Researchers at Xi’an Jiaotong University have devised a potential solution for the self-transport of gas underwater using a femtosecond laser. The team created a superhydrophobic surface with a hollow microchannel less than 100 µm wide. The microchannel enabled underwater self-transportation of gases and bubbles at the microscopic level. The researchers believe the advancement supports applications including chemical manufacturing, environmental protection, and use of microfluidic chips. In demonstrations, when a microchannel connected two underwater bubbles, the gas spontaneously traveled from the small bubble to the large bubble along the microchannel. Gas self-transportation can be extended to functions related to manipulating bubbles underwater, such as unidirectional gas passage and the separation of water and gas, the research team said. The team used femtosecond lasers to directly write microgrooves on a polytetrafluoroethylene (PTFE) substrate. The nano- and microstructures on a microgroove’s inner wall made the microgroove water-repellent, giving the substrate superhydrophobic properties in air and superaerophilic — or oxygen-motivated — properties underwater. Immersing the PTFE substrate in water generated hollow microchannels between the substrate and the water medium, through which underwater gas flowed. “Superhydrophobic microstructures have great water repellence, allowing the materials to repel liquids,” researchers Feng Chen and Jiale Yong said. “If a microgroove has superhydrophobic micro- and nanostructures on its inner wall, the microgroove will not be wetted by water when the groove-structured surface is immersed in water. Therefore, a hollow microchannel is formed between the substrate and water medium.” The femtosecond laser-induced hierarchical micro- and nanostructures promote superhydrophobicity (water-repelling quality) in air and excellent underwater superaerophilicity (attracting air) on the PTFE surface. Immersing the PTFE surface with superhydrophobic microgrooves in water generates hollow microchannels between the PTFE substrate and the water medium. Underwater gas can flow through this channel. Courtesy of Jiale Yong et al. When two different superhydrophobic regions — located underwater — are connected by the microgrooves, gas spontaneously traveled along the hollow microchannel from a small to a large region. The width of the laser-induced microgroove determined the width of the hollow microchannel, which was less than 100 μm and enabled the team to realize gas self-transportation at a microscopic level. To extend gas self-transportation and achieve anti-buoyancy unidirectional penetration, the researchers used a femtosecond laser drilling process to integrate microholes into a thin PTFE sheet. The asymmetric morphology of the microholes and the surface superwettability of the PTFE sheet caused gas bubbles to travel unidirectionally through the PTFE sheet when it was submerged in water. The unidirectional transport of gas through the superwetting, porous sheet can be used to manipulate the direction of gas transport in order to prevent gas backflow, and the PTFE sheets can also be used to separate water from gas. Chen and Yong relied on the special characteristics of the femtosecond laser, including its extremely high peak intensity, ultrashort pulse width, high spatial resolution, and ability to ablate almost any material. The technique to transport gas underwater does not depend on the chemical composition of the gas and can be used with a range of gases. The researchers said further investigation is needed to determine how factors like the size of the microgrooves, the length of the channel, and the volume of gas affect gas transportation. The research was published in the International Journal of Extreme Manufacturing (www.iopscience.iop.org/article/10.1088/2631-7990/ac466f).