (such as fluid shear) and find out solely the effect of electric field on the cell viability. The variation in the %cell viability with electric field is presented in Fig. 5a. It is observed that within the range of electric fields used, there is no considerable variation in the cell viability with increase in the electric field. The cell viability was found to be >97% which indicates minimal effect of electric field on cell viability. Thus the device can be safely used for the extraction of cells encapsulated in droplets into an aqueous stream. The effect of flow rate (fluid shear) of the cell sample on the cell viability was determined. HeLa and DU145 cells at concentration of 7.9×105 and 5×105 cells per ml respectively were infused at flow rate in the range 5.0 to 20.0 µl/min at a fixed electric field of 4.2×105 V/m (i.e. 105 V). At each flow rate, the cells were collected at the device outlet and the cell viability was measured. The cell viability without application of electric field was also determined to find out solely the effect of fluid shear on the cell viability. Fig. 5b shows the variation in the %cell viability with flow rate and it is observed that %cell viability decreases with increase in flow rate (>99% at 5.0 µl/min to ~94% at 20.0 µl/min). The reduced cell viability with increase in flow rate can be attributed to the hydrodynamic shear acting on the cell surface30. The results show that the device need to operate at a continuous flow rate below 18 µl/min to obtain cell viability >96%.
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Fig. 4 Experimental results showing electro-coalescence of a cell-encapsulating droplet at different time instants, before coalescence the cell is encapsulated inside droplet (DP), after coalescence the cell in in the aqueous stream CP2, droplet diameter 90 µm and velocity 2.19 mm/s, 4% surfactant, =7.6×104 V/m. (b) 100
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Fig. 5 (a) The variation in the %cell viability with electric field, flow rate of 5µl/min (b) The variation in the %cell viability with flow rate, electric field of 4.2×105 V/m (i.e. 150 V). Size based selective droplet demulsification The electrocoalescence phenomena was then used for size based selective demulsification of droplets (Fig. 1c). Droplets of two different diameters, 37 µm and 42 µm, were generated at two different T-junctions at the upstream of the inlet 1. The flow rate ratio of CP1 and CP2 was adjusted such that the 42 µm droplet was in macroscopic contact with the planar CP1CP2 interface whereas the 37 µm droplet did not make contact (within the electric field region). At a droplet velocity of 9.81 mm/s for 37 µm and 8.36 mm/s for 42 µm, surfactant concentration of 4% in CP1, when the corresponding critical electric field of 2.1×105 V/m for the 42 µm droplet was applied, the larger 42 µm droplets coalesced while the smaller 37 µm droplets continued to flow along CP1. Experimental images showing size based selective demulsification of droplets at few time instants is depicted in Fig. 6. With the proposed design, droplets of above a critical size can be selectively demulsified by appropriately varying the flow rate ratio of the continuous phases. Similarly, in case of an emulsion containing droplets of a range of sizes, the technique can be used to demulsify droplets of larger sizes with CP2 and obtain smaller droplets in the CP1. It is to be noted that if the size of the droplets is equal or larger than the width of the oil stream, the droplets would always be in contact with the interface and get coalesced with the aqueous stream as soon as it enters the high electric field region close to the electrodes. However, if the size of the droplets is smaller than the width of the oil stream, such smaller droplets are not in contact with the interface initially. However, due to small Reynolds numbers, the droplets are subjected to noninertial lift force, and may eventually come in contact with the interface. However, with the given design and operating
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conditions used in the present work, we estimated that the lateral migration of droplets (of different sizes) before arriving at the high electric field region i.e. over the residence time available for the droplets, is insignificant. Thus, the only the droplets whose size is equal or larger than the width of the oil stream get coalesced while the droplets that smaller than the width of the oil stream do not get coalesced.
