Direct and opposite droplet ejections from metal films induced by nanosecond laser pulses: experimental observation and lattice Boltzmann modeling

Laser-induced forward transfer is a versatile method for the fabrication of surface microstructures. In this paper, the material ejection from copper films induced by nanosecond laser pulses was studied. Two reversely oriented ejections were induced simultaneously by a single laser shot. The opposite ejected droplets were significantly smaller than the direct ejected ones, and were separated from the remnant donor material clearly, which might be utilized to fabricate microstructures on the donor substrates. Three distinct ejection regimes: no ejection, stable ejection and sputtering were characterized with respect to the laser intensity. The thermal field and the phase transition within the donor film were analyzed with a finite element model. Correlations between the phase transitions and the ejection regimes were noticed, suggesting that the material ejection was mainly caused by the cavitation effect. The dynamics of the laser-induced cavitation bump and the formation of the jets were simulated with a vapor-liquid lattice Boltzmann model. The formation of the bump and the material ejections were clearly demonstrated. It was concluded that the jets were mainly induced by the collision of the cumulating flows, which were generated within the bump shell by the asymmetrical cavitation of the bump. The dynamics of the vapor also plays an important role in the development of the jets. The simulation results agree well with the present experimental observations.