Facile Low-Oxidation\nEmulsification of Liquid Metal\nUsing Polyvinylpyrrolidone for Highly Viscoelastic Heat Conductive\nPastes

Low-melting-point metals, known as liquid metals (LMs),\nhave recently\nattracted significant interest owing to their high conductivity and\nfluidity. “Emulsification” of LMs into colloidal microdroplets\nin immiscible carrier fluids confers a variety of unique opportunities\nin terms of their processability as well as functionality; however,\nachieving emulsification at high LM loads while significantly modifying\nthe rheology of the resulting emulsions presents a considerable challenge.\nFurthermore, the formation of a surface oxide skin on emulsified LM\ndroplets complicates their interfacial dynamics and often deteriorates\nthe performance of the resulting emulsions. In this study, we demonstrate\nthat polyvinylpyrrolidone (PVP), which can coordinate-bond with LM,\nmarkedly increases the emulsification efficiency of LM in ethanol\n(EtOH), thereby enabling the formation of highly viscoelastic LM-in-EtOH\nemulsion pastes via simple shear mixing using a mortar and pestle.\nThe growth of the oxide layer is controlled by the surface-adsorbed\nPVPs, which form an interdroplet percolation network. The resulting\nPVP-mediated “structured” emulsions exhibit significantly\nhigher thermal conductivities than their additive-free counterparts\nunder a given LM load, owing to the formation of an effective thermal\ntransport network of interconnected conductive LM droplets with controlled\ngrowth of insulating oxide skin. Industry-relevant blade coating using\nthese LM-in-EtOH emulsions is demonstrated, during which LM droplets\ncoated on nonstretchable substrates readily develop anisotropy under\napplied shear, which may be potentially useful for directed thermal\ntransport in relevant applications. Lastly, the performance of the\nLM droplets coated with PVP as thermal interface materials is evaluated.