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Surfactants in Droplet-based Microfluidics

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The surfactant stabilizes the interface between the continuous oil phase and the aqueous droplet.[1] Also, the surfactants can reduce the adhesion of aqueous droplets to the channel wall, by decreasing the surface tension at the aqueous-oil interface.[2]

Surfactants play an important role in droplet-based microfluidics.[3] The main purpose of using a surfactant is to reduce the interfacial tension between the dispersed phase (droplet phase, typically aqueous) and continuous phase (carrier liquid, typically oil) by adsorbing at interfaces and preventing droplets from coalescing with each other, therefore stabilizing the droplets in a stable emulsion state, which allows for longer storage times in delay-lines, reservoirs, or vials.[3][4] Without using surfactants, the unstable emulsions will eventually evolve into separate phases to reduce the overall energy of the system.[5] Surface chemistry cannot be ignored in microfluidics as the interfacial tension becomes a major consideration among microscale droplets.[6] Linas Mazutis and Andrew D. Griffiths presented a method that used surfactants to achieve a selective and highly controllable coalescence without external manipulation.[7] They manipulate the contact time and the interfacial surfactant coverage of a drop pair to control droplet fusion. The larger the difference percentage of the interfacial surfactant coverage between two droplets, the less likely coalescence will occur. This method allowed researchers to add reagents to droplets in a different way and further study the emulsification.[7]

The hydrophobic tails on the surfactant keep the emulsion stable and prevent droplet coalescence together during the incubation time.[8] The larger/longer the hydrophobic tails are, the better the biocompatibility and the better the solubility in oil phase, also the better the insolubility in aqueous phase.[9][10]


Microfluidics is widely used for biochemical experiments, so it is important that surfactants are biocompatible when working with living cells and high-throughput analysis.[11][5] Surfactants used in living cell research devices should not interfere with biochemical reactions or cellular functions. Hydrocarbon oil is typically not used in cell microfluidic research because it is not compatible with cells and damages cell viability.[12] Hydrocarbon oil also extracts organic molecules from the aqueous phase.[12] However, fluorosurfactants with fluorinated tails, for example, are used as a compatible droplet emulsifier that stabilizes droplets containing cells inside without harming or altering the cells.[3] Fluorosurfactants are soluble in a fluorinated oil (continuous phase) but insoluble in the aqueous phase, which results in decreasing the aqueous-fluorous interfacial tension.[6] For example, a triblock copolymer surfactant containing two perfluoropolyether (PFPE) tails and a polyethylene glycol (PEG) block head group is a fluorosurfactant with great biocompatibility and excellent droplet stability against coalescence.[11][13][14] Another example are the fluorinated linear polyglycerols, which can be further functionalized on their tailored side-chains and are more customizable compared to the PEG-based copolymer.[15] Surfactants can be purchased from many chemical companies, such as RainDance Technologies (now through BioRad)[4] and Miller-Stephenson[11].

