Multiscale stability analysis of electric-field-based air nanobubbles under varying pH and generation time using experimental and molecular dynamics insights.

Electric-field-based gas nanobubbles (NBs) generation provides a green, chemical-free, and energy-efficient alternative for different environmental applications such as water treatment; however, despite their unique physicochemical properties, a sustainability-oriented understanding of their stability, optimum operating conditions, and the kinetic pathways governing their interfacial evolution remain largely unresolved. This study presents a comprehensive multiscale investigation of air NBs produced under different generation duration and pH conditions, integrating long-term experimental monitoring, hydrodynamic analysis, and molecular dynamics (MD) simulation. The results demonstrate that NB stability and population are optimized when bubbles are generated for approximately 30 min under neutral to mildly alkaline conditions, where enhanced electrostatic repulsion, reduced buoyant rise velocity, and suppressed coalescence collectively prolong NB lifetime while simultaneously minimizing energy demand. Furthermore, MD simulation provided mechanistic insights into NB evolution under different pH conditions, directly reinforcing the experimental observations. The analysis revealed that higher electric-field strength increases NB clustering population while reducing individual NB size and stability duration. Examination of interfacial properties, including surface density, charge distribution, and electrostatic potential, shows that alkaline environments promote the formation of a more ordered hydrogen-bond network driven by ion-induced electrostatic structuring. Conversely, acidic solutions exhibited chloride enrichment and stronger surface charge heterogeneity, whereas the more balanced presence of Na+ and OH- in alkaline media results in a more uniform and near-neutral interfacial environment, consistent with improved stability. This study establishes a predictive mechanistic link between interfacial ion layering and nanobubble longevity, offering valuable guidance for the energy-efficient design of NB technologies in water and environmental engineering technologies, and resource recovery.