Microfluidic devices have been downscaled to dimensions of 10–1000 nm. Manipulation methods for femtoliter samples are important for realizing novel analytical devices. In the present study, we developed a microfluidic device that utilizes two-step flow focusing by air flows to generate femtoliter liquid droplets that float in the gas phase with size and trajectory control. The device includes a branched and stepped hydrophobic microchannel with four air inlets to exploit the instability of the gas–liquid interface. We succeeded in the generation of 704 fL (11.0 ± 0.01 μm) droplets of pure water and a shooting frequency of 24 kHz at a sample flow rate of 1 μL/min. In addition to pure water, we succeeded in generating acetonitrile droplets. The device operation was stable even at a sample flow rate of 10¹ pL/s, which is similar to that used in recent nanofluidic analytical devices. The results confirm that the instability of the gas–liquid interface is the dominant factor in femtoliter droplet generation as designed. Based on the experimental results, our method has a potential to generate droplets with the minimum volume of 123 fL (6.1 μm) in case of pure water, which can be achieved by minimizing the channel size. The present study provides a method based on gas–liquid micro/nanofluidics for the generation of uniform femtoliter droplets with trajectory control. This method is suitable for various applications, such as a size interface for the transport of femtoliter samples from nanofluidic devices to analytical instruments.

微流体设备已缩小到 10-1000 nm 的尺寸。飞升样品的操作方法对于实现新型分析设备很重要。在本研究中，我们开发了一种微流体装置，它利用气流的两步流动聚焦来产生飞升液滴，这些液滴漂浮在气相中，具有尺寸和轨迹控制。该装置包括一个带有四个空气入口的分支和阶梯式疏水微通道，以利用气液界面的不稳定性。我们成功地生成了 704 fL (11.0 ± 0.01 μm) 的纯水液滴，并且以 1 μL/min 的采样流速以 24 kHz 的频率发射。除了纯水，我们还成功生成了乙腈液滴。即使在样品流速为 10 ^{1 时}，设备运行也稳定pL/s，类似于最近的纳米流体分析设备中使用的 pL/s。结果证实，气液界面的不稳定性是飞升液滴产生的主要因素。根据实验结果，我们的方法有可能在纯水的情况下产生最小体积为 123 fL (6.1 μm) 的液滴，这可以通过最小化通道尺寸来实现。本研究提供了一种基于气液微/纳米流体的方法，用于生成具有轨迹控制的均匀飞升液滴。该方法适用于各种应用，例如用于将飞升样品从纳米流体装置传输到分析仪器的尺寸接口。