Long-Term Stability of Nanobubbles Generated via Pressure Oscillation-Hydrodynamic Cavitation: A Rapid Assessment by UV–Vis Spectrophotometry

The long-term stability of bulk nanobubbles is crucial for their functional applications; however, understanding the evolution of their size distribution remains a significant challenge. While conventional characterization methods, such as Dynamic Light Scattering and Nanoparticle Tracking Analysis, provide size information, they are often sample-intensive and expensive, making them ill-suited for high-throughput or long-term dynamic monitoring of size distribution polydispersity. This research validated UV-Vis spectrophotometry as a simple, powerful tool for tracking these dynamic changes. Air nanobubbles generated via pressure oscillation-hydraulic cavitation were systematically monitored over 30 days using correlative DLS, NTA, and UV-Vis spectroscopy. A distinct two-stage evolution was identified: an initial "purification" phase marked by the dissolution of unstable bubbles, followed by a long-term "maturation" phase governed by Ostwald ripening. The Ångström exponent (<i>n</i>), derived from the full extinction spectrum, is a highly sensitive descriptor of this process. The evolution of <i>n</i> traced a unique V-shaped trajectory, which resulted in a pronounced hysteresis loop when plotted against the mean diameter from DLS. This hysteresis reveals that systems with identical mean diameters can possess vastly different distribution morphologies, which are inaccessible through traditional sizing methods alone. This research establishes full-spectrum UV-Vis analysis as a robust methodology, enabling rapid and efficient assessment of nanobubble stability and providing a deeper mechanistic understanding of their complex evolution.