Liquid metal consists of alloys (particularly those of gallium) with very low melting points which form a eutectic that is liquid at room temperature. For instance, eutectic gallium–indium (EGaIn) has attracted increasing attention credit to its important potential applications in liquid robots and flexible circuits. Ideally, the fundamental technology for achieving these applications is to control the shape and adhesion of liquid metal and even obtain complex liquid metal patterns. Liquid metals, however possess liquid-like property and are therefore susceptible to adhere to a solid substrate due to the high adhesion between solid surface and liquid metal. Consequently, this makes the liquid metal difficult to control. In addition, liquid metal is very different from water in the aspect of hydrophilicity/hydrophobicity, no matter the chemical composition or the physical/mechanical property. Therefore, establishing a principle for endowing any materials with remarkable repellence to liquid metals has an extremely vital significance.
To date, the preparation of liquid metal-repellent surface by a simple and widely applicable way is really a great challenge in the field of liquid metal applications. Liquid metal possess high adhesion that make it very difficult to control the liquid metal shape and prepare fine liquid metal patterns. To address this, a team of researchers from the Xi’an Jiaotong University: Professor Jiale Yong, Dr. Chengjun Zhang, Dr. Xue Bai, Dr. Jingzhou Zhang, Professor Qing Yang, Professor Xun Hou, and Professor Feng Chen investigated the influence of the surface chemistry and microstructure on the wettability of a solid surface to liquid metal. Their goal was to demonstrated that the wettability of liquid metal on a substrate is very different from the water wettability. Their work is currently published in the research journal, Advanced Materials Interfaces.
In their approach, micro/nanoscale structures were prepared on the silicon and polydimethylsiloxane (PDMS) surfaces by simple femtosecond laser (fsL) processing. After additional chemical modification/treatment, the researchers were able to obtain both superhydrophobic and superhydrophilic Si/PDMS surfaces. The researchers characterized the morphology of the fsL-ablated sample surfaces using various techniques.
The authors reported that all the obtained structured surfaces had excellent liquid metal repellence in spite of superhydrophobicity or superhydrophilicity. Indeed, the difference between liquid metal wettability and water wettability of a solid surface was revealed from the aspects of experimental comparison and contact model analysis, with water and liquid metal droplets on different substrates. The laser-induced surface microstructures are able to greatly reduce the adhesion between liquid metals and solid substrate. Such remarkable liquid metal-repellent property is defined as “supermetalphobicity”.
The supermetalphobic microstructure has many potential applications, such as reducing the adhesion at the interface of liquid metal and solid, controlling the shape of liquid metal, designing liquid metal pattern, etc. FsL pulses have the characteristics of extremely short pulse width and ultrahigh peak intensity, so fsL can process almost all of the known materials. Micro/nanoscale surface structure can be created on any substrates by direct fsL processing. Therefore, supermetalphobicity is potentially obtained on the surfaces of various materials after fsL processing.
In summary, the study demonstrated the influence of surface chemistry and microstructure on achieving supermetalphobicity. The results presented indicated that both the superhydrophilic and the superhydrophobic Si/PDMS surfaces had similar supermetalphobicity; that is, the supermetalphobicity did not depend on the superhydrophobicity or superhydrophilicity for a solid substrate. In a statement to Advances in Engineering, Professor Jiale Yong highlighted that they anticipated that the established principle for endowing any materials with remarkable repellence to liquid metals would have important significance, which will accelerate the application progress of liquid metal materials in flexible circuits and liquid robots.
Jiale Yong, Chengjun Zhang, Xue Bai, Jingzhou Zhang, Qing Yang, Xun Hou, Feng Chen. Designing “Supermetalphobic” Surfaces that Greatly Repel Liquid Metal by Femtosecond Laser Processing: Does the Surface Chemistry or Microstructure Play a Crucial Role? Advanced Material Interfaces 2020, 7, 1901931.