Buried Liquid Metal Interfaces: Oxides and Surface Water-Driven Hydrogen Bubbles

This paper investigates the interface between liquid metal and air-impermeable substrates. Liquid metals, such as eutectic gallium indium (EGaIn), are low-viscosity liquids at room temperature that can be injected or dispensed against surfaces to form electrical or thermal contacts. While advancing EGaIn across a surface, its native oxide might roll like a "tank-tread" onto the substrate. In this case, there will be oxide between the metal and the substrate. Alternatively, the oxide might 'slip' across the substrate while EGaIn advances, resulting in an oxide-free, metallic contact to the substrate. Here, time-of-flight secondary ion mass spectrometry (TOF-SIMS) and macroscopic fluid dynamic experiments show that the oxide exists between EGaIn and the walls of both air-impermeable Si₃N₄ and glass substrates after injecting EGaIn across these surfaces. Surprisingly, the liquid metal reacts with water molecules present at the interface between EGaIn and the substrate, thereby producing hydrogen bubbles. The formation and size of these bubbles are governed by the surrounding relative humidity, the substrate's surface chemistry, and its gas permeability. These findings provide insights into the interface between liquid metal and various substrates, such as glasses, polymers, and metals, used commonly in electrical applications of liquid metal.