Analysis of nano bubble collapse process based on molecular simulation method

Abstract

A molecular dynamics simulation method is used to study the process of nanobubbles under the action of shock waves, which gradually depresses and develops to collapse, and this paper focuses on the mechanism of the impact velocity and bubble size on the kinetic properties of nanobubble collapse. The results show that the collapse of nanobubbles goes through three stages. The preferred stage is the compression of water molecules on the outside of the bubble, followed by the destruction of the stable structure of the liquid film caused by the shock wave, and finally develops to the stage of complete bubble collapse; when the impact velocity is larger, the smaller size bubbles have shorter bubble collapse time under the action of the stronger impact effect; the nanobubbles form a bulge at the right end of the velocity contour after the collapse of the high-speed jet, and the degree of the bulge increases with the increase of the bubble size and the impact velocity, and water molecules move to the center of the bubbles. Large, water molecules to the center of the bubble convergence, the formation of vortex structure above and below the bubble, effectively enhancing the fluid internal mass transfer; with the increase in bubble size and impact velocity, the density around the bubble is also gradually increasing, the bubble completely collapsed when the local density up to 1.5 g/cm³ nearby; water hammer impact time in the bubble volume attenuation of 50% after the increase in bubble size and impact velocity, water hammer impact is more and more important, the water hammer impact is more and more important. With the increase of bubble size and impact velocity, the water hammer impact is more and more obvious, for <i>u_p</i>=3.0 km/s, <i>D</i>=10 nm nano-bubble structure after the collapse of the jet water hammer impact formed by the local pressure up to 30 GPa.