Microfluidic Laser-Induced Nucleation of Air Microbubbles and Crystals in Urea–Isopropanol Solutions

The conditions for the laser-induced nucleation of urea in isopropanol were investigated. A capillary-based microfluidic system was used to expose supersaturated urea–isopropanol to nanosecond pulses of 1064 nm light. We determined the critical supersaturation of the system to be S = 2.2 (equivalent to 6.4 g of urea per 100 g of isopropanol with respect to saturation at 21.5 °C), where crystals spontaneously nucleated in flow and needle crystals were observed. Smaller aspect-ratio prismoid crystals were observed after irradiation of supersaturated solutions with a supersaturation of S = 1.65. We noted that a few microbubbles were moving with the crystals and found that microbubbles can be readily generated in the urea–isopropanol solutions by the addition of iron(II, III) oxide nanoparticles. In flow experiments, the bubbles had an average diameter of 19 μm. Using an inline degasser, we were able to suppress the formation of bubbles, confirming that they are formed from dissolved air in the solution. Needle crystals were observed to transform into smaller aspect-ratio prismoid crystals as the solution warmed to room temperature (21.5 °C). This suggests that the needle crystals are the kinetically favored habit at high supersaturations in urea–isopropanol solutions. We discuss our results in the context of the impurity-heating mechanism for nonphotochemical laser-induced nucleation (NPLIN). The findings presented here provide additional support for the impurity-heating mechanism of NPLIN.