Facile Low-Oxidation Emulsification of Liquid Metal Using Polyvinylpyrrolidone for Highly Viscoelastic Heat Conductive Pastes

Low-melting-point metals, known as liquid metals (LMs), have recently attracted significant interest owing to their high conductivity and fluidity. "Emulsification" of LMs into colloidal microdroplets in immiscible carrier fluids confers a variety of unique opportunities in terms of their processability as well as functionality; however, achieving emulsification at high LM loads while significantly modifying the rheology of the resulting emulsions presents a considerable challenge. Furthermore, the formation of a surface oxide skin on emulsified LM droplets complicates their interfacial dynamics and often deteriorates the performance of the resulting emulsions. In this study, we demonstrate that polyvinylpyrrolidone (PVP), which can coordinate-bond with LM, markedly increases the emulsification efficiency of LM in ethanol (EtOH), thereby enabling the formation of highly viscoelastic LM-in-EtOH emulsion pastes via simple shear mixing using a mortar and pestle. The growth of the oxide layer is controlled by the surface-adsorbed PVPs, which form an interdroplet percolation network. The resulting PVP-mediated "structured" emulsions exhibit significantly higher thermal conductivities than their additive-free counterparts under a given LM load, owing to the formation of an effective thermal transport network of interconnected conductive LM droplets with controlled growth of insulating oxide skin. Industry-relevant blade coating using these LM-in-EtOH emulsions is demonstrated, during which LM droplets coated on nonstretchable substrates readily develop anisotropy under applied shear, which may be potentially useful for directed thermal transport in relevant applications. Lastly, the performance of the LM droplets coated with PVP as thermal interface materials is evaluated.