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Mass Transfer via Femtoliter Droplets in Ping-Pong Mode

Droplets with diameters in the micrometer range find numerous applications, for example, as biological reaction compartments. In that context, adding and subtracting the smallest amounts of liquid in a well-controlled manner is a key step. Here, we demonstrate a principle that allows the transfer of tiny amounts of dissolved species to an aqueous femtoliter droplet reciprocating between two aqueous reservoirs under the influence of a dc electric field. Mass transfer is size selective and adaptive, for example, via tuning the viscosity of the surrounding oil phase or the electric-field strength. We provide a map of the dynamic regimes, indicating under which conditions the reciprocating droplet motion occurs. We also formulate a model based on diffusive mass transfer that describes the amount of species taken up and transferred by the droplet. Overall, our results significantly help to facilitate the handling and manipulation of femtoliter droplets.