OsciDrop: A versatile on-demand droplet generator

Droplet microfluidics is a powerful tool in many biological and clinical applications. Microfluidic chips, such as flow-focusing droplet generators, have been extensively used to high-throughput encapsulate reactions with single-cell and single-molecular resolutions. However, microfabrication is expensive and precision-demanding, preventing it from widespread use in biomedical laboratories and clinical facilities. Herein, we present a versatile chip-free droplet generator, OsciDrop, for generating size-tunable droplets on demand, with high uniformity. OsciDrop segments the fluid flowing out of the orifice of a micropipette tip into droplets by oscillating the tip under the surface of a continuous oil phase. We investigated the factors influencing droplet generation by examining several control parameters. Results show that flow rate, oscillating amplitude, and frequency are key parameters to generate monodisperse droplets on demand. And OsciDrop is able to generate droplets in a flexible and repeatable manner. Importantly, using an optimal asymmetrical oscillation waveform, OsciDrop can controllably generate monodisperse droplets spanning a wide volume range (200 pL - 2 L). To demonstrate the ability of OsciDrop for chip-free droplet assays, a digital loop-mediated isothermal amplification (dLAMP) was performed to absolutely quantify African swine fever virus (ASFV). The OsciDrop method opens up a feasible and versatile avenue to perform droplet-based assays, exhibiting full accessibility for chip-free droplet microfluidics.