MOLECULAR MODELLING OF AQUEOUS NANOBUBBLES UNDER NON-EQUILIBRIUM SITUATIONS

Abstract

Nanobubbles have attracted considerable attention due to their unique physicochemical properties and potential applications in various fields including water treatment and biomedical sciences. Thiing with lower density nanobubble. For higher initial densis study presents a molecular dynamics simulation framework to investigate the stability and behavior of oxygen nanobubble in SPC/E water using equilibrium molecular dynamic and non-equilibrium molecular dynamic methods under shear flow conditions. The initial configuration of the system was set using Packmol software to create a spherical arrangement of oxygen molecules with radius of 2 nm surrounded by water molecules within a cubic box with dimensions of 10×10×10 nm. The oxygen nanobubble systems with different initial densities (6.2 nm⁻³ and 4.14 nm⁻³) was studied to analyze performance of the bulk nanobubble at periodic boundary conditions. Large-scale atomic-molecular massively parallel simulator (LAMMPS) was used to conduct an NVT-NPT-NVT ensemble sequence for a total of 3 ns for each three runs with a time step of 1 fs. Thermodynamic properties such as temperature, pressure and density were monitored throughout the simulation to ensure equilibrium was reached. The results showed that higher initial gas density resulted in stable nanobubble over the simulation time, comparing with lower density nanobubble. For higher initial density nanobubble NEMD simulations were used to study the effect of various shear rates along the xz plane, in order to evaluate the behavior of nanobubble. Shear flow can significantly influence nanobubble structural and dynamic properties, affecting their stability, dissolution, and interactions with the surrounding medium. Under low shear conditions, nanobubbles tend to remain stable. Even at a shear rate of 10⁻⁶ fs⁻¹, which is still high for real systems is equal to 109 when converting to s-1, the bubble is stable within the 10 ns time scale of our simulation, suggesting that this is a threshold below which shear forces do not have a significant effect on bubble stability for this size of oxygen nanobubble. However, increasing shear rate (10-5, 10-4, 5×10-5 fs-1) can lead to increased gas dissolution and reducing bubble lifetime. High shear forces can deform nanobubble, changing their shape especially at shear rate equal to 10-3 fs-1.