Effect of size and interparticle distance of nanoparticles on the formation of bubbles induced by nanosecond laser

When a laser beam is irradiated on nanoparticles (optical absorbers) in a liquid, massive heat is generated, which causes vaporization of a nearby liquid, followed by the formation of bubbles. Despite the potential application of these bubbles for cancer diagnosis and therapy, solar energy harvesting, and local chemical reaction enhancement, little is known on how the size and interparticle distance of optical absorbers affect laser-induced bubble formation. Additionally, observing the bubble dynamics near nanomaterials is challenging due to their fast growth and immediate collapse. In this study, we trapped bubbles using hydrogels and investigated how their diameter changed with the size and interparticle distance of nanoparticles or their agglomerates and the energy level of laser irradiation. We found that the size of trapped bubbles increased with softer hydrogels, higher laser energy level, larger agglomerate size, and smaller interparticle distance among agglomerates. Based on these observations, we suggest that nanobubbles initially formed on nanoparticles merged to become microbubbles.