Fig. 6 Experimental images showing size based selective demulsification of droplets at few time instants, two different sized droplets 37 µm and 42 µm, the larger (42 µm) droplets coalesce with the continuous aqueous phase, the (37 µm) smaller droplets moves past the electric field in CP2, surfactant concentration in CP1 4%. CONCLUSIONS In this work, we presented demulsification of non-ionic surfactant-stabilized droplets in a microchannel using electric field. In the absence of electric field, the coalescence of the aqueous droplet with the aqueous stream is prevented due to the disjoining pressure resulting from the tail-tail interaction between the surfactant molecules present on the droplet and the interface. When the electric field is above a critical value, the electric stress overcomes the disjoining pressure leading to the coalescence of the droplet with the aqueous stream. In addition, Marangoni stress may also have an effect on the droplet electro-coalescence process, however, estimation of Marangoni stress in the present case is extremely challenging since the interfacial tension changes dynamically. Effect of the surfactant concentration, droplet size and velocity on the critical electric field required for the coalescence was studied. It is to be noted that if the surfactant concentration is smaller than the CMC, then the droplets coalesce with the continuous aqueous phase even in the absence of the electric field, since in that case the dynamic interfacial tension is high. For a fixed droplet diameter and velocity, as the surfactant concentration increases, the critical electric field required for the droplet coalescence increases due to increase in the disjoining pressure with surfactant concentration. For a fixed surfactant concentration and droplet velocity, the critical electric field is lower for a larger diameter droplet due to the smaller thickness of the thin film between the droplet and the interface as compared to that for a smaller droplet. Similarly, for a fixed surfactant concentration and droplet size, as the droplet velocity is increased, the critical electric field required for droplet coalescence is higher. The proposed technique enabled extraction of biological cells at an electric field ~104 to 105 V/m, which significantly lower than the electric fields used in the works reported in the literature. The technique was also applied to the size based electrocoalescence of aqueous droplets by adjusting the location of the interface. The droplets of size larger than the width of the CP1 stream contact the interface and get coalesced while the smaller droplets continue to move in CP1 thus enabling sorting of droplets. In conclusion, a new paradigm of droplet electrocoalescence and content extraction is presented that would find significant applications in biology and chemistry. SUPPORTING INFORMATION Video S1 – dynamics of the electrocoalescence of a DI water droplet of size 145 µm at various time instants. AUTHOR INFORMATION
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Corresponding Author: Dr. Ashis Kumar Sen Email:
[email protected] Notes: The authors declare no competing for financial interest.
ACKNOWLEDGEMENTS This work was supported by the Science and Engineering Research Board (SERB), Department of Science and Technology (DST), India via grant no. EMR/2014/001151 and IIT Madras via project no. MEE1516843RFTPASHS. The authors acknowledge the CNNP, IIT Madras for supporting the photolithography work. The authors cordially thank the Interdisciplinary Program at IIT Madras which enabled this work. PATENTS/INTELLECTUAL PROPERTY Patent titled “MicroFACS for detection and isolation of target cells”, with IITM ICSR IDF number 1517, and Application no. 201741012180 filed on April 05, 2017. REFERENCES (1) Theberge, A. B.; Courtois, F.; Schaerli, Y.; Fischlechner, M.; Abell, C.; Hollfelder, F.; Huck, W. T. S. Microdroplets in Microfluidics: An Evolving Platform for Discoveries in Chemistry and Biology. Angew. Chemie Int. Ed. 2010, 49 (34), 5846–5868. (2) Nayak, R.; Liu, J.; Sen, A. K.; Knapp, D. R. Dual