Cover story: Zhang DS, Li XZ, Fu Y, Yao QH, Li ZG et al. Liquid vortexes and flows induced by femtosecond laser ablation in liquid governing formation of circular and crisscross LIPSS. Opto-Electron Adv 5, 210066 (2022).

Laser induced periodic surface structure (LIPSS) has always been a hot topic in the field of laser micro-nano processing. Despite decades of experimental exploration and theoretical deepening, its formation mechanism is still not very clear. It is generally believed that light field plays a dominant role in the formation of LIPSS, so the mainstream way of regulating LIPSS is based on the adjustment of the spatial and temporal distribution of laser light field. Through the combination of experimental observation and theoretical simulation, this work reveals for the first time that in the liquid environment, femtosecond laser ablation can induce the formation of complex fluids such as liquid vortex, which will break the limitation of the light field to regulate the direction of LIPSS, causing the formation of novel circular and crisscross LIPSS structures. The resulting vortex flow field also exerts ultra-high pressure to its influential region and can give rise to sidewall etching, induce the formation of macropores and cause structural fragmentation. In addition to further enriching the diversity of LIPSS, the newly discovered flow-field-based LIPSS formation mechanism provides a new interdisciplinary perspective to re-examine the complexity and particularity of femtosecond laser ablation in liquids.