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A versatile technology for droplet-based microfluidics: thermomechanical actuation† Cite this: DOI: 10.1039/c5lc00110b
Vincent Miralles,‡ Axel Huerre,‡ Hannah Williams, Bastien Fournié and Marie-Caroline Jullien* We report on a versatile technique for microfluidic droplet manipulation that proves effective at every step: from droplet generation to propulsion to sorting, rearrangement or break-up. Non-wetting droplets are Received 29th January 2015, Accepted 20th March 2015
thermomechanically actuated in a microfluidic chip using local heating resistors. Controlled temperature variation induces local dilation of the PDMS wall above the resistor, which drives the droplet away from the hot (i.e. constricted) region (B. Selva, I. Cantat and M.-C. Jullien, Phys. Fluids, 2011, 23, 052002). Adapted
DOI: 10.1039/c5lc00110b
placing and actuation of such resistors thus allow us to push forward, stop, store and release, or even break up droplets, at the price of low electric power consumption ( 0 (as is the case here, ∂Tγ = 1.4 10−4 N m−1 measured by the pendant drop technique). Furthermore, to ensure mass conservation, this converging surface flow towards the neck leads to an elongational flow from the neck towards the droplet extremities in the bulk of the droplet (see schematics in Fig. 11a). Such an elongational flow drives the droplet extremities away from the heating zone, favoring the droplet to break up. The physical mechanism at play is thus rather a thermally-induced hydrodynamic pinching (Fig. 11). Furthermore, it is admittedly fastidious to position a droplet at the saddle point of an elongational flow, while in the present mechanism this saddle point is within the droplet. As the physical mechanism is not purely thermomechanical, experiments using water-in-oil droplets were successfully performed to validate this functionality (see the ESI†). On a side note, this experiment also provides us with an estimation for the transient time T over which the imposed temperature is established in the system, which corresponds to the heat diffusion time through the different materials (PDMS thin layer, water in the channel, PDMS above the channel). In the experiment showed in Fig. 11, the resistor is switched on at t = 0.48 s and the actuation is functional at t = 0.53 s, which gives an upper estimate of T ~ 50 ms.
5.2 Droplet production Though the above sections have focused on actuating droplets that had been previously generated by conventional methods (e.g. flow focusing), droplet production can also be ensured thermomechanically. To tune the size or the
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frequency of the droplets, most state-of-the-art strategies rely on modifying the flow rates of both phases or the geometry of the channels, which can be inconvenient for versatility purposes.2 Conversely, our technique allows us to tune both the size and the frequency of generated droplets without changing the channels or flow rates. Droplet delivery at a T-junction is controlled via a resistor placed at the entrance of the junction (see Fig. 12). Increasing the applied power without changing the flow rates leads to an increase in the production frequency and a decrease in the droplet size. The evolution of the droplet frequency and size (normalized by their value f0 and L0 without thermomechanical actuation, at P = 0 W) with the applied power is presented in Fig. 12 for given flow rates; we find that these parameters can be respectively increased or decreased by 50% with a very small power input. Note that in this case, droplet production takes place in the dripping regime, for which no formal model exists to rationalize the droplet size and generation frequency;30,31 we may expect the mechanisms at play to be a non trivial combination of droplet production in the dripping regime enhanced by thermally-induced hydrodynamic pinching, as described in section 5.1.
6. Conclusion By integrating heating resistors into a microfluidic chip, we are able to generate droplets, propel them with or without an outer-plase flow, and direct them towards specific locations or in specific arrangements. A full collection of functionalities have been achieved based on two mechanisms: thermomechanical effect and thermally-induced hydrodynamic pinching, for both water-in-oil and oil-in-water droplets. We believe our technique thus constitutes a very useful and versatile tool, all the more so since it is very technically accessible: the chip is easy to fabricate as it needs a single layer of conductive metal, and the actuation is low power consuming. We hope that thermomechanical actuation opens a new route towards fully portable and integrated assays. In this sense, our versatile technology constitutes a building block for lowcost and built-in lab on chips.
