Droplet ejection in laser-induced forward transfer: mechanism for droplet fragmentation

Laser-induced forward transfer is a direct-write method suitable for precision printing of various materials. However, occasional defects (i.e. contamination of the receiver due to the impact of multiple small droplets instead of a single droplet) hamper a widespread application of this method. As the ejection mechanism has not been visualized in detail, the cause of these defects is not understood as yet. Therefore, this article presents an experimental study on the ejection process mechanisms of copper-based picosecond laser-induced forward transfer. Images were obtained using bright field illumination by a 6 ns pulsed laser and a 50× long-distance microscope objective. For laser fluences just above the transfer-threshold, the release of a single droplet is frequently (97%) observed. The typical droplet radius in these cases is estimated to be 3 μm. However, images acquired at a later time in time show multiple droplets in the majority (86%) of the observations. The droplet fragments usually follow the main droplet. Two mechanisms to explain these fragments are proposed: i) break-up of “threads” between the donor layer and the ejected droplet; ii) contraction of the ejected droplet. As the phase of the ejected copper is not identified completely, the exact mechanism is not yet known and will be subject of further research.