The invention provides an alternating current electrospray technology that can generate micron sized droplets in oil at very high throughput for emulsion or digital PCR (Polymerase Chain Reaction). This technology outperforms the throughput of the current gold standard in droplet generation using flow-focusing technology by at least a factor of 100. The design is simple and can generate a billion to a trillion monodispersed droplets in about one hour. This is much faster than flow-focusing which is limited to a few million droplets per hour. The droplet size and generation rate can also be easily adjusted by changing the voltage of the AC electric field. The range of produced droplet sizes is about 1-100 microns, wherein the droplets are monodispersed in size.