Numerical study of ultrashort laser-induced microjet formation on the metal film based on the Navier–Stokes equation

A two-temperature model (TTM) for the electron-phonon thermal equilibrium is used to determine the heat distribution and laser fluence threshold for melting a thin metal film coated on a glass substrate and irradiated by an ultrashort laser pulse. This study proposes a novel model based on the Navier–Stokes equation to explain the formation of jet-shaped structures in the film's molten region. By solving this equation and obtaining the temporal evolution of the velocity distribution and displacement in the molten region, the Marangoni convection effect can be numerically demonstrated, and the circular motion of the fluid can describe the formation of a jet-shaped structure in the central region of the radiation. The results are compared to those obtained by numerically solving the thermo-elastoplastic equations, and also, to the previously reported experimental results to ensure the accuracy of the microjet height calculated by the Navier–Stokes equation. Good agreement is observed, particularly when the temperature of the irradiated area is significantly over the film's melting temperature. In addition, several calculations are performed for various pulse fluences. In both models, increasing the pulse fluences leads to an increase in the height of microjets.