Slingshot droplet generator for clogging-free and tunable production of small uniform drops in air

Submillimeter air-borne drops of well defined size are notoriously difficult to produce in the lab. Here, we demonstrate that any simple droplet dripper, in which millimetric droplets are created by letting them drip under gravity from a hollow needle, is readily turned into a versatile droplet generator capable of producing drops smaller than the needle diameter. This is achieved by vibrating the needle base at a frequency close to a flexing resonance of the needle tube. Through high-speed imaging and detailed image analysis we show that the droplets flung off one-by-one from the needle tip are uniform in size. The produced drop size is mainly set by the driving frequency and is practically independent of needle geometry or driving amplitude, making the generator robust and easily tunable. Like for a simple dripper, the drop ejection rate is independently controlled through the flow-rate. Since the needle diameter can be much larger than the produced drop size, clogging is not an issue. By using needles with different resonant frequencies we could produce droplets with radii anywhere between 100 microns to 1 mm with a high reproducibility (the production of smaller drops is possible in principle). We propose a mass-spring model for the vibrated pendant drop that captures all our observations.