Tuning surface texture of liquid metal particles by exploiting material metastability

Abstract

Surface roughness, and associated changes in properties, often dictate how a material is utilized or manifests unprecedented capabilities to well-known materials. Most approaches to engineer surface texture are, however, predominantly based on additive and/or subtractive routes that are often either; i) inefficient (arduous, lengthy and expensive), or ii) inaccessible (due to a need for specialized equipment and skilled manpower). Understanding a materials interface structure at the sub-nanometer scale, and pairing this with its thermodynamic landscape, offers a new frugal approach to engineer its surface texture. Herein, we demonstrate thermal-driven evolution of surface morphology by exploiting inherent structural complexity, and metastability, of the ultra-thin passivating oxide on liquid metal particles. We achieve tunable surface texture via thermal-triggered oxidation of the liquid, whereby structural order on the surface was controlled through kinetics (reduction potentials of constituent elements) and stoichiometry, the latter being driven by interfacial phase-segregation upon oxidation. Release of the underlying phasesegregated components lead to inversion in the composition of the surface of these metal oxides, with concomitant growth in the thickness of the passivating layer. We divide our study in two parts viz; i) utilizing a thermodynamically stable core with a metastable shell, and ii) engineering liquid metal particles with metastable core and a metastable interface. For the former case, we obtained particles characterized by crumples, patches and multi-tiered roughness with increase in processing temperature and time. The overall structure of the particle evolves from a smooth sphere into a crumpled sphere and eventually a textured surface with increasing temperature while retaining the liquid core. At temperatures >1173 K, the liquid core is not observed but a hollow solid particle is formed with significant inter-particle sintering. For particles with metastable core vi and metastable interfaces, we observed greater effects of thermal stress due to phase change. The particles undergo spinodal decomposition upon solidification while differences in thermal expansivities of the constituent elements led to dendritic growth, albeit after a specific trigger temperature.

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

“…We thank Dr. Dapeng Jing and Dr. James Anderegg for their help and support in XPS and Auger spectroscopy experiments, respectively. This work was carried out as part of J.C.’s graduate dissertation …”

“…Herein, we demonstrate that temperature-driven oxidation of EGaIn core−shell particles can be utilized to form tunable surface texture and invert composition by taking advantage of the elasticity and interfacial metastability of the passivating oxide layer. 84 We further show that surface modification is achieved via two processes (Figure 1a): (i) expansion−induced diffusion-limited oxidation (EDO), and (ii) thermo-mechanical fracture leakage and oxidation (TFO). The core−shell particle consisting of an EGaIn core and a Ga 2 O 3 shell are illustrated in gray, whereas segregated In is represented in white.…”

Effects of particle size and oxide shell on variable stiffness performance of phase-changing materials

“…Generally, secure bonding corresponds to higher material strength, 15 and poor bonding has been shown to cause early fracture in previous examples of FM composites. 23 While there exist many methods to achieve high bonding in Field's metal or other core-shell particles, including chemical, heat, and mechanical treatments, 24 they often disrupt the eutectic balance of the alloy or increase oxide growth which inhibits the stiffness change performance. As such, this section remains strictly focused on mechanical bonding strength due to particle size, which will be applicable to variable stiffness fillers regardless of material.…”