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On-chip manipulation of free droplets

Tiny free-floating drops can be driven across a liquid medium by an electric field.


'Lab-on-a-chip' systems resemble factories with permanently rigged pipes, but their prefabricated microchannels could have problems in delivering materials such as suspended particles, biological cells or proteins, which may adhere to the walls and clog the channels. More flexible microfluidic systems allow liquids to be transported as droplets on a solid surface1,2,3,4,5,6,7,8,9,10, but these suffer from similar drawbacks where the droplets are in contact with solid walls. Here we describe a liquid–liquid microfluidic system for manipulating freely suspended microlitre- and nanolitre-sized droplets of water or hydrocarbon, which float on a denser, perfluorinated oil and are driven by an alternating or constant electric field applied by arrays of electrodes below the oil. These microfluidic chips could be used as a versatile tool in microscale transport and mixing and in chemical and materials synthesis.

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Figure 1: Schematics of the dielectrophoretic transporter used for the operation of a liquid–liquid microfluidic chip with freely suspended droplets.
Figure 2: Behaviour of freely suspended droplets above a liquid–liquid microfluidic chip (for movies, see supplementary information).

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Correspondence to Orlin D. Velev.

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The authors declare no competing financial interests.

Supplementary information


Supplementary Movie 1: Controlled mixing of droplets - 750 nL droplets containing white polystyrene latex and gold nanoparticles merge at the convergence of the two tracks of electrodes, and the mixed droplet moves on the single track. Compare with Fig. 2a-b from the communication. (MP4 158 kb)


Supplementary Movie 2: Mixing of droplets with aggregated particles on a matrix - Two droplets containing polystyrene (white) and magnetic (brown) latex have been mixed by moving them in arbitrary direction on an electrode array to temporarily form anisotropic polymer aggregate. Compare with Fig. 2c from the paper. (AVI 937 kb)


Supplementary Movie 3: Multistage process - mixing and encapsulation - Droplets of aqueous suspensions of gold nanoparticles and of white polystyrene latex (750 nL each) were mixed and subsequently encapsulated inside a 1000 nL dodecane droplet transported separately. Encapsulation is facilitated by 2 mM Na-dodecyl sulfate dissolved inside the aqueous droplets. The columns of electrodes are energized at 400V/200Hz. Compare with Fig. 2d from the paper. (MP4 173 kb)

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Velev, O., Prevo, B. & Bhatt, K. On-chip manipulation of free droplets. Nature 426, 515–516 (2003).

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