Femtosecond laser-induced hierarchical micro/nanostructures promote superhydrophobicity in air and excellent underwater superaerophilicity on the polytetrafluoroethylene (PTFE) surface. Immersion of the PTFE surface with superhydrophobic microgrooves in water generates hollow microchannels between the PTFE substrate and the aqueous medium. Underwater gas can flow through this channel. When a microchannel connects two underwater bubbles, gas is spontaneously transported from the small bubble to the large bubble along this hollow microchannel. Gas self-transport can be extended to more functions related to underwater bubble manipulation, such as unidirectional gas passage and water/gas separation. Credit: Jiale Yong et al
The manipulation and use of gas in water has wide applications in energy use, chemical production, environmental protection, agricultural cultivation, microfluidic chips, and health care. The ability to drive underwater bubbles to move directionally and continuously over a given distance via unique gradient geometries has been successfully archived, opening the way for more research on this exciting topic. In many cases, however, the gradient geometry is microscopic and unsuitable for microscopic gas transport, as most microscale gradient structures provide insufficient driving force. This makes the underwater self-transport of bubbles and gases at the microscopic level a major challenge.
In a new paper published in International Journal of Extreme Manufacturing, a team of researchers led by Prof. Feng Chen from the School of Electronic Science and Engineering, Xi’an Jiaotong University, China, proposed a novel strategy for underwater gas self-transport on a femtosecond laser-induced open superhydrophobic surface with sub-100 µm microchannel width. The microgroove with superhydrophobic and underwater superaerophilic micro/nanostructures on its inner wall cannot be wetted by water, so a hollow microchannel is formed between the substrate and water when the groove-structured surface is immersed in water. The gas can flow freely through the underwater microchannel; this means that this microchannel allows gas transport in the water. Superhydrophobic microgrooves enable the self-transport of bubbles and gases at the microscopic level.
Femtosecond (10-15 s) laser technology has emerged as a promising solution for preparing such a superhydrophobic microgroove. Taking advantage of their two key characteristics: extremely high peak intensity and ultrashort pulse width, femtosecond lasers have become an essential tool for today’s extreme and ultra-precision manufacturing. Femtosecond laser processing has the characteristics of high spatial resolution, small heat-affected zone, and non-contact manufacturing. In particular, a femtosecond laser can ablate almost any material, resulting in microstructures on the surface of the material. Thus, the femtosecond laser is a viable tool to create superhydrophobic microstructures on material surfaces, which is essential for realizing gas self-transport at the microscopic level.
Hierarchical micro/nanostructures are easily fabricated on the intrinsically hydrophobic polytetrafluoroethylene (PTFE) substrate by femtosecond laser processing, giving the PTFE surface excellent superhydrophobicity and underwater superaerophilicity. The femtosecond laser-induced superhydrophobic and underwater superaerophilic microgrooves strongly repel water and can support underwater gas transport because a hollow microchannel is formed between the surface of PTFE and the aqueous medium in water. Underwater gas is easily transported through this hollow microchannel.
Interestingly, when superhydrophobic microgrooves connect different superhydrophobic regions in water, gas spontaneously transfers from a small region to a large region. A unique laser drilling process can also integrate the microholes into the superhydrophobic and underwater superaerophilic PTFE sheet.
The asymmetric morphology of the femtosecond laser-induced “Y”-shaped microholes and the unique superwettability of the PTFE sheet surface allow gas bubbles to pass unidirectionally through the porous superwetting PTFE sheet (from the small hole side to the large hole side) into the water.
One-way anti-buoyancy penetration was achieved; this means that the gas overcomes the buoyancy of the bubble and self-transports downward. Similar to the diode, the one-way gas passage function of the superwetting porous sheet was used to determine the direction of gas transport in underwater gas manipulation, preventing gas backflow.
The Laplace pressure difference drives the processes of spontaneous gas transport and one-way bubble passage. Superhydrophobic and underwater superaerophilic porous sheets have also been successfully used for water-gas separation based on gas self-transport behavior.
Professor Feng Chen (Director of the Ultrafast Photonic Laboratory, UPL) and Associate Professor Jiale Yong identified the significance of the research and the potential applications of this technology (undersea gas self-transport) as follows:
“How to think about using superhydrophobic microgrooves for gas transport?”
“Superhydrophobic microstructures have high water repellency, which enables materials to repel liquids. If the microgroove has superhydrophobic micro/nanostructures on its inner wall, the microgroove will not be wetted by water because the groove-structured surface is immersed in water. Therefore, between the substrate and the water medium, a hollow microchannel is formed. This microchannel allows the transport of gas in water, so that the gas can flow freely along the underwater microchannel. The femtosecond laser can easily produce such a superhydrophobic microgroove. The width of the laser-induced microgroove determines the width of the the hollow microchannel, which is smaller than 100 μm, allowing us to realize gas self-transport at the microscopic level.”
“Why was a femtosecond laser used to prepare such a superhydrophobic microgroove for gas self-transport?”
“The laser is one of the greatest inventions of the 20th century. In recent years, the femtosecond laser has become an essential tool for today’s extreme and ultra-precision manufacturing. Femtosecond laser processing is a flexible technology that can directly record superhydrophobic and underwater superaerophilic microgrooves on the surface of a solid substrate and drill open microholes through a thin film. Furthermore, the traces of the open microgrooves and the location of the open microholes can be precisely designed by the control program during laser processing.”
“Do gas species affect the self-transport of bubbles and gases at the microscopic level?”
“Although only the simple air bubble was studied, it should be noted that the driving force for gas transport does not involve the chemical composition of the gas. Therefore, the gas manipulation reported in this paper is applicable to other gases as long as they do not dissolve completely in the respective liquids.”
“What are the potential applications of the technology to achieve self-transport and bubble/gas manipulation based on femtosecond laser-written superhydrophobic microgrooves?”
“We believe that the reported methods of gas-in-water self-transport along femtosecond laser-patterned superhydrophobic microchannels will open up many new applications in energy utilization, chemical production, environmental protection, agricultural cultivation, microfluidic chips, healthcare, etc. “
The researchers also point out that this self-transporting gas strategy based on the superhydrophobic microgrooves, although validated, is still in its infancy. The effect of various factors (such as microgroove size, channel length, and gas volume) on gas transport efficiency needs further research. Practical applications based on the gas self-transport feature also need to be developed.
New technology could help repel water, save lives through improved medical devices More information: Jiale Yong et al, Underwater self-transport of gas along femtosecond laser-written open superhydrophobic surface microchannels (International Journal of Extreme Manufacturing (2021). DOI : 10.1088/2631 -7990/ac466f
Courtesy of International Journal of Extreme Manufacturing
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