Development of liquid metal-coated polymer beads with stimuli-responsive electrical conductivity

Eutectic gallium–indium alloy (EGaIn) is a liquid metal (LM) with a melting point of approximately 15 °C and has been applied to soft composite materials involving electrically and thermally conductive elastomers. Most composite materials were fabricated by simple mixing of EGaIn with polymer matrices or their precursors, or by coating EGaIn bulk onto polymer substrates. However, it remains challenging to coat spherical particles with EGaIn while the core remains chemically or physically accessible. Such a core–shell particle could provide a novel stimuli-responsive electric material thanks to the cooperative responsiveness of the core and shell. Recently, we developed a simple method for preparing liquid metal-based powders (LMPs) composed of EGaIn microparticles stabilized with small-molecule surfactants. In this research, we prepared LMP-coated liquid marbles of water and EGaIn, oil-in-water droplets, and polymer beads by taking advantage of the hydrophobic surfaces of the LMPs. Importantly, the LMP-coated polymer beads were not electrically conductive, while the sintered LMP-coated (SLMP-coated) polymer beads, which were prepared by partial mechanical sintering of the LMP surface, were conductive. Finally, we demonstrate that treating the SLMP-coated polymer beads with toluene vapor also alters electrical conductivity, caused by swelling and shrinking of the beads during organic solvent fuming, which can penetrate the remaining LMP domains. Although the sensitivity and repeatability are still low, these results suggest that the SLMP-coated polymer beads could be potentially applicable as sensors for organic solvents and volatile organic compounds (VOCs).