Interface-Controlled Phase Separation of Liquid Metal-Based Eutectic Ternary Alloys

Liquid metals (LMs) are immiscible in many common electrolytic solutions and, when immersed in them, establish phase boundaries that display intriguing interfacial characteristics. The application of a cathodic potential to such interfaces may trigger phase separation of solute elements out of the LMs. Here, we investigate this possibility in two of the most researched and industrially used eutectic ternary LMs of Galinstan (Ga-In-Sn) and Field's metal (FM, In-Bi-Sn). We observe that upon surface perturbation by an applied electric potential, solute elements compete to segregate out of the LM alloys according to their energy levels. The nature of the electrolytic solutions plays a key role in the separation process as they dictate whether solute metals are expelled selectively in their pure form or as binary compounds. For example, in a phosphate-based aqueous electrolyte, nano-sized Sn-based entities are selectively expelled from Galinstan, while only Bi-based structures leave the surface of FM. In contrast, in a non-aqueous electrolyte, nano-sized binary compounds of Sn-In and Bi-Sn are separated from the surfaces of Galinstan and FM, respectively. We show that selectivity in the surface separation process, achieved by the alteration of the electrolytic solutions, is due to the interplay between the electrodynamic interactions and the electrocapillary effect. This study presents two key findings: (a) it is essential to carefully consider the possibility of component separation in electrochemical systems based on LMs and (b) it demonstrates interfacial metallurgical pathways to process alloys for refining metals into specific purities, component ratios, and dimensions.