  1. ^ Baret, Jean-Christophe; Kleinschmidt, Felix; El Harrak, Abdeslam; Griffiths, Andrew D. (2009-06-02). "Kinetic Aspects of Emulsion Stabilization by Surfactants: A Microfluidic Analysis". Langmuir. 25 (11): 6088–6093. doi:10.1021/la9000472. ISSN 0743-7463.
  2. ^ Roach, L. Spencer; Song, Helen; Ismagilov, Rustem F. (2005-02). "Controlling Nonspecific Protein Adsorption in a Plug-Based Microfluidic System by Controlling Interfacial Chemistry Using Fluorous-Phase Surfactants". Analytical Chemistry. 77 (3): 785–796. doi:10.1021/ac049061w. ISSN 0003-2700. PMC 1941690. PMID 15679345. {{cite journal}}: Check date values in: |date= (help)CS1 maint: PMC format (link)
  3. ^ a b c Baret, Jean-Christophe (2012-01-10). "Surfactants in droplet-based microfluidics". Lab on a Chip. 12 (3): 422–433. doi:10.1039/C1LC20582J. ISSN 1473-0189.
  4. ^ a b Mazutis, Linas; Gilbert, John; Ung, W. Lloyd; Weitz, David A.; Griffiths, Andrew D.; Heyman, John A. (2013-05). "Single-cell analysis and sorting using droplet-based microfluidics". Nature Protocols. 8 (5): 870–891. doi:10.1038/nprot.2013.046. ISSN 1750-2799. {{cite journal}}: Check date values in: |date= (help)
  5. ^ a b Shang, Luoran; Cheng, Yao; Zhao, Yuanjin (2017-06-28). "Emerging Droplet Microfluidics". Chemical Reviews. 117 (12): 7964–8040. doi:10.1021/acs.chemrev.6b00848. ISSN 0009-2665.
  6. ^ a b Roach, L. Spencer; Song, Helen; Ismagilov, Rustem F. (2005-02). "Controlling Nonspecific Protein Adsorption in a Plug-Based Microfluidic System by Controlling Interfacial Chemistry Using Fluorous-Phase Surfactants". Analytical Chemistry. 77 (3): 785–796. doi:10.1021/ac049061w. ISSN 0003-2700. PMC 1941690. PMID 15679345. {{cite journal}}: Check date values in: |date= (help)CS1 maint: PMC format (link)
  7. ^ a b Mazutis, Linas; Griffiths, Andrew D. (2012-04-24). "Selective droplet coalescence using microfluidic systems". Lab on a Chip. 12 (10): 1800–1806. doi:10.1039/C2LC40121E. ISSN 1473-0189.
  8. ^ Baret, Jean-Christophe; Kleinschmidt, Felix; El Harrak, Abdeslam; Griffiths, Andrew D. (2009-06-02). "Kinetic Aspects of Emulsion Stabilization by Surfactants: A Microfluidic Analysis". Langmuir. 25 (11): 6088–6093. doi:10.1021/la9000472. ISSN 0743-7463.
  9. ^ Baret, Jean-Christophe; Kleinschmidt, Felix; El Harrak, Abdeslam; Griffiths, Andrew D. (2009-06-02). "Kinetic Aspects of Emulsion Stabilization by Surfactants: A Microfluidic Analysis". Langmuir. 25 (11): 6088–6093. doi:10.1021/la9000472. ISSN 0743-7463.
  10. ^ Mazutis, Linas; Gilbert, John; Ung, W. Lloyd; Weitz, David A.; Griffiths, Andrew D.; Heyman, John A. (2013-05). "Single-cell analysis and sorting using droplet-based microfluidics". Nature Protocols. 8 (5): 870–891. doi:10.1038/nprot.2013.046. ISSN 1750-2799. {{cite journal}}: Check date values in: |date= (help)
  11. ^ a b c Holtze, C.; Rowat, A. C.; Agresti, J. J.; Hutchison, J. B.; Angilè, F. E.; Schmitz, C. H. J.; Köster, S.; Duan, H.; Humphry, K. J.; Scanga, R. A.; Johnson, J. S. (2008-09-09). "Biocompatible surfactants for water-in-fluorocarbon emulsions". Lab on a Chip. 8 (10): 1632–1639. doi:10.1039/B806706F. ISSN 1473-0189.
  12. ^ a b Chen, Fangyuan; Zhan, Yihong; Geng, Tao; Lian, Hongzhen; Xu, Peisheng; Lu, Chang (2011-11-15). "Chemical Transfection of Cells in Picoliter Aqueous Droplets in Fluorocarbon Oil". Analytical Chemistry. 83 (22): 8816–8820. doi:10.1021/ac2022794. ISSN 0003-2700.
  13. ^ Köster, Sarah; Angilè, Francesco E.; Duan, Honey; Agresti, Jeremy J.; Wintner, Anton; Schmitz, Christian; Rowat, Amy C.; Merten, Christoph A.; Pisignano, Dario; Griffiths, Andrew D.; Weitz, David A. (2008-06-27). "Drop-based microfluidic devices for encapsulation of single cells". Lab on a Chip. 8 (7): 1110–1115. doi:10.1039/B802941E. ISSN 1473-0189.
  14. ^ Skhiri, Yousr; Gruner, Philipp; Semin, Benoît; Brosseau, Quentin; Pekin, Deniz; Mazutis, Linas; Goust, Victoire; Kleinschmidt, Felix; Harrak, Abdeslam El; Hutchison, J. Brian; Mayot, Estelle (2012-10-03). "Dynamics of molecular transport by surfactants in emulsions". Soft Matter. 8 (41): 10618–10627. doi:10.1039/C2SM25934F. ISSN 1744-6848.
  15. ^ Wagner, Olaf; Thiele, Julian; Weinhart, Marie; Mazutis, Linas; Weitz, David A.; Huck, Wilhelm T. S.; Haag, Rainer (2015-12-15). "Biocompatible fluorinated polyglycerols for droplet microfluidics as an alternative to PEG-based copolymer surfactants". Lab on a Chip. 16 (1): 65–69. doi:10.1039/C5LC00823A. ISSN 1473-0189.
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