Elucidating ejection regimes of metal microdroplets in voxel-based laser-induced forward transfer

Voxel-based laser-induced forward transfer (VB-LIFT) is a promising micro additive manufacturing process to efficiently print microstructures by ejecting a metal microdroplet array. Elucidating the ejection regimes of microdroplets is crucial to manipulating the printing process. This study investigates the ejecting behaviors of microdroplets induced by irradiating a femtoliter metal voxel with an ultraviolet nanosecond laser. A time-resolved dark-field imaging system was developed to capture the ejecting microdroplets. The ejecting velocity increased linearly with the laser fluence but was independent of the voxel side length. The analysis of the laser energy conversion revealed that one-thousandth of the laser energy was converted to the kinetic energy of the ejecting microdroplets, and the majority of the remaining were converted into internal energy. Meanwhile, the relaxation of the thermal compressive stress induced by laser heating drove the metal voxel detaching from the donor glass to generate a microdroplet. Furtherly, the ejection map in the VB-LIFT is proposed, including the non-detaching, detaching without melting, detaching and melting, and broken regimes. This study provides insights to eject debris-free metal microdroplets with the desired temperature and velocity in laser-induced forward transfer.