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Interfacial Chemistry of Liquid Metals: Eutectic Gallium Indium (E-GaIn)


Gallium-based liquid metal alloys have garnered attention for their use in self-healing and flexible electronics, soft robotics, catalysis, and biomedicine. Advances in these next-generation materials have only been made possible through understanding the liquid-metal interfaces and the interactions of matter with these liquids. We propose that liquid metal chemistry of gallium and gallium-based alloys can be subdivided into a series of interfacial phenomenon. To study the interfacial properties of liquid metal, eutectic gallium indium (E-GaIn), a non-toxic liquid metal alloy comprised of ~75% gallium and ~25% indium, and has a melting temperature of 15.5 �C, was employed for its liquid state at room temperature.

With an overview of the breadth of chemistry involved within liquid metal systems, we first consider the oxidation of liquid metals in air. Room-temperature liquid metals, their interactions in air, and the kinetics of oxidation are critical first steps towards understanding the materials properties of liquid metals, such as surface-energy/surface-tension that originate at the metal-oxide interface. Once understood, this oxidation can be prevented via surface-bound ligands. We employed gallium-thiol chemistry and bi-functional thiol molecules as capping ligands for the surface stabilization of gallium-based liquid-metal nanostructures. These surface stabilization effects, within mixtures of liquid metal and polar solution, such as water and ethanol, are examined as a new type of emulsified system. These emulsified systems, with the help of surface-active molecules, can exist in both their singly and doubly emulsified forms. Like mercury and other liquid metals, E-GaIn has a high surface tension, which when broken can form ultra-small droplets, ~4 nm. These ultra-small liquid metal droplets were found to undergo higher order assemblies in the form of fractal aggregates under evaporation of the surrounding solvent.

When examining the doubly emulsified form, these liquid metal droplets present a novel approach for the encapsulation of cargo. The material properties of E-GaIn present an interesting avenue for the study of nanocarriers. Eutectic gallium indium, which has low toxicity, enable considering this liquid metal for drug-delivery applications. Furthermore, where traditional nanoparticle carriers typically have cargo decorated on the surface, or within pores and imperfections; liquid-metal double emulsions, which have a non-metallic phase embedded within the core of the liquid metal droplets, allow for cargo loading much larger than the surface of the nanostructure. Lastly, competition for interfaces and the role of self-assembly at metallic interfaces presents a scaffold for the post-encapsulation functionalization of liquid metal carriers.

The ‘skin’ of liquid E-GaIn, whether comprised of metal oxide, metal thiolate, or pristine surface can act as a barrier for reaction kinetics, such as in the case of surface oxidation and the prevention of it with thiol/thiolate assemblies at the liquid metal interface, or a regenerative reactive interface, as in the case of surface-initiated galvanic reduction. Galvanic reduction with gallium, as the name implies, can act as a free-electron surface for the reduction of simple metal salts. Similar to hanging mercury drop experiments, liquid metals provide a pristine, self-regenerating, liquid metal surface. We explore the galvanic reduction of silver salts on nanoscopic liquid metal seeds via solution-liquid-solid-growth of silver nanovines. We found these arboriform liquid metal structures were grown as diffusion-limited aggregates and we were able to impart control on both the growth front thickness and aspect ratio.

The manipulation of liquid metals with surface-active molecules will have applications in next-generation devices and soft robotics. By incorporating liquid metal as the conductive contacts, we enable malleable devices. Liquid metals enable new applications of traditional conductive materials with certain advantages. We envision robots, sensors, and a variety of future prospects where liquid metal can open new doors to a variety of applications. In an effort to explore these devices, we have shown that E GaIn is an ideal contact material for an ultra-thin indium oxide based field-effect transistor (FET). These liquid-metal-enabled devices provide lower barriers to charge transfer, yielding lower power devices as compared to gold.

Liquid-metal-enabled emulsions, materials, and devices present new chapters in inorganic chemistry, materials chemistry, engineering, nanoscience, and medicine. By looking at the interfacial phenomenon that dictates the interactions of liquid metal with the environment, we explore these avenues of control and application. With the rise in availability for three-dimensional (3-D) printers, liquid metal and hybrid materials comprised of liquid-metal-based emulsions are a new frontier. The field of liquid-metal-enabled materials remains relatively unexplored.

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