“Aerofluidics” for Transporting and Manipulating Trace Gases

Microfluidics is an integrated system that uses microchannels with a size of only tens to hundreds of microns to process or manipulate tiny fluids. The characteristics of miniaturization and integration enable microfluidic chips to perform a range of complex microprocess and micromanipulation, which are challenging to accomplish with traditional large-scale test and analysis instruments. Microfluidic technology has been widely applied in chemical and biological analysis (e.g., genomic and proteomic research), cellular manipulation and detection, medical and health, high-throughput pharmaceutical screening, and integrated optics. Conventional microfluidic technology mainly focuses on processing and manipulating liquids; nevertheless, little attention is given to gases.

Since gas is involved in many chemical reactions, analysis and detection objects, the transportation and manipulation of trace gases also have great application potential. Like liquid microfluidics, tiny volumes of gas, from nanoliters to attoliters, can also be transported and manipulated through microchannels. Such a multifunctional system can be defined as “aerofluidics,” which manipulates trace gases at the microscopic scale to build highly versatile integrated systems based on gas‒gas or gas‒liquid microinteractions. Aerofluidics will have pioneering applications in gas-involved microanalysis, microdetections, biomedical engineering, sensors, and environmental protection. However, it is still a technical challenge to realize the transport of trace gas without the preparation of a closed channel inside solid substrates and without external energy consumption.

In a new study published in the peer-reviewed Journal of Advance Science, Professor Jiale Yong, Yubin Peng, Xiuwen Wang, Professor Jiawen Li, Professor Yanlei Hu, Professor Jiaru Chu, and Professor Dong Wu from University of Science and Technology of China presented the new concept of underwater aerofluidics which involved using superhydrophobic surface microgrooves to create a hollow closed channel between the substrate and the water environment, allowing gas to flow freely and giving the chip an aerofluidic function in a liquid medium. The Laplace pressure, which was the differential pressure across the gas-liquid interface, was used to move gas. This pressure occurred in the form of little bubbles, with the smaller the bubble, the greater the pressure. For gas transport on underwater aerofluidic systems, the Laplace pressure was exploited as a passive pumping source. The internal Laplace pressure of a small bubble was determined analytically using the interfacial free energy between water and gas, the radius of curvature of the bubble, the height of the undersea bubble, and the radius of the bottom dot. The small underwater bubble’s Laplace pressure provided the driving power for gas movement in underwater aerofluidic systems.

Superhydrophobic microgrooves the authors used refer to a surface texture or pattern that is designed to repel water and resist wetting. The superhydrophobic microgrooves are created by femtosecond laser direct writing technology. The microgrooves are extremely small, typically on the order of micrometers in size, and can be arranged in a specific pattern. When water comes into contact with a surface that has superhydrophobic microgrooves, it sits on top of the surface rather than spreading out or wetting the microgrooves, trapping a layer of air between the water and the surface. This is because the microgrooves prevents the water from making direct contact with the superhydrophobic microstructures.

Professor Jiale Yong and colleagues also discussed the performance of an underwater aerofluidic device for gas transportation, with a focus on the influencing factors. They reported that the magnitude of the superhydrophobic inlet and outlet dots, as well as the width, depth, and length of the laser-induced microgrooves, have significant impact on the gas transportation performance. Specifically, the larger the outlet dot’s surface area, the more conducive it was to gas transport, and the deeper and broader the microgrooves, the quicker the gas transport process. These devices could transport gas along a variety of complex paths, including curved surfaces and 3D spirals, in addition to linear microgrooves. These devices’ pliability allowed them to be used as flexible devices that could adapt to more complex working environments. By connecting superhydrophobic microgrooves in series, the distance a gas can travel can be progressively increased.

The authors demonstrated various gas manipulations in underwater aerofluidic devices, including merging, splitting, and array formation. As demonstrated by a proof-of-concept experiment, these devices may also serve as microreactors for gas-gas and gas-liquid reactions. The first author, Professor Jiale Yong in a statement to Advances in Engineering, pointed out that aerofluidics enables precise and complex manipulation of trace gases and the flexible characteristics of advanced femtosecond laser processing provide infinite imagination space for preparing multifunctional aerofluidic devices. This new technology could revolutionize our comprehension of transporting and manipulating trace gases among gas-involved applications.