Speaker
Description
Recent advances in microfabrication have enabled the development of microscale platforms for biotechnological and chemical applications, opening new perspectives also for the controlled exploitation of hydrodynamic cavitation. Compared to traditional macroscale systems, microfluidic configurations provide enhanced surface-to-volume ratios, improved control of operating conditions, and stronger coupling between interfacial phenomena and flow dynamics. These features make microscale cavitation particularly attractive for applications such as intensified oxidation, improved micromixing, accelerated mass transfer, and enhanced chemical and biological reaction efficiency. In this work, cavitation development in a micro-Venturi channel is experimentally investigated, with the aim of identifying the distinctive characteristics of microscale cavitating flows and their departure from classical macroscale cavitation shedding. The study of cavitation is of growing interest in the context of microscale fluid mechanics representing one of the few examples of turbulent-like dynamics in microchannels and plays a key role in enhancing interfacial transport phenomena beyond purely diffusive mechanisms. The analysis of the two-phase flow reveals a strong coupling between phase change, confinement effects, surface forces, and hydrodynamic instabilities, leading to a flow regime dominated by Kelvin–Helmholtz instability. The study is part of a broader multiphysics investigation aimed at evaluating the effectiveness of cavitation-assisted processes for the decolourization of selected azo dyes, which are of significant concern for water safety in the textile industry. Both cavitation in pure water and cavitation with the addition of H₂O₂ are analysed. Flow dynamics were characterized through the combined analysis of high-speed imaging and pressure measurements, allowing direct correlation between cavitation structures and pressure fluctuations. The comparison between these two cases provides the foundation for the subsequent qualitative and quantitative assessment of the results. Image- and pressure-derived signals supported the formulation of a qualitative hypothesis of the differences observed between the flow regimes, which was subsequently proven using quantitative signal-processing approaches not commonly applied in this field: Fragmentation Level and Wavelet Frequency Analysis. The fragmentation dynamics of cavitation structures were further investigated using image-processing algorithms based on fractal dimension estimation, which proved to be a robust quantitative descriptor of structure morphology. Higher tendency of bubble structures fragmentation is proven in this way for cases having H₂O₂ addition. These results were systematically correlated with Wavelet Frequency spectral analyses of pressure signals, revealing the occurrence of intense bubble collapse events and distinct high-frequency pressure emissions depending on the shape and evolution of the cavitating structures reaching up to 30-80 kHz. Moreover, no high frequency contribution is demonstrated when small vapor structures implode or proximity to big vapor region cause damping effect. Overall, this work provides new insight into how cavitation phenomena evolve when transitioning from macro to microscale systems, highlighting the critical role of confinement and interfacial effects. In addition, it introduces data analysis methodologies that are transferable to a broader class of microscale multiphase flow systems beyond the specific case investigated here.