Desorption Electrospray Ionization-Laser Desorption Ionization Mass Spectrometry on a Common Nanoporous Alumina Platform for Enhanced Shotgun Proteomic Analysis. Anal. Chem. 2008, 80 (22), 8840–8844. (3) Sen, A. K.; Nayak, R.; Darabi, J.; Knapp, D. R. Use of Nanoporous Alumina Surface for Desorption Electrospray Ionization Mass Spectrometry in Proteomic Analysis. Biomed. Microdevices 2008, 10 (4), 531–538. (4) Joensson, H. N.; Andersson Svahn, H. Droplet Microfluidics-A Tool for Single-Cell Analysis. Angew. Chemie - Int. Ed. 2012, 51 (49), 12176–12192. (5) Sajeesh, P.; Manasi, S.; Doble, M.; Sen, A. K. A Microfluidic Device with Focusing and Spacing Control for Resistance-Based Sorting of Droplets and Cells. Lab Chip 2015, 15 (18), 3738–3748. (6) Xu, S.; Nie, Z.; Seo, M.; Lewis, P.; Kumacheva, E.; Stone, H. A.; Garstecki, P.; Weibel, D. B.; Gitlin, I.; Whitesides, G. M. Generation of Monodisperse Particles by Using Microfluidics: Control over Size, Shape, and Composition. Angew. Chemie - Int. Ed. 2005, 44 (5), 724–728. (7) Mazutis, L.; Gilbert, J.; Ung, W. L.; Weitz, D. A.; Griffiths, A. D.; Heyman, J. A. Single-Cell Analysis and Sorting Using Droplet-Based Microfluidics. Nat. Protoc. 2013, 8 (5), 870–891. (8) de Gennes, P.-G.; Brochard-Wyart, F.; Quéré, D. Capillarity and Wetting Phenomena. Capillarity and Wetting Phenemona: Drops, Bubbles, Pearls, and Waves 2004, 292. (9) Xu, B.; Nguyen, N.-T.; Neng Wong, T. Droplet Coalescence in Microfluidic Systems. Micro Nanosyst. 2011, 3 (2), 131–136. (10) Bibette, J.; Leal-Calderon, F. Surfactant-Stabilized Emulsions. Curr. Opin. Colloid Interface Sci. 1996, v. 1, 746– 751. (11) Stringer, J.; Derby, B. Limits to Feature Size and Resolution in Ink Jet Printing. J. Eur. Ceram. Soc. 2009, 29 (5), 913–918. (12) Mhatre, S.; Vivacqua, V.; Ghadiri, M.; Abdullah, A. M.; Al-Marri, M. J.; Hassanpour, A.; Hewakandamby, B.; Azzopardi, B.; Kermani, B. Electrostatic Phase Separation: A Review. Chem. Eng. Res. Des. 2015, 96, 177–195. (13) Chen, T. Y.; Mohammed, R. A.; Bailey, A. I.; Luckham, P. F.; Taylor, S. E. Dewatering of Crude Oil Emulsions 4. Emulsion Resolution by the Application of an Electric Field. Colloids Surfaces A Physicochem. Eng. Asp. 1994, 83 (3), 273–284. (14) Cambiella, A.; Benito, J. M.; Pazos, C.; Coca, J.; Saether, Ø.; Sjöblom, J.; Coca, J. Centrifugal Separation Efficiency in the Treatment of Waste Emulsified Oils. Chem. Eng. Res. Des. 2006, 84 (1), 69–76. (15) Hempoonsert, J.; Tansel, B.; Laha, S. Effect of Temperature and pH on Droplet Aggregation and Phase Separation Characteristics of Flocs Formed in Oil-Water Emulsions after Coagulation. Colloids Surfaces A Physicochem. Eng. Asp. 2010, 353 (1), 37–42. (16) Thiam, A. R.; Bremond, N.; Bibette, J. Breaking of an Emulsion under an Ac Electric Field. Phys. Rev. Lett. 2009, 102 (18), 1–4. (17) Owe Berg, T. G.; Fernish, G. C.; Gaukler, T. A. The Mechanism of Coalescence of Liquid Drops. Journal of the Atmospheric Sciences. 1963, pp 153–158. (18) Velasco, D.; Tumarkin, E.; Kumacheva, E. Microfluidic Encapsulation of Cells in Polymer Microgels. Small 2012, 8 (11), 1633–1642. (19) Pan, J.; Stephenson, A. L.; Kazamia, E.; Huck, W. T.; Dennis, J. S.; Smith, A. G.; Abell, C. Quantitative Tracking
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Electrodes (AC supply)
Inlet 1
Mineral oil
Main channel
Electrocoalescence
Inlet 2
Surfactant molecules
DI water
(a) Demulsification at oil-aqueous interface based on electrocoalescence
IFT
(i)
Electrocoalescence Electrocoalescence (c) Size based selective (b) Extraction of microparticles from demulsification droplets into an aqueous stream
(ii)
(d-i) Droplet and interface in contact (d-ii) droplet and interface some distance away
Fig. 1 (a) Schematic showing microfluidic device for electrocoalescence based droplet demulsification (b) Extraction of microparticles from aqueous droplets into an aqueous stream (c) Size based selective demulsification of aqueous droplets (di) Droplet and interface in contact (d-ii) droplet and interface located at some distance from each other.
(a)
Electrode
t=0 s
t=0.518 s
t=0.236 s Coalescence
t=0.552 s
t=0.554 s
t=0.556 s
t=0.560 s
t=0.564 s
t=0.616 s
(b) Droplet High electric field in the thin film region Interface
Electric field: surface
Electric field: surface (zoomed view)
Fig. 2 (a) Dynamics of the electrocoalescence of a DI water droplet of size 145 µm at various time instants, flow rates of the mineral oil as the primary continuous phase (CP1), DI water as the discrete phase (DP) and DI water stream as the secondary continuous phase (CP2) were 𝑄𝐶𝑃1 ⁄𝑄𝐶𝑃2 = 0.37, 𝑄𝐷𝑃 = 1.17 µl/min, respectively, sinusoidal AC voltage of 5 V was applied (at 80 kHz) to give rise to an electric field of 2×104 V/m (b) Simulation results showing high electric field in the thin film between the droplet and interface when the droplet is located near the electrode region.
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(a)
(b) 4.5 30 µm droplet 40 µm droplet
1.6
Electric field(x105 V/m)
Electric field(x105 V/m)
4.0 1.2
0.8
0.4
0.0
Mineral oil, 5% Span 85 Mineral oil, 4% Span 85 Mineral oil, 3% Span 85
75
80
85 90 95 Droplet diameter(m)
3.5 3.0 2.5 2.0 1.5
100
2
3
4
5
6
7
8
9
10
Droplet velocity (mm/s)
Fig. 3 (a) Variation in the critical electric field required for droplet coalescence with droplet diameter and surfactant concentration in CP1, flow rate ratio of the continuous phases 𝑄𝐶𝑃1 ⁄𝑄𝐶𝑃2 = 0.22 and discrete phase flow rate is varied in the range 𝑄𝐷𝑃 = 1.66 𝑡𝑜 2.45 µ𝑙/𝑚𝑖𝑛. (b) The variation of electric field with droplet velocity for droplets of two different sizes (30 µm and 40 µm), surfactant concentration 4%. Cell encapsulating droplet
CP1 Electrode CP2
Cell in DP
t=0 s
Cell Electrocoalescence
t=0.027 s
Cell in CP 2 t=0.051 s
(a) 100
(b) 100 Cell Viability(%) Rate (%)
Fig. 4 Experimental results showing electro-coalescence of a cell-encapsulating droplet at different time instants, before coalescence the cell is encapsulated inside droplet (DP), after coalescence the cell in in the aqueous stream CP 2, droplet diameter 90 µm and velocity 2.19 mm/s, 4% surfactant, 𝐸=7.6×104 V/m.
Cell Viability(%) 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 49 50 51 52 53 54 55 56 57 58 59 60
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98 96 94 HeLa cells DU145 cells
92 90
0
5
1x10
5
2x10
5
3x10
5
4x10
5
5x10
5
6x10
98 96 94 DU145 cells HeLa cells
92 90
4
Electric Field Intensity (V/m)
6
8
10
12
14
16
18
20
Flow Rate (µl/min)
Fig. 5 (a) The variation in the %cell viability with electric field, flow rate of 5µl/min (b) The variation in the %cell viability with flow rate, electric field of 4.2×105 V/m (i.e. 150 V).
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Fluid interface t = 0s
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Larger droplets makes contact with interface t = 0.205 s
Electrode
42 µm dia.
37 µm dia.
50µm
Electrocoalescence
Smaller droplets do not make contact with interface
t = 0.207 s
t = 0.251 s
Fig. 6 Experimental images showing size based selective demulsification of droplets at few time instants, two different sized droplets 37 µm and 42 µm, the larger (42 µm) droplets coalesce with the continuous aqueous phase, the (37 µm) smaller droplets moves past the electric field in CP2, surfactant concentration in CP1 4%.
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