**4. Conclusion**

In this study, the microfluidic system based on the aqueous two-phase system (ATPS) droplets was developed. Taking advantage of the benefits that microfluidics can provide, this study was focused on the mass transfer between two liquid-liquid phases. Droplets generated and manipulated in microfluidic system can provide unique opportunities to handle fluids in a segmented flow, offering novel platform of miniaturized system for chemical and biological processes. The two-phase that has been employed in this experimental work is the ATPS formed by tetrabutylammonium bromide (TABA)/ammonium sulfate (AS). Among the numerous possible liquid-liquid two-phase systems available, only a few of them can be operated in microfluidic system both in the stratified laminar flow and in segmented droplet-based flow. This unique feature comes from the moderate interfacial tension between the TBAB-rich phase and AS-rich phase.

Generation of TBAB/AS ATPS droplets by electrohydrodynamic method was studied. Initially fed as laminar flow, the AS-rich phase was destabilized and eventually dispersed as droplets in the continuous TBAB-rich phase at the T-junction in the microfluidic system when the d. c. electric potential difference was applied as pulses. Based on the pH dependency of the threshold electric voltage required to generate a single droplet, the electrophoretic mobility of the AS-rich phase in TBAB-rich phase was discussed and a model for the mechanism was suggested. Applying the same electrohydrodynamic principle, manipulation of droplet movement direction was demonstrated in a different microfluidic device. This technique can be a fundamental method for the operation of the ATPS-based microfluidic system.

TBAB/AS ATPS droplet-based microextraction was also investigated. Ruthenium red was successfully extracted from the TBAB-rich phase to the AS-rich phase droplet in the microfluidic extraction device. The mass transportation between the two liquid phases was discussed assuming two different steps, from the continuous phase to the droplet surface and from the droplet surface to the center of the droplet, applying diffusion equation for each step. Further, the method for stopping microextraction which is essential for controlling solute concentration inside a droplet was suggested by means of the electrohydrodynamic manipulation of droplet movement direction. Here showed is just one example of TBAB/AS ATPS droplet-based microextraction. The compatibility of TBAB/AS ATPS with stable biological materials and other water-soluble inorganic molecules can provide potentials for further application in chemistry and biology. It is expected that the next applications of this ATPS in microfluidic should be in the analysis of charged biomolecules, separation of surface-treated nanoparticles and reaction of inorganic molecules in coordinate chemistry.

In addition, future development of ATPS droplet handling technique will add the merits of this microfluidic system. For example, precisely controlled coalescence of ATPS droplets can lead to reaction of the reactants confined inside the droplets. Appropriate surface treatment of the microfluidic device and proper fabrication which takes the phase properties into account can enable one to alternate the flow pattern from segmented flow to continuous flow depending on the need of the process. Application of additives that can give affinity for efficient separation and aqueous three-phase system which allows more variety can be considered as the topics for future research. Alternative aqueous twophase-forming materials which can be used for multi-phase system in low concentrations can widen the application in which relatively susceptible biological molecules are involved.

### **5. References**

820 Mass Transfer - Advanced Aspects

In this study, the microfluidic system based on the aqueous two-phase system (ATPS) droplets was developed. Taking advantage of the benefits that microfluidics can provide, this study was focused on the mass transfer between two liquid-liquid phases. Droplets generated and manipulated in microfluidic system can provide unique opportunities to handle fluids in a segmented flow, offering novel platform of miniaturized system for chemical and biological processes. The two-phase that has been employed in this experimental work is the ATPS formed by tetrabutylammonium bromide (TABA)/ammonium sulfate (AS). Among the numerous possible liquid-liquid two-phase systems available, only a few of them can be operated in microfluidic system both in the stratified laminar flow and in segmented droplet-based flow. This unique feature comes from the moderate interfacial

Generation of TBAB/AS ATPS droplets by electrohydrodynamic method was studied. Initially fed as laminar flow, the AS-rich phase was destabilized and eventually dispersed as droplets in the continuous TBAB-rich phase at the T-junction in the microfluidic system when the d. c. electric potential difference was applied as pulses. Based on the pH dependency of the threshold electric voltage required to generate a single droplet, the electrophoretic mobility of the AS-rich phase in TBAB-rich phase was discussed and a model for the mechanism was suggested. Applying the same electrohydrodynamic principle, manipulation of droplet movement direction was demonstrated in a different microfluidic device. This technique can be a fundamental method for the operation of the

