Film-free laser microprinting of transparent solutions

Abstract

Liquids laser printing has been usually addressed through laser-induced forward transfer, a technique that allows the deposition of microdroplets with good resolution and control. However, it presents a significant drawback that can compromise its future industrial implementation: the need for the preparation of the liquid to be printed in thin film form. Such constraint results especially detrimental when very high degrees of resolution need to be achieved. In order to overcome the problem, we have recently shown that in the case of solutions transparent or weakly absorbing to the laser radiation, liquid printing is possible directly from the liquid contained in a reservoir, without the requirement of thin film preparation. The principle of operation of the film-free laser printing technique is the tight focusing of ultrashort laser pulses underneath the free surface of a liquid. Subsurface absorption leads to the formation of a cavitation bubble through optical breakdown, and the subsequent bubble expansion displaces some liquid towards the substrate, where the pattern is formed. Though the feasibility of the technique for microdroplets printing has already been proved, there is not much insight yet in the mechanisms of liquid ejection and transport. In this work we investigate the mechanisms of liquid printing during film-free laser forward printing. The study, essential for the optimization of the technique, reveals that the process is complex: the bubble expansion-collapse cycle results in the formation of two consecutive jets which display completely different dynamics, and which behavior is strongly dependent on the laser pulse energy density.