Conversion of Polymer Surfaces into Nonwetting Substrates for Liquid Metal Applications

Interface of gallium-based liquid metals: oxide skin, wetting, and applications

Gallium-based liquid metals (GaLMs) are promising for a variety of applications-especially as a component material for soft devices-due to their fluidic nature, low toxicity and reactivity, and high electrical and thermal conductivity comparable to solid counterparts. Understanding the interfacial properties and behaviors of GaLMs in different environments is crucial for most applications. When exposed to air or water, GaLMs form a gallium oxide layer with nanoscale thickness. This "oxide nano-skin" passivates the metal surface and allows for the formation of stable microstructures and films despite the high-surface tension of liquid metal. The oxide skin easily adheres to most smooth surfaces. While it enables effective printing and patterning of the GaLMs, it can also make the metals challenging to handle because it adheres to most surfaces. The oxide also affects the interfacial electrical resistance of the metals. Its formation, thickness, and composition can be chemically or electrochemically controlled, altering the physical, chemical, and electrical properties of the metal interface. Without the oxide, GaLMs wet metallic surfaces but do not wet non-metallic substrates such as polymers. The topography of the underlying surface further influences the wetting characteristics of the metals. This review outlines the interfacial attributes of GaLMs in air, water, and other environments and discusses relevant applications based on interfacial engineering. The effect of surface topography on the wetting behaviors of the GaLMs is also discussed. Finally, we suggest important research topics for a better understanding of the GaLMs interface.

1000 °C High-Temperature Wetting Behaviors of Molten Metals on Laser-Microstructured Metal Surfaces

The melting of metals at high temperatures is common and important in many fields, e.g., metallurgy, refining, casting, welding, brazing, even newly developed batteries, and nuclear fusion, which is thus of great value in modern industrialization. However, the knowledge of the wetting behaviors of molten metals on various substrate surfaces remains insufficient, especially when the temperature is over 1000 °C and with microstructured metal substrate surfaces. Herein, we selected molten cerium (Ce) on a tantalum (Ta) substrate as an example and investigated in detail its wetting at temperatures up to 1000 °C by modulating the microstructures of the substrate surfaces via laser processing. We discovered that the wetting states of molten Ce on Ta surfaces at temperatures over 900 °C could be completely altered by modifying the laser-induced surface microstructures and the surface compositions. The molten Ce turned superlyophilic with its contact angle (CA) below 10° on the only laser-microstructured surfaces, while it exhibited lyophobicity with a CA of about 135° on the laser-microstructured plus oxidized ones, which demonstrated remarkably enhanced resistance against the melt with only tiny adhesion in this circumstance. In contrast, the CA of molten Ce on Ta substrate surfaces only changed from ∼25 to ∼95° after oxidization without laser microstructuring. We proved that modulating the substrate surface microstructures via laser together with oxidization was capable of efficiently controlling various molten metals' wetting behaviors even at very high temperatures. These findings not only enrich the understanding of molten metal high-temperature wettability but also enable a novel practical approach to control the wetting states for relevant applications.

Controlling the oxidation and wettability of liquid metal via femtosecond laser for high-resolution flexible electronics

Liquid metal-based electronic devices are attracting increasing attention owing to their excellent flexibility and high conductivity. However, a simple way to realize liquid metal electronics on a microscale without photolithography is still challenging. Herein, the wettability and adhesion of liquid metal are controlled by combining the stirring method, femtosecond laser microfabrication, and sacrificial layer assistant. The adhesive force of liquid metal is dramatically enhanced by adjusting its oxidation. The wetting area is limited to a micro-pattern by a femtosecond laser and sacrificial layer. On this basis, a high-resolution liquid metal printing method is proposed. The printing resolution can be controlled even less than 50 μm. The resultant liquid metal pattern is applied to electronic skin, which shows uniformity, flexibility, and stability. It is anticipated that this liquid metal printing method will hold great promise in the fields of flexible electronics.

Acoustic Wave-Driven Liquid Metal Expansion

In this paper, we report a volume expansion phenomenon of a liquid metal droplet naturally oxidized in an ambient environment by applying an acoustic wave. An oxidized gallium-based liquid metal droplet was placed on a paper towel, and a piezo-actuator was attached underneath it. When a liquid metal droplet was excited by acoustic wave, the volume of liquid metal was expanded due to the inflow of air throughout the oxide crack. The liquid metal without the oxide layer cannot be expanded with an applied acoustic wave. To confirm the effect of the expansion of the oxidized liquid metal droplet, we measured an expansion ratio, which was calculated by comparing the expanded size in the x (horizontal), y (vertical) axis to the initial size of the liquid metal droplet, using a high-speed camera. For various volumes of the droplet, when we applied various voltages in the range of 5~8 Vrms with 18.5~24.5 kHz using the piezo-actuator, we obtained a maximum expansion ratio of 2.4 in the x axis and 3.8 in the y axis, respectively. In addition, we investigated that the time to reach the maximum expansion in proportion to the volume size of liquid metal differed by five times from 4 s to 20 s, and that the time to maintain the maximum expansion differed from 23 s to 2.5 s, which was inversely proportional to the volume size. We also investigated the expansion ratios depending on the exposure time to the atmosphere. Finally, a circuit containing LED, which can be turned on by expanded liquid metal droplet, was demonstrated.

Morphogenesis of Liquid Indium Microdroplets on Solid Molybdenum Surfaces during Solidification at Normal Pressure and under Vacuum Conditions

Electrical and optical applications based on micro- and nanoparticles have specific demands on their interfacial properties. These properties are strongly related to atmospheric conditions to which the particles were exposed during their formation. In this study, metallic In microparticles are synthesized by solidification of In droplets on an amorphous Mo substrate at normal pressure and under vacuum conditions. The influence of ambient pressure on the interface and surface shape is investigated. While solidification at atmospheric pressure leads to collapsed particles with undisturbed contact to the substrate, low pressures result in smooth spherical particles but with cavities inside. Numerical simulations with COMSOL Multiphysics reveal different temperature profiles and heat flux in particles during solidification for both cases. This indicates different starting conditions of the solidification, which leads to the described phenomenon eventually. The investigation of the varying process conditions on the particle shape in combination with the calculated and measured temperature curves over time gives valuable insights into new approaches to synthesize micro- and nanoparticles with defined interfacial properties. Both ambient pressure and cooling rate provide well-controllable and reliable parameters for the realization of different interfacial shapes.