Soft Energy Harvesters and Actuators Using Liquid Metal

This talk will discuss recent progress in utilizing gallium-based liquid metals for energy harvesting and actuation. The promise of soft devices—wearable electronics, implantables, soft robotics, e-skins, sensors—has accelerated the demand for soft, flexible, and stretchable energy sources and actuators. This talk will discuss a completely soft and stretchable (> 400% strain) energy harvester based on variable-area electrical double layer capacitors. Electrical double layers form spontaneously at the interface between liquid metal electrodes and hydrogel encapