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.

中文翻译：

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在乒乓球模式下通过飞升液滴进行传质

直径在微米范围内的微滴有许多应用，例如作为生物反应隔室。在这种情况下，以可控的方式添加和减少最少量的液体是关键步骤。在这里，我们演示了一个原理，该原理允许在直流电场的影响下，将少量溶解的物质转移到在两个水库之间往复运动的飞升液滴中。传质是尺寸选择的和适应性的，例如通过调节周围油相的粘度或电场强度。我们提供了动态范围的地图，指出了在哪些条件下往复液滴运动发生。我们还基于扩散质量转移制定了一个模型，该模型描述了液滴吸收并转移的物种数量。