Role of dissolved CO₂ in hydrodynamic cavitation on a chip

This study presents an experimental investigation of hydrodynamic cavitation (HC) in two microfluidic chips (microscale HC chips) under varying flow patterns (upstream pressure and local temperature), with and without dissolved CO2. High-speed imaging and spectral analysis were used to characterize cavitation inception, vapor cloud formation, void fraction, and bubble dynamics (frequency spectra) under the effect of the dissolved gas in micro domains. The results show that higher upstream pressure substantially intensifies cavitation, while the presence of dissolved CO2 lowers the pressure threshold for cavitation inception and amplifies cavitation activity. The micro-step chip (Reactor 1) presented more intense cavitation and a greater vapor void fraction than the long-diaphragm chip (Reactor 2) across all conditions. Notably, dissolved CO2 suppressed high-frequency bubble-collapse fluctuations and induced a transition from violent cloud-shedding cavitation to a stable, continuous bubbly flow regime. Additionally, cavitation facilitated significant CO2 degassing, removing up to similar to 30% of the dissolved gas in Reactor 1 (versus similar to 11% in Reactor 2) under the similar conditions. The results also show that temperature significantly influenced CO2 removal efficiency, with the highest elimination (52%) occurring at 25 degrees C, where high gas solubility and low vapor pressure were optimally balanced. These findings highlight the coupled influence of pressure, temperature, and dissolved gas on "HC on a chip" concept and provide fundamental insights into multiphase flow dynamics and bubble-fluid interactions, offering guidance for controlling microscale cavitation and bubble-mediated transport phenomena.