A molecular dynamics simulation of the abrupt changes in the thermodynamic properties of water after formation of nano-bubbles / nano-cavities induced by passage of charged particles

Abstract

We present a multi-scale formalism that accounts for the formation of nano-scale bubbles/cavities owing to a burst of water molecules after the passage of high energy charged particles that leads to the formation of hot non-ionizing excitations or thermal spikes (TS). We demonstrate the coexistence of a rapidly growing condensed state of water and a hot spot that forms a stable state of diluted water at high temperatures and pressures, possibly at a supercritical phase. Depending on the temperature of TS, the thin shell of a highly dense state of water grows by three to five times the speed of sound in water, forming a thin layer of shock wave (SW) buffer, wrapping around the nano-scale cylindrical symmetric cavity. The stability of the cavity, as a result of the incompressibility of water at ambient conditions and the surface tension, allows the transition of supersonic SW to a subsonic contact discontinuity and dissipation to thermo-acoustic sound waves. We further study the mergers of nanobubbles that lead to fountain-like or jet-flow structures at the collision interface. We introduce a time delay in the nucleation of nano-bubbles, a novel mechanism, responsible for the growth and stability of much larger or even micro-bubbles, possibly relevant to FLASH ultra-high dose rate (UHDR). The current study is potentially significant at FLASH-UHDRs. Our analysis predicts the black-body radiation from the transient supercritical state of water localized in nano-cavities wrapping around the track of charged particles can be manifested in the (indirect) water luminescence spectrum.