Acknowledgements
Fig. 12 Thermally-tuned droplet production. The red and blue curves correspond respectively to the normalized production frequency IJf/f0) and normalized length IJL/L0) of the droplets generated at the T-junction, as a function of the electric power applied to the heating resistor. L0 and f0 refer respectively to the length and frequency of a droplet generated when the heating resistor is switched off. For each value of the applied power, the corresponding snapshot is the last picture taken before the droplet breaks up. The scale bar is valid for the five snapshots and represents 90 μm. For each experiment, Qw = 15.7 μL min−1 and Qoil = 0.99 μL min−1.
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This work has been patented under the reference PCT/ EP2014/079337, and supported by CNRS, Direction Générale des Armées (DGA), IPGG Equipex ANR-10-EQPX-34, ESPCI ParisTech, Dim-NanoK région Ile de France, and ANR under the grant 13-BS09-0011-01. We acknowledge valuable discussions with Marine Bezagu and Marie Leman.
References 1 A. A. Darhuber and S. M. Troian, Annu. Rev. Fluid Mech., 2005, 37, 425–455. 2 R. Seemann, M. Brinkmann, T. Pfohl and S. Herminghaus, Rep. Prog. Phys., 2012, 75, 016601.
This journal is © The Royal Society of Chemistry 2015
View Article Online
Published on 26 March 2015. Downloaded by UNIVERSITY OF NEBRASKA on 09/04/2015 17:51:59.
Lab on a Chip
3 C. N. Baroud, F. Gallaire and R. Dangla, Lab Chip, 2010, 10, 2032. 4 A. Huerre, V. Miralles and M.-C. Jullien, Soft Matter, 2014, 10, 6888–6902. 5 L. Mazutis and A. D. Griffiths, Appl. Phys. Lett., 2009, 95, 204103. 6 K. Ahn, C. Kerbage, T. P. Hunt, R. Westervelt, D. R. Link and D. Weitz, Appl. Phys. Lett., 2006, 88, 024104. 7 J.-C. Baret, O. J. Miller, V. Taly, M. Ryckelynck, A. El-Harrak, L. Frenz, C. Rick, M. L. Samuels, J. B. Hutchison, J. J. Agresti, D. R. Link, D. A. Weitz and A. D. Griffiths, Lab Chip, 2009, 9, 1850–1858. 8 W. Wang, C. Yang, Y. Liu and C. M. Li, Lab Chip, 2010, 10, 559–562. 9 J.-H. Choi, S.-K. Lee, J.-M. Lim, S.-M. Yang and G.-R. Yi, Lab Chip, 2010, 10, 456–461. 10 N. R. Beer, K. A. Rose and I. M. Kennedy, Lab Chip, 2009, 9, 841–844. 11 C. N. Baroud, M. Robert de Saint Vincent and J.-P. Delville, Lab Chip, 2007, 7, 1029. 12 T. H. Ting, Y. F. Yap, N.-T. Nguyen, T. N. Wong, J. C. K. Chai and L. Yobas, Appl. Phys. Lett., 2006, 89, 234101. 13 M. Chabert, K. D. Dorfman and J.-L. Viovy, Electrophoresis, 2005, 26, 3706–3715. 14 D. J. Im, J. Noh, D. Moon and I. S. Kang, Anal. Chem., 2011, 83, 5168–5174. 15 P. Abbyad, R. Dangla, A. Alexandrou and C. N. Baroud, Lab Chip, 2011, 11, 813–821. 16 F. Mugele, Soft Matter, 2009, 5, 3377.
This journal is © The Royal Society of Chemistry 2015
Paper
17 S. K. Chung, K. Rhee and S. K. Cho, Int. J. Precis. Eng. Man., 2010, 11, 991–1006. 18 P. Brunet, M. Baudoin, O. B. Matar and F. Zoueshtiagh, Phys. Rev. E: Stat., Nonlinear, Soft Matter Phys., 2010, 81, 036315. 19 L. Y. Yeo and J. R. Friend, Biomicrofluidics, 2009, 3, 012002. 20 A. A. Darhuber, J. P. Valentino, S. M. Troian and S. Wagner, J. Microelectromech. Syst., 2003, 12, 873–879. 21 A. A. Darhuber, J. P. Valentino and S. M. Troian, Lab Chip, 2010, 10, 1061. 22 B. Selva, I. Cantat and M.-C. Jullien, Phys. Fluids, 2011, 23, 052002. 23 Y. Xia and G. M. Whitesides, Annu. Rev. Mater. Sci., 1998, 28, 153–184. 24 B. Selva, J. Marchalot and M.-C. Jullien, J. Micromech. Microeng., 2009, 19, 065002. 25 R. Dangla, F. Gallaire and C. N. Baroud, Lab Chip, 2010, 10, 2972–2978. 26 S. Hodges, O. Jensen and J. Rallison, J. Fluid Mech., 2004, 501, 279–301. 27 A. M. Leshansky and L. M. Pismen, Phys. Fluids, 2009, 21, 023303. 28 M.-C. Jullien, M.-J. T. M. Ching, C. Cohen, L. Ménétrier and P. Tabeling, Phys. Fluids, 2009, 21, 072001. 29 A. M. Leshansky, S. Afkhami, M.-C. Jullien and P. Tabeling, Phys. Rev. Lett., 2012, 108, 264502. 30 M. De Menech, P. Garstecki, F. Jousse and H. Stone, J. Fluid Mech., 2008, 595, 141–161. 31 A. Gupta and R. Kumar, Microfluid. Nanofluid., 2010, 8, 799–812.
Lab Chip
Published on 26 March 2015. Downloaded by UNIVERSITY OF NEBRASKA on 09/04/2015 17:51:59.
PAPER
View Journal
A versatile technology for droplet-based microfluidics: thermomechanical actuation† Cite this: DOI: 10.1039/c5lc00110b
Vincent Miralles,‡ Axel Huerre,‡ Hannah Williams, Bastien Fournié and Marie-Caroline Jullien* We report on a versatile technique for microfluidic droplet manipulation that proves effective at every step: from droplet generation to propulsion to sorting, rearrangement or break-up. Non-wetting droplets are Received 29th January 2015, Accepted 20th March 2015
thermomechanically actuated in a microfluidic chip using local heating resistors. Controlled temperature variation induces local dilation of the PDMS wall above the resistor, which drives the droplet away from the hot (i.e. constricted) region (B. Selva, I. Cantat and M.-C. Jullien, Phys. Fluids, 2011, 23, 052002). Adapted
DOI: 10.1039/c5lc00110b
placing and actuation of such resistors thus allow us to push forward, stop, store and release, or even break up droplets, at the price of low electric power consumption ( 0 (as is the case here, ∂Tγ = 1.4 10−4 N m−1 measured by the pendant drop technique). Furthermore, to ensure mass conservation, this converging surface flow towards the neck leads to an elongational flow from the neck towards the droplet extremities in the bulk of the droplet (see schematics in Fig. 11a). Such an elongational flow drives the droplet extremities away from the heating zone, favoring the droplet to break up. The physical mechanism at play is thus rather a thermally-induced hydrodynamic pinching (Fig. 11). Furthermore, it is admittedly fastidious to position a droplet at the saddle point of an elongational flow, while in the present mechanism this saddle point is within the droplet. As the physical mechanism is not purely thermomechanical, experiments using water-in-oil droplets were successfully performed to validate this functionality (see the ESI†). On a side note, this experiment also provides us with an estimation for the transient time T over which the imposed temperature is established in the system, which corresponds to the heat diffusion time through the different materials (PDMS thin layer, water in the channel, PDMS above the channel). In the experiment showed in Fig. 11, the resistor is switched on at t = 0.48 s and the actuation is functional at t = 0.53 s, which gives an upper estimate of T ~ 50 ms.
5.2 Droplet production Though the above sections have focused on actuating droplets that had been previously generated by conventional methods (e.g. flow focusing), droplet production can also be ensured thermomechanically. To tune the size or the
Lab on a Chip
frequency of the droplets, most state-of-the-art strategies rely on modifying the flow rates of both phases or the geometry of the channels, which can be inconvenient for versatility purposes.2 Conversely, our technique allows us to tune both the size and the frequency of generated droplets without changing the channels or flow rates. Droplet delivery at a T-junction is controlled via a resistor placed at the entrance of the junction (see Fig. 12). Increasing the applied power without changing the flow rates leads to an increase in the production frequency and a decrease in the droplet size. The evolution of the droplet frequency and size (normalized by their value f0 and L0 without thermomechanical actuation, at P = 0 W) with the applied power is presented in Fig. 12 for given flow rates; we find that these parameters can be respectively increased or decreased by 50% with a very small power input. Note that in this case, droplet production takes place in the dripping regime, for which no formal model exists to rationalize the droplet size and generation frequency;30,31 we may expect the mechanisms at play to be a non trivial combination of droplet production in the dripping regime enhanced by thermally-induced hydrodynamic pinching, as described in section 5.1.
6. Conclusion By integrating heating resistors into a microfluidic chip, we are able to generate droplets, propel them with or without an outer-plase flow, and direct them towards specific locations or in specific arrangements. A full collection of functionalities have been achieved based on two mechanisms: thermomechanical effect and thermally-induced hydrodynamic pinching, for both water-in-oil and oil-in-water droplets. We believe our technique thus constitutes a very useful and versatile tool, all the more so since it is very technically accessible: the chip is easy to fabricate as it needs a single layer of conductive metal, and the actuation is low power consuming. We hope that thermomechanical actuation opens a new route towards fully portable and integrated assays. In this sense, our versatile technology constitutes a building block for lowcost and built-in lab on chips.
Acknowledgements
Fig. 12 Thermally-tuned droplet production. The red and blue curves correspond respectively to the normalized production frequency IJf/f0) and normalized length IJL/L0) of the droplets generated at the T-junction, as a function of the electric power applied to the heating resistor. L0 and f0 refer respectively to the length and frequency of a droplet generated when the heating resistor is switched off. For each value of the applied power, the corresponding snapshot is the last picture taken before the droplet breaks up. The scale bar is valid for the five snapshots and represents 90 μm. For each experiment, Qw = 15.7 μL min−1 and Qoil = 0.99 μL min−1.
Lab Chip
This work has been patented under the reference PCT/ EP2014/079337, and supported by CNRS, Direction Générale des Armées (DGA), IPGG Equipex ANR-10-EQPX-34, ESPCI ParisTech, Dim-NanoK région Ile de France, and ANR under the grant 13-BS09-0011-01. We acknowledge valuable discussions with Marine Bezagu and Marie Leman.
References 1 A. A. Darhuber and S. M. Troian, Annu. Rev. Fluid Mech., 2005, 37, 425–455. 2 R. Seemann, M. Brinkmann, T. Pfohl and S. Herminghaus, Rep. Prog. Phys., 2012, 75, 016601.
This journal is © The Royal Society of Chemistry 2015
View Article Online
Published on 26 March 2015. Downloaded by UNIVERSITY OF NEBRASKA on 09/04/2015 17:51:59.
Lab on a Chip
3 C. N. Baroud, F. Gallaire and R. Dangla, Lab Chip, 2010, 10, 2032. 4 A. Huerre, V. Miralles and M.-C. Jullien, Soft Matter, 2014, 10, 6888–6902. 5 L. Mazutis and A. D. Griffiths, Appl. Phys. Lett., 2009, 95, 204103. 6 K. Ahn, C. Kerbage, T. P. Hunt, R. Westervelt, D. R. Link and D. Weitz, Appl. Phys. Lett., 2006, 88, 024104. 7 J.-C. Baret, O. J. Miller, V. Taly, M. Ryckelynck, A. El-Harrak, L. Frenz, C. Rick, M. L. Samuels, J. B. Hutchison, J. J. Agresti, D. R. Link, D. A. Weitz and A. D. Griffiths, Lab Chip, 2009, 9, 1850–1858. 8 W. Wang, C. Yang, Y. Liu and C. M. Li, Lab Chip, 2010, 10, 559–562. 9 J.-H. Choi, S.-K. Lee, J.-M. Lim, S.-M. Yang and G.-R. Yi, Lab Chip, 2010, 10, 456–461. 10 N. R. Beer, K. A. Rose and I. M. Kennedy, Lab Chip, 2009, 9, 841–844. 11 C. N. Baroud, M. Robert de Saint Vincent and J.-P. Delville, Lab Chip, 2007, 7, 1029. 12 T. H. Ting, Y. F. Yap, N.-T. Nguyen, T. N. Wong, J. C. K. Chai and L. Yobas, Appl. Phys. Lett., 2006, 89, 234101. 13 M. Chabert, K. D. Dorfman and J.-L. Viovy, Electrophoresis, 2005, 26, 3706–3715. 14 D. J. Im, J. Noh, D. Moon and I. S. Kang, Anal. Chem., 2011, 83, 5168–5174. 15 P. Abbyad, R. Dangla, A. Alexandrou and C. N. Baroud, Lab Chip, 2011, 11, 813–821. 16 F. Mugele, Soft Matter, 2009, 5, 3377.
This journal is © The Royal Society of Chemistry 2015
Paper
17 S. K. Chung, K. Rhee and S. K. Cho, Int. J. Precis. Eng. Man., 2010, 11, 991–1006. 18 P. Brunet, M. Baudoin, O. B. Matar and F. Zoueshtiagh, Phys. Rev. E: Stat., Nonlinear, Soft Matter Phys., 2010, 81, 036315. 19 L. Y. Yeo and J. R. Friend, Biomicrofluidics, 2009, 3, 012002. 20 A. A. Darhuber, J. P. Valentino, S. M. Troian and S. Wagner, J. Microelectromech. Syst., 2003, 12, 873–879. 21 A. A. Darhuber, J. P. Valentino and S. M. Troian, Lab Chip, 2010, 10, 1061. 22 B. Selva, I. Cantat and M.-C. Jullien, Phys. Fluids, 2011, 23, 052002. 23 Y. Xia and G. M. Whitesides, Annu. Rev. Mater. Sci., 1998, 28, 153–184. 24 B. Selva, J. Marchalot and M.-C. Jullien, J. Micromech. Microeng., 2009, 19, 065002. 25 R. Dangla, F. Gallaire and C. N. Baroud, Lab Chip, 2010, 10, 2972–2978. 26 S. Hodges, O. Jensen and J. Rallison, J. Fluid Mech., 2004, 501, 279–301. 27 A. M. Leshansky and L. M. Pismen, Phys. Fluids, 2009, 21, 023303. 28 M.-C. Jullien, M.-J. T. M. Ching, C. Cohen, L. Ménétrier and P. Tabeling, Phys. Fluids, 2009, 21, 072001. 29 A. M. Leshansky, S. Afkhami, M.-C. Jullien and P. Tabeling, Phys. Rev. Lett., 2012, 108, 264502. 30 M. De Menech, P. Garstecki, F. Jousse and H. Stone, J. Fluid Mech., 2008, 595, 141–161. 31 A. Gupta and R. Kumar, Microfluid. Nanofluid., 2010, 8, 799–812.
Lab Chip
