Producing molten metal droplets with a pneumatic droplet-on-demand generator

A pneumatic droplet generator to produce molten metal droplets on demand is described. It consists of a cylindrical heated chamber with a small nozzle set into its bottom surface, connected to a gas cylinder through a solenoid valve. Opening and closing the valve for approximately 10–12 ms imposes a pressure pulse on the molten metal in the chamber, ejecting a single droplet through the nozzle. Pressure in the chamber then drops rapidly as gas escapes through a vent hole and draws back liquid from the nozzle, preventing more droplets from escaping. By adjusting the duration of the pressure pulse we can obtain a single droplet each time a pulse is applied. Droplets of molten indium, tin, lead, and zinc were produced with the droplet generator. Drops with diameters ranging from 0.17 to 0.60 mm were formed using nozzles 0.076–0.254 mm in diameter. It was important to keep the oxygen content in the test chamber below 150 ppm, otherwise metal oxidized as it emerged and blocked the nozzle. By coordinating droplet formation with the movement of an x–y stage placed beneath the droplet generator, droplets could be place in arbitrary patterns such as a straight line, circle, triangle or grid.