TBAB/AS ATPS droplet-based microextraction was also investigated. Ruthenium red was successfully extracted from the TBAB-rich phase to the AS-rich phase droplet in the microfluidic extraction device. The mass transportation between the two liquid phases was discussed assuming two different steps, from the continuous phase to the droplet surface and from the droplet surface to the center of the droplet, applying diffusion equation for each step. Further, the method for stopping microextraction which is essential for controlling solute concentration inside a droplet was suggested by means of the electrohydrodynamic manipulation of droplet movement direction. Here showed is just one example of TBAB/AS ATPS droplet-based microextraction. The compatibility of TBAB/AS ATPS with stable biological materials and other water-soluble inorganic molecules can provide potentials for further application in chemistry and biology. It is expected that the next applications of this ATPS in microfluidic should be in the analysis of charged biomolecules, separation of surface-treated nanoparticles and reaction of inorganic

In addition, future development of ATPS droplet handling technique will add the merits of this microfluidic system. For example, precisely controlled coalescence of ATPS droplets can lead to reaction of the reactants confined inside the droplets. Appropriate surface treatment of the microfluidic device and proper fabrication which takes the phase properties into account can enable one to alternate the flow pattern from segmented flow to continuous flow depending on the need of the process. Application of additives that can give affinity for efficient separation and aqueous three-phase system which allows more variety can be considered as the topics for future research. Alternative aqueous twophase-forming materials which can be used for multi-phase system in low concentrations

tension between the TBAB-rich phase and AS-rich phase.

ATPS-based microfluidic system.

molecules in coordinate chemistry.

**4. Conclusion** 


Microdroplets for the Study of Mass Transfer 823

Squires, T. M. & Quake, S. R. (2005). Microfluidics: Fluid physics at the nanoliter scale,

Stone, H. A., Stroock, A. D. & Ajdari, A. (2004). Engineering flows in small devices:

Surmeian, M., Slyadnev, M. N., Hisamoto, H., Hibara, A., Uchiyama, K. & Kitamori, T.

Tan, Y.-C., Ho, Y. L. & Lee, A. P. (2008) Microfluidic sorting of droplets by size, *Microfluidics* 

Tice, J. D., Song, H., Lyon, A. D. & Ismagilov, R. F. (2003). Formation of droplets and mixing

Tokeshi, M., Minagawa, T. & Kitamori, T. (2000). Integration of a microextraction system on

Tokeshi, M., Minagawa, T. & Kitamori, T. (2000). Integration of a microextraction system:

Xu, J. H., Tan, J., Li, S. W. & Luo, G. S. (2008). Enhancement of mass transfer performance of

Walter, H., Brooks, D. E. & Fisher, D. (Eds.) (1985). *Partitioning in Aqueous Two Phase Systems:* 

Walter, H. & Johansson, G. (1986). Partitioning in aqueous two-phase systems: An overview,

Walter, H., Johansson, G. & Brooks, D. E. (1991). Partitioning in aqueous two-phase systems:

Whitesides, G. M. (2006). The origins and the future of microfluidics, *Nature*, Vol. 442,

Yamada, M., Kasim, V., Nakashima, M., Edahiro, J. & Seki, M. (2004). Continuous cell

Zagnoni, M. & Cooper, J. M. (2009). On-chip electrocoalescence of microdroplets as a

Zheng, B., Roach, L. S. & Ismagilov, R. F. (2003). Screening of protein crystallization

microchip, *Journal of Chromatography A*, Vol.894, No. 1-2, pp. 19-23

molecular transport, *Analytical Chemistry*, Vol.74, No.9, pp. 2014-2020 Tan, Y.-C., Ho, Y. L. & Lee, A. P. (2007). Droplet coalescence by geometrically mediated

microfluidics toward a lab-on-a-chip, *Annual Review of Fluid Mechanics*, Vol.36,

(2002). Three-layer flow membrane system on a microchip for investigation of

flow in microfluidic channels, *Microfluidics and Nanofluidics*, Vol.3, No.4, pp. 495-

in multiphase microfluidics at low values of the Reynolds and the capillary

a glass chip: Ion-pair solvent extraction of Fe(II) with 4,7-diphenyl-1,10 phenanthrolinedisulfonic acid and tri-n-octylmethylammonium chloride, *Analytical* 

Solvent extraction of a Co-2- nitroso-5-dimethylaminophenol complex on a

liquid-liquid system by droplet flow in microchannels, *Chemical Engineering Journal*,

*Theory, Methods, Uses and Applications to Biotechnology*, Academic Press, Orlando, FL.

partitioning using an aqueous two-phase flow system in microfluidic devices,

function of voltage, frequency and droplet size, *Lab on a Chip - Miniaturisation for* 

conditions on microfluidic chip using nanoliter-size droplets, *Journal of the American* 

