Direct Observation of the Concentration Gradient upon Laser-Induced Crystallization by Using High-Speed Shadowgraphy

We performed high-speed observation of the concentration gradient induced by focused irradiation of a supersaturated solution of anthracene with femtosecond laser pulses, which could lead to crystallization. We found that anthracene crystals were formed in the area where a laser-induced cavitation bubble was formed. The high-speed shadowgraph and Schlieren imaging with about 500,000 fps visualized the formation of large refractive index gradients upon the shrinkage and collapse of a laser-induced ellipsoidal cavitation bubble. The refractive index gradients were diffused in about 200 μs with long-lasting bubbles, which were generated after the cavitation bubble collapsed. We then discussed the impact of concentration and temperature on the refractive index change from the viewpoints of the presence of solute in solution, crystal growth rate, and molecular diffusion coefficient. This revealed that the refractive index gradients were mainly caused by a concentration increase related to the cavitation bubble shrinkage and collapse processes, which could not be predicted by previously reported theoretical works. These results provide clear insights into the underlying mechanism of the laser-induced crystallization on the basis of local and transient concentration changes via cavitation bubble behaviors.