Formation of microtower structures on nanosecond laser ablation of liquid metals

Impacts of Ambient and Ablation Plasmas on Short- and Ultrashort-Pulse Laser Processing of Surfaces

“…The limiting factors for the length are discussed below. Similar microtowers formed in different ambient gases at 1 bar (SF6, air, and N2) exhibit strong dependence of the growth rate on chemical activity of the ambient gas [70]. The largest rate for liquid gallium in SF6 environment (~9 μm/pulse) was at least ten times higher than in a nitrogen atmosphere.…”

“…In this section, we discuss the importance of chemical reactions on the irradiated surface on the example of recently discovered phenomenon, the growth of microstructures upon laser ablation of liquid metals [70]. Up to date, very few systematic studies are known on laser ablation of liquid metals.…”

“…Some examples include comparison of ablation dynamics for metals in solid and liquid states [71][72][73], thermal modeling of molten gallium ablation to understand the causes of high energies of the expanding ablation products [74], experiments on imaging the laser ablation dynamics and the irradiated surface response [75,76], generation of hot electrons and X-rays under fs laser ablation of molten metals [77], and GaN film deposition by pulsed laser ablation of liquid gallium in a nitrogen atmosphere [78]. In [70], during multishot ns laser irradiation (IR and UV) of liquid metals in the presence of reactive ambient gases, a tower-like structure (called below "microtower") with the diameter somewhat exceeding the irradiation spot size was growing on the irradiated surface at an average rate of 3-20 μm per pulse. The growth rate depends on laser fluence and metal type.…”

“…The tip unsealed by the laser is shown in Figure 8c, demonstrating a layer of ~50-μm thick oxide, under which the pure SnPb alloy with initial composition is seen. The scenario of tower-like structure growth has been revealed in [70], which can be described on the example of ablation in air as follows. A drop of liquid metal being exposed to a chemically active atmosphere is covered by a solid layer of oxide.…”

“…The main parameter that limits the microtower length is the strength of the chemically produced solid film covering the microstructure. At reaching a definite "critical" length, specific for each pair "metal-ambient gas", the microtower breaks or is even sinking into the molten target [70]. It was found that the higher is laser fluence, the longer structure can be grown.…”