Molecular Approach for Understanding the Stability, Collision, and Coalescence of Bulk Nanobubbles

Though long-lived nanobubbles (NBs) have been reported by multiple researchers, the underlying reason behind their stability is still obscure. Some of the conjectured reasons include diffusive shielding, the presence of surface charges, and stability due to contamination. Still, the stability of NBs against coalescence and Ostwald ripening is not confirmed. Using molecular dynamics simulations, the present study aims to understand the stabilization effects due to diffusive shielding and the presence of an electrical double layer at the surface of NBs. Accumulation of charges on NBs for different concentrations of ions is discussed. Also, the collision of equal-sized NBs with different approach velocities and offset distances is simulated. A regime map is predicted on the basis of initial approach velocity and offset distance. The transition in regime obtained upon increasing the offset distance is discussed, which differs from the collision characteristics of macroscopic bubbles and drops. The merging of NBs is initiated through the bridge formation, for which the temporal evolution rate along with the scaling argument is presented. The stress terms involved and the corresponding regimes are predicted based on the fluid properties. For all the cases where merging is observed, the estimated probability is observed to be low, which suggests the stability of NBs against coalescence.