Femtosecond laser bionic fabrication enabling bubble manipulation

The manipulation and use of gas in water have broad applications in energy utilization, chemical manufacturing, environmental protection, agricultural breeding, microfluidic chips, and health care. The possibility of driving underwater bubbles to move directionally and continuously over a given distance via unique gradient geometries has been successfully archived, opening room for more research on this exciting topic. In many cases, however, the gradient geometry is microscope and unsuitable for transporting gas at microscope level because most microscale gradient structures provide the insufficient driving force. This makes underwater self-transportation of bubbles and gases at the microscopic level a big challenge.

In a new paper published in the 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, have proposed an innovative strategy for underwater self-transportation of gas along a femtosecond laser-induced open superhydrophobic surface with a microchannel width less than 100 µm. The microgroove with superhydrophobic and underwater superaerophilic micro/ nanostructures on its inner wall cannot be wetted by water, so a hollow microchannel forms between the substrate and water as the groove-structured surface is immersed in water. Gas can freely flow along the underwater microchannel; that is, this microchannel enables gas transport in water. The superhydrophobic microgrooves make it possible to self-transport bubbles and gases at the microscopic level.

Femtosecond (10⁻¹⁵ s) laser technology has emerged as a promising solution to prepare such a superhydrophobic microgroove. Leveraging on its two key features: extremely high peak intensity and ultrashort pulse width, femtosecond lasers have become an essential tool for modern 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, the femtosecond laser can ablate almost any material, resulting in microstructures on the material’s surface. Thus, the femtosecond laser is a viable tool for creating superhydrophobic microstructures on material surfaces, which is essential for realizing gas self-transportation at microscopic level.

Hierarchical micro/nanostructures were easily produced on the inherently hydrophobic polytetrafluoroethylene (PTFE) substrate by femtosecond laser processing, endowing the PTFE surface with excellent superhydrophobicity and underwater superaerophilicity. The femtosecond laser-induced superhydrophobic and underwater superaerophilic microgrooves greatly repel water and can support gas transportation underwater because a hollow microchannel formed between the PTFE surface and water medium in water. Underwater gas was easily transported through this hollow microchannel.

Interestingly, when superhydrophobic microgrooves connect different superhydrophobic regions in water, the 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 surface superwettability of the PTFE sheet allowed the gas bubbles to unidirectionally pass through the porous superwetting PTFE sheet (from the small-holes side to the big-holes side) in the water.

Anti-buoyancy unidirectional penetration was achieved; that is, the gas overcame the buoyance of the bubble and self-transported downward. Similar to a diode, the function of the unidirectional gas passage of the superwetting porous sheet was used to determine the gas’s transporting direction in manipulating underwater gas, preventing gas backflow.

The Laplace pressure difference drove the processes of spontaneous gas transportation and unidirectional bubble passage. The superhydrophobic and underwater superaerophilic porous sheets were also successfully used to separate water and gas based on the behavior of gas self-transportation.

Professor Feng Chen (Director of Ultrafast Photonic Laboratory, UPL) and Associate Professor Jiale Yong have identified the significance of the research and the potential applications of this technology (underwater gas self-transportation) as follows:

“How to think of using superhydrophobic microgrooves for gas transportation?”

“Superhydrophobic microstructures have great water repellence, allowing the materials to repel liquids. If a microgroove has superhydrophobic micro/nanostructures on its inner wall, the microgroove will not be wetted by water as the groove-structured surface is immersed in water. Therefore, a hollow microchannel forms between the substrate and water medium. This microchannel enables gas transport in water so that gas can freely flow along the underwater microchannel. The femtosecond laser can easily fabricate such a superhydrophobic microgroove. The width of the laser-induced microgroove determines the width of the hollow microchannel, which is less than 100 μm, enabling us to realize gas self-transportation at microscopic level.”

“Why was femtosecond laser used to prepare such a superhydrophobic microgroove for gas self-transportation?”

“The laser is one of the greatest inventions of the 20^th century. In recent years, the femtosecond laser has become an essential tool for modern extreme and ultra-precision manufacturing. Femtosecond laser processing is a flexible technology that can directly write superhydrophobic and underwater superaerophilic microgrooves on the surface of a solid substrate and drill open microholes through a thin film. Furthermore, the track of the open microgrooves and the location of the open microholes can be accurately designed by the control program during laser processing.”

“Does the types of the gas affect the self-transportation of bubbles and gases at microscopic level?”

“Although just the ordinary air bubble has been studied, it should be noticed that the driving force for gas transportation does not involve the chemical composition of the gas. Therefore, the manipulation of gas reported in this paper is applicable to other gases as long as they do not completely dissolve into the corresponding liquids.”

“What are the potential applications of the technology achieving bubble/gas self-transportation and manipulation based on the femtosecond laser-written superhydrophobic microgrooves?”

“We believe the reported methods of self-transporting gas in water along femtosecond laser-structured superhydrophobic microchannels will open up many new applications in energy utilization, chemical manufacturing, environmental protection, agricultural breeding, microfluidic chips, health care, etc.”

Researchers also point out that this strategy for self-transporting gas based on the superhydrophobic microgrooves, while validated, is still in its infancy. The influence of various factors (such as the size of the microgrooves, the length of the channel, and the volume of the gas) on the performance of gas transportation needs further research. The practical applications based on the gas self-transportation function also need to be developed.

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International Journal of Extreme Manufacturing