Channel-Free Transportation of Liquid Metal Droplets by Magnetically Active Microwall Arrays

Recently, gallium-based liquid metal has attracted great attention as a non-toxic conducting material in soft electronics for alternatives to mercury. Transportation of liquid metal droplets can act as on/off switch in manipulating electric conduction. Previous studies reported methods to transport liquid metal droplet by generating electric field-induced surface tension gradient. However, these methods were only operated in a confined path, and thus limited application for electronics without channels. Herein, we suggest on-demand channel-free control of liquid metal droplets through bending actuation of microwall arrays with a height gradient. The magnetically active microwall, consisted with polydimethylsiloxane (PDMS) and iron particle, is designed to have a high height to width aspect ratio (~7) to achieve large degree of bending actuation. The top parts of the microwalls bend in response to an external magnetic field, while the bottom parts of the microwalls are immobilized by a magnetically inert PDMS substrate. To control liquid metal droplet transportation without constraint on the paths, we investigated various interwall-spacing parameters related to the bending actuation of the microwalls. Furthermore, we demonstrate collective merger of independently separated liquid metal droplets by manipulating each droplet to move in opposite directions.