Droplet formation on metallic surfaces during low-fluence laser irradiation

We have developed a technique to numerically investigate laser-induced modification of metal surfaces at low laser fluences, where plasma effects and thermal sputtering can be neglected. In this contribution we focus on the technique and the model employed to solve the corresponding Navier-Stokes equations for laser irradiation with an axially symmetric intensity profile. All equations are numerically solved using the method of finite differences. Specifically, the linearized set of the Navier-Stokes equations for an inviscid and incompressible fluid is solved using a slightly modified MAC (marker and cell) method, whereas the thermal behavior of the solid-liquid system is computed using the ADI (alternating direction implicit) algorithm. Under the assumptions made, the heat conduction in the solid and the melt can be computed separately from the actual melt dynamics. The results obtained by solving the heat conduction equation, where the phase transition solid-liquid has been taken into account, are parametrically considered in the MAC module. Our results show that the surface does not remain flat in general after laser irradiation of gold targets for low laser fluences. Small asperities, whose height increases with each consecutive laser shot, are observed.