Pneumatic dispensing of nano- to picoliter droplets of liquid metal with the StarJet method for rapid prototyping of metal microstructures

Abstract

This study presents a new, simple and robust, pneumatically actuated method for the generation of liquid metal micro droplets in the nano-to picoliter range. The so-called StarJet dispenser utilizes a star-shaped nozzle geometry that stabilizes liquid plugs in its center by means of capillary forces. Single droplets of the liquid metal can be pneumatically generated by the interaction of the sheathing gas flow in the outer grooves of the nozzle and the liquid metal. For experimental validation, a print head was build consisting of silicon chips with a star-shaped nozzle geometry and a heated actuator (up to 280°C). The silicon chips are fabricated by Deep Reactive Ion Etching (DRIE). Chip designs with different star-shaped geometries were able to generate droplets with diameters in the range of the corresponding nozzle diameters. The StarJet can be operated in two modes: Either continuous droplet dispensing mode or drop on demand (DoD) mode. The continuous droplet generation mode for a nozzle with 183 lm diameter shows tear-off frequencies between 25 and 120 Hz, while droplet diameters remain constant at 210 lm for each pressure level. Metal columns were printed with a thickness of 0.5-1.0 mm and 30 mm height (aspect ratio [30), to demonstrate the directional stability of droplet ejection and its potential as a suitable tool for direct prototyping of the metal microstructures.

Enhanced Liquid Metal Micro Droplet Generation by Pneumatic Actuation Based on the StarJet Method

“…Because the design changes described before, the StarJet droplet generator is now able to dispense at significantly higher frequencies in continuous operation mode. Therefore, it is no longer feasible to measure the droplet generation frequency using the acoustic method applied in [15]. We therefore mounted a high speed camera (imagine source, MINI 2) next to the nozzle outlet for determining the dispensing frequencies visually.…”

“…The body of the previously published actuator modules of the StarJet droplet generators V0 and V1 were made of brass providing good properties in terms of thermal conductivity and ease of mechanical machining [15,16]. During the experimental characterization of these devices, however, we observed the brass being corroded after contact with molten solder.…”

“…However, the frequency is also influenced by the design of the nozzle chip and fluid properties, like the viscosity and surface tension. A detailed description of the method can be found in [15]. All connections are sealed by high temperature stable materials.…”

Plug-in nanoliter pneumatic liquid dispenser with nozzle design flexibility

“…4,5 To dispense liquids in minute volumes, dispensers have been developed for use with different actuating methods such as thermal-bubble, piezoelectric, pneumatic, and solenoid actuation. [6][7][8][9][10] Although commercial dispensers have high reliability and accuracy, there is the inevitable issue of channel clogging induced as a result of the narrow liquid paths inside the dispenser and various liquid properties. 11,12 Therefore, frequent maintenance is required.…”

“…Specifically, clamping and thread-tightening have been chosen previously. [8][9][10] However, the reported integration procedures are cumbersome and time-consuming and hinder prompt operation. For this reason, the need for simplicity of integration has been increasing.…”