Beyond Gallium Oxide—Modifying Liquid Metal Core-Shell Mechanical Properties via SiO₂-Coatings

Responsive materials utilizing room-temperature liquid metals (LM) are gaining strong interest in the flexible electronics, 3D printing, and biomedical therapeutics. The facile production of LM core-shell particles is enabled through the rapid formation of a passivating surface oxide that prevents LM droplets from coalescing. Rupture of these oxides can facilitate mechanically-responsive switching from electrically insulating to conducting states. While the relative ease of gallium oxide deformation is attractive for intrinsically stretchable electronics, much of the LM mechanical properties is dominated by the mechanics of the oxide and the kinetics of the LM-air interface. Previous works show that this oxide matures over time under ambient conditions, which increases the stiffness of the LM particles.¹ Chemical functionalization of LM particles have demonstrated ways to disrupt the oxide formation or hinder its growth.² However, the lack of complete control over the LM oxidation may introduce mechanical hysteresis into these materials systems. Furthermore, environmental conditions and dynamics of particle strain also play a large role in influencing this hysteresis. In this work, we demonstrate the effects of oxygen content and strain rate on the viscoelastic behavior of eutectic Ga-In LM particles observed during in situ compression tests. Through performing single-particle compression, we show that adjusting the oxygen concentration affects how quickly the oxide reforms to stabilize the LM droplet. This cyclic rupture-reformation of the oxide heavily influences the LM particle onset rupture stress, especially at low strain rates. By coating LM particles with materials having well-known mechanical properties (i.e., silica), we can supersede the properties of the gallium oxide and develop LM particles with predictable and tunable mechanical behavior under ambient conditions. SiO₂-eGaIn particles show an order of magnitude higher stiffness than oxide-coated eGaIn while fracturing in an abrupt and brittle manner. The silica shell thickness can be modified to change the mechanical properties of eGaIn particles as desired. Overall, the results from this investigation will promote further exploration in tuning the surface properties of LM particles beyond gallium oxide.<br/>References:<br/>1. Farrell, Zachary J., and Christopher Tabor. "Control of gallium oxide growth on liquid metal eutectic gallium/indium nanoparticles via thiolation." Langmuir 34.1 (2018): 234-240.<br/>2. Morris, Nicholas J., Zachary J. Farrell, and Christopher E. Tabor. "Chemically modifying the mechanical properties of core–shell liquid metal nanoparticles." Nanoscale 11.37 (2019): 17308-17318.