Controllable micro-/nanostructures on titanium surface induced by femtosecond laser for underwater air bubble manipulation

Bubbles exist widely in nature and industrial process. The research on bubbles has attracted the attention of many scientists due to the importance of practical application and a large number of studies on bubbles have been carried out since last century. In some cases, the gas in water is often in the form of bubbles, and these bubbles will have some adverse effects on the surface wettability. However, the presence of underwater bubbles also has beneficial effects on natural organisms and human beings. Therefore, it is necessary to study the surface construction of superaerophobicity or superaerophilicity and the manipulation of bubbles. A series of multiscale metallic micro-/nanostructures have been fabricated on typical titanium surface with different morphological features by using self-developed femtosecond laser scanning processing system, which can be controlled through parameters such as laser power, scanning time and scanning intervals. On this basis, the reversible wetting characteristics of multiscale micro-/nanostructures and underwater air bubble manipulation were experimentally investigated by using strategies such as ultraviolet light exposure and heating. The tuning mechanism of reversible wettability was clarified from the perspective of interface chemistry. The research results show that: the original titanium surface wettability has been amplified by multiscale micro-/nanostructures induced by femtosecond laser ablation and solid-liquid contact angle changed to 1° from 48°, being superhydrophilic and bubble contact angle changed to 156° from 136°, being superaerophobic, with bubble sliding angle only being 2° and an extremely low bubble adhesion force. However, under the condition of auxiliary heating, solid-liquid contact angle increased gradually to be superhydrophobic, with liquid sliding angle only being 8°, the surface adhesion force dropping sharply and underwater bubble contact angle rising gradually. The liquid contact angle on the multiscale micro-/nanostructures gradually decreased after the exposure of superhydrophobic surface under the ultraviolet lamp and finally the reversible tuning of superhydrophobic to superhydrophilic could be realized. Furthermore, there is a trend that the reversible tuning characteristics of the liquid contact angle and the bubble contact angle show the opposite way, which is closely related to the moving mechanism of the solid-liquid-gas contact line. The surface micro-/nanostructures with tunable bubble affinity and wettability are prepared by combining the femtosecond laser micromachining technology and the cutting-edge bubble manipulation technology. At the same time, the chemical mechanism of the underwater affinity and wettability of the bubble is studied. The theoretical model of the wettability of the bubble is established and the potential application of the bubble collection is explored so as to solve the problem of the manipulation of the bubble in the aqueous medium. Through our research, the inspirations will be found for the operation of bubbles in industrial production and daily life, which is of great value for dealing with a series of problems caused by bubbles and driving the generation of new bubble manipulation topics. The research results of this paper are also of great significance to the design and fabrication of micro-/nanostructures on typical metal surface and improving the application of metallic surface with reversible wetting characteristics in the fields of underwater bubble manipulation and collection and sewage treatment.