From Soft to Stiff: Nanoparticle Jamming Governs the Interfacial Behavior of Liquid Metals under Electrochemical Oxidation.

Electrochemical oxidation significantly lowers the effective interfacial tension of liquid metals in electrolytes, while the resulting oxide layer exhibits a unique combination of fluid- and solid-like characteristics. Here, we uncover the mechanisms underlying these phenomena by isolating the oxide layer via a novel extraction process and probing its microstructure. We find that the oxide layer comprises densely packed nanoparticles, each consisting of a liquid metal core surrounded by an oxide shell. Based on this insight, we propose a nanoparticle jamming mechanism to explain both the dramatic interfacial tension reduction and the fluid-solid duality of the oxide layer. The oxidation-induced volume expansion of these core-shell nanoparticles generates compressive stress within the oxide layer, counterbalancing the intrinsic interfacial tension of the liquid metal and thereby substantially lowering its effective interfacial tension. Concurrently, the jamming transition among nanoparticles imparts the oxide layer with a coexistence of fluidity and solidity. These results reveal a fundamental physical mechanism governing liquid metal interfaces under electrochemical conditions with broad implications for interface engineering and the design of soft functional materials.