Molecular dynamics simulation of bulk nanobubbles

The long-term stability of nanobubbles with high internal gas pressure is a puzzling question for nanobubble researchers. The classical Molecular Dynamics simulation based on LAMMPS software was used to evaluate the performance of an oxygen nanobubble with high gas density. This research consists of two main cases. In the first case, bubble behavior was examined with 3247 O₂ molecules embedded in a 4.5 nm radius spherical volume to represent the bubble and was surrounded by 438,490 H₂O molecules with 1 g/cm₃ density. In the second case, the parametric study was conducted while maintaining the same parameters where the amount of O₂ molecules changed at the initial configuration. Hence six different O₂ bubble configurations were simulated under two different temperature settings, 20 ℃ and 30 ℃. The Lennard Jones potentials are used for molecular interactions. The simulations were run first under the NVT and then changed to the NPT ensemble. Simulation results were analyzed for bubble size, pressure, surface tension, gas diffusion, the influence of internal initial gas densities, gas concentration, and temperature conditions. The high initial gas concentration with high initial internal gas density causes a more stable bubble condition under both NVT and NPT ensembles. This stability can be attributed to the gas supersaturation conditions. The systems with low initial internal gas densities transferred to a smaller radius gas cluster with a high internal density as they shifted to the NPT ensemble. The system with a higher temperature causes elevated system pressures at NVT and volume expansion during NPT. Under NVT simulation, bubble size was higher at lower temperatures. Under NPT conditions, bubble size increases for larger initial density cases and decreases for lower density cases. Further increased temperature causes faster gas diffusion, and higher internal bubble pressure leads to unstable conditions.