Cavitation induced by pulsed and continuous-wave fiber lasers in confinement

Bubbles generated with lasers in liquids under confinement have been investigated for their potential use as the driving mechanism for liquid micro-jets in various microfluidic devices, such as needle-free jet injectors. Here, we report on the study of bubble formation by a continuous-wave (CW) and a pulsed laser inside an open-ended microfluidic capillary. This results in a direct comparison between bubbles generated by laser sources emitting light in different time scales (ms and ns). The bubble kinetics represents an important parameter because it determines the available kinetic energy for a subsequent liquid jet. We show that the bubble growth rate increases linearly with the delivered energy for both the CW and the pulsed laser. Experiments show that at equal absorption coefficient, the bubble growth for both lasers is similar, which indicates that they can be used interchangeably for jet generation purposes. However, bubbles generated by a CW laser require more optical energy, which is due to heat dissipation. Furthermore, the bubbles generated by the CW laser show a slightly larger variation in size and growth rate for identical initial conditions, which we attribute to the stochastic nature of thermocavitation.