*Reviews of Modern Physics*, Vol.77, No.3, pp. 977-1026

*and Nanofluidics*, Vol.4, No.4, pp. 343-348

numbers, *Langmuir*, Vol.19, pp.9127-9133

*Chemistry*, Vol.72, No.7, pp. 1711-1714

*Analytical Biochemistry*, Vol.155, No.2, pp. 215-242

Recent results, *Analytical Biochemistry*, Vol.197, No. 1, pp. 1-18

*Biotechnology and Bioengineering*, Vol.88, No.4, pp. 489-494

*Chemistry and Biology*,Vol. 9, pp. 2652-2658

*Chemical Society*, Vol.125, pp.11170-11171

Vol.141, No.1-3, pp. 242-249

pp.368-373

pp.381-411

499


Frenz, L., Harrak, A. E., Pauly, M., Bégin-Colin, S., Griffiths, A. D. & Baret, J. (2008). Droplet-

Hatti-Kaul, R. (2000) *Aqueous Two-Phase Systems: Methods and Protocols*, Humana Press,

Hisamoto, H., Shimizu, Y., Uchiyama, K., Tokeshi, M., Kikutani, Y., Hibara, A. & Kitamori,

Jullien, M.-C., Tsang Mui Ching, M.-J., Cohen, C., Menetrier, L. & Tabeling, P. (2009) Droplet

Kamholz, A. E., Weigl, B. H., Finlayson, B. A. & Yager, P. (1999). Quantitative analysis of

Kim, D. H. (2004). Microfluidic systems: state of the art, *Korean Chemical Engineering Research*,

Link, D. R., Anna, S.I ., Weitz, D. A. & Stone, H. A. (2004). Geometrically Mediated Breakup

Link, D. R., Grasland-Mongrain, E., Duri, A., Sarrazin, F., Cheng, Z., Cristobal, G.,

Mary, P., Studer, V. & Tabeling, P. (2008). Microfluidic droplet-based liquid-liquid

Ménétrier-Deremble, L. & Tabeling, P. (2006). Droplet breakup in microfluidic junctions of

Nam, K.-H., Chang, W.-J., Hong, H., Lim, S.-M., Kim, D.-I. & Koo, Y.-M. (2005). Continuous-

Ozen, O., Aubry, N., Papageorgiou, D. T. & Petropoulos, P. G. (2006). Electrohydrodynamic

Ozen, O., Aubry, N., Papageorgiou, D. T. & Petropoulos, P. G. (2006). Monodisperse drop

Prakash, M. & Gershenfeld, N. (2007). Microfluidic bubble logic, *Science*, Vol.315, No.5813,

Song, Y. S., Choi, Y. H. & Kim, D. H. (2007). Microextraction in a tetrabutylammonium

extraction, *Analytical Chemistry*, Vol.80, pp.2680-2687

extraction, *Biomedical Microdevices*, Vol.7, No.3, pp. 189-195

*Angewandte Chemie International Edition*, Vol.47, pp.6817-6820

microchip, *Analytical Chemistry*, Vol. 75, No.2, pp. 350-354

Totowa, New Jersey

Vol.21, No.7, art. no. 072001

Vol.71, No.23, pp. 5340-5347

Vol.74, No.3, art. no. 035303

No.25, pp. 5316-5323

Vol.1162, pp. 180-186.

144501, pp. 1-4

pp. 832-835

Vol. 42, pp.375-386

545034

2560

based microreactors for the synthesis of magnetic iron oxide nanoparticles,

T. (2003). Chemicofunctional membrane for integrated chemical processes on a

breakup in microfluidic T-junctions at small capillary numbers, *Physics of Fluids*,

molecular interaction in a microfluidic channel: The T-sensor, *Analytical Chemistry*,

of Drops in Microfluidic Devices, *Physical Review Letters*, Vol.92, No.5, pp. 545031-

Marquez, M. & Weitz, D. A. (2006). Electric control of droplets in microfluidic devices, *Angewandte Chemie - International Edition*, Vol.45, No.16, pp. 2556-

arbitrary angles, *Physical Review E - Statistical, Nonlinear, and Soft Matter Physics*,

flow fractionation of animal cells in microfluidic device using aqueous two-phase

linear stability of two immiscible fluids in channel flow, *Electrochimica Acta*, Vol.51,

formation in square microchannels, *Physical Review Letters*, Vol.96, No.14, art. no.

bromide/ammonium sulfate aqueous two-phase system and electrohydrodynamic generation of a micro-droplet, *Journal of Chromatography A*,


Ziemecak I., Steijn V., Koper G. J. M., Rosso M., Brizard A. M., Esch J. H. & Kreutzer M. T. (2011). Monodisperse hydrogel microspheres by forced droplet formation in aqueous two-phase system, *Lab on a Chip - Miniaturisation for Chemistry and Biolog*y, Vol.11, pp. 620-624

Ziemecak I., Steijn V., Koper G. J. M., Rosso M., Brizard A. M., Esch J. H. & Kreutzer M. T.

Vol.11, pp. 620-624

(2011). Monodisperse hydrogel microspheres by forced droplet formation in aqueous two-phase system, *Lab on a Chip - Miniaturisation for Chemistry and Biolog*y,
