Formation of microstructure on liquid metal surface under nanosecond laser ablation

We report a new phenomenon, formation of microstructures, observed at multipulsed nanosecond laser ablation of liquid metals (Ga, In, Sn-Pb alloy, Wood's metal). Laser irradiation of liquid metal targets was carried out in a gas chamber equipped with a heater. In contrast to vacuum conditions or an inert atmosphere when a crater is formed which is healed after termination of irradiation, ablation in a reactive ambient gas (air, nitrogen, sulfur hexafluoride, nitrogen trifluoride) leads to a horn-like structure growing on the irradiated surface with the rate of 3-20 μm per pulse depending on laser fluence and the types of metal and ambient gas. The interplay between different processes in a heat-affected zone of the irradiated samples is analyzed, including ablation, thermal expansion, temperature variations of viscosity, surface tension, thermal stresses, capillary and plasma effects, and surface chemistry. A clear picture of microstructure origin has been established and a qualitative modeling representation is given to explain the growth process of microstructures. The optimal conditions of microstructure growth have been determined and perspective applications of the discovered effect are discussed.

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