High-speed imaging of non-photochemical laser-induced nucleation in aqueous cesium chloride

A study of non-photochemical laser-induced nucleation (NPLIN) of cesium chloride in aqueous supersaturated solutions is presented. Single, unfocused laser pulses (duration 5 ns, peak power density 180 MW cm^-2) of 532 nm laser light were used to induce crystal nucleation, and the resulting dynamics were studied using imaging at frame rates up to 250 000 frames per second (4 μs per frame). Thermocavitation events were observed in a surrounding index-matching fluid, both in the bulk and at the exterior walls of vials. These events were attributed to heating of solid particles by the laser light. The cavities were observed to oscillate in size, with the initial expansion and collapse lasting approximately 50 μs. In some cases, a small persistent gas bubble (lifetime >1 s) was observed after the cavitation event. Within the supersaturated salt solutions, new objects were observed in the image frame of the laser pulse, which faded within 20 μs. These objects were attributed to cavitation: this is the first time that thermocavitation has been observed during NPLIN using an unfocused laser pulse. Crystals were observed to grow at the locations of cavitation events, and the growth was faster at higher supersaturations. Crystals were sometimes observed to form in the location of particles that were observed before the laser pulse, which we consider to be impurity particles that trigger NPLIN. The results provide direct evidence for the nanoparticle-heating mechanism for NPLIN, which begins with thermocavitation. The possible role of stable gas bubbles in NPLIN of crystals is discussed.