Autonomous Thermal-Oxidative Composition Inversion and Texture Tuning of Liquid Metal Surfaces

Abstract

Droplets capture an environment-dictated equilibrium state of a liquid material. Equilibrium, however, often necessitates nanoscale interface organization, especially with formation of a passivating layer. Herein, we demonstrate that this kinetics-driven organization may predispose a material to autonomous thermal-oxidative composition inversion (TOCI) and texture reconfiguration under felicitous choice of trigger. We exploit inherent structural complexity, differential reactivity, and metastability of the ultrathin (∼0.7-3 nm) passivating oxide layer on eutectic gallium-indium (EGaIn, 75.5% Ga, 24.5% In w/w) core-shell particles to illustrate this approach to surface engineering. Two tiers of texture can be produced after ca. 15 min of heating, with the first evolution showing crumpling, while the second is a particulate growth above the first uniform texture. The formation of tier 1 texture occurs primarily because of diffusion-driven oxide buildup, which, as expected, increases stiffness of the oxide layer. The surface of this tier is rich in Ga, akin to the ambient formed passivating oxide. Tier 2 occurs at higher temperature because of thermally triggered fracture of the now thick and stiff oxide shell. This process leads to inversion in composition of the surface oxide due to higher In content on the tier 2 features. At higher temperatures (≥800 °C), significant changes in composition lead to solidification of the remaining material. Volume change upon oxidation and solidification leads to a hollow structure with a textured surface and faceted core. Controlled thermal treatment of liquid EGaIn therefore leads to tunable surface roughness, composition inversion, increased stiffness in the oxide shell, or a porous solid structure. We infer that this tunability is due to the structure of the passivating oxide layer that is driven by differences in reactivity of Ga and In and requisite enrichment of the less reactive component at the metal-oxide interface.

Ga‐Based Liquid Metal Micro/Nanoparticles: Recent Advances and Applications

“…Several research groups have created micro/nanoparticles of LMs by shearing bulk LMs into complex particles in a process denoted as SLICE, an extension of the droplet emulsion technique . In the SLICE approach, which combines mechanical and chemical principles to create a core‐shell structure, the forces applied to the LM droplet include shear (γ), gravity ( F g ), drag ( F d ), centrifugal forces ( F c ), and buoyancy ( F b , a negligible contributor due to the density difference between the aqueous solution and EGaIn particles; the density of EGaIn is 6.25 g mL −1 at 25 °C) (Figure g).…”

Rapid Prototyping of Reconfigurable Microfluidic Channels in Undercooled Metal Particle-Elastomer Composites

“…A caveat with heat-induced reconfiguration lies in the concomitant alterations on the surface oxide of the metal, 30 which can alter the properties of the composite upon repeated reuse. Furthermore, applications that require elevated temperatures could induce undesired deformation in the composite.…”

Synthesis of Iron Oxide Nanostructures via Carbothermal Reaction of Fe Microspheres Generated by Infrared Pulsed Laser Ablation

“…The size and length of these nanowires are kinetically and thermodynamically controlled by the diffusion of Fe and the availability of carbon in air. It is important to note that in most surface-diffusion induced growth of nanowires, such as ZnO, the nanowires are prepared with small Au clusters on GaN substrates [53,54] and/or in Zn acetate seeds on amorphous substrates, where the diffusion of Zn ions controls the length of the nanowires [55]. A thorough discussion on the growth of ZnO nanowires from Zn-based seeds can be found in [55].…”

“…It is important to note that in most surface-diffusion induced growth of nanowires, such as ZnO, the nanowires are prepared with small Au clusters on GaN substrates [53,54] and/or in Zn acetate seeds on amorphous substrates, where the diffusion of Zn ions controls the length of the nanowires [55]. A thorough discussion on the growth of ZnO nanowires from Zn-based seeds can be found in [55]. However, iron oxide nanowires were observed to grow on the surface of microspheres even without a catalyst as long as a sufficient amount of carbon was present, which was supplied by heating the activated carbon at high temperatures.…”