StarJet: Pneumatic Dispensing of Nano- to Picoliter Droplets of Liquid Metal

In this work we present a novel, simple and robust, pneumatically actuated dispenser for nano- to picoliter sized droplets of liquid metals. The so called StarJet dispenser utilizes a star-shaped nozzle geometry that stabilizes plugs of liquid in the centre of the nozzle by capillary force. This minimizes the wall contact of the liquid plug and reduces contact line friction. Individual droplets of liquid metal can be pneumatically generated by interplay of the sheathing gas flow in the outer grooves of the nozzle and the liquid metal. The working principle was first discovered and studied by computational fluid dynamic (CFD) simulations. For experimental validation silicon chips with the star-shaped geometry were fabricated by deep reactive ion etching (DRIE) and assembled into a printhead. With different nozzle chips volumes between 120 pl and 3.6 nl could be generated at natural frequencies of 90 Hz and 400 Hz. The StarJet can either be operated as drop on demand or as continuous droplet dispenser. We printed columns of metal with 0,5 to 1,0 mm width and 40 mm height (aspect ratio >40) to demonstrate the directional stability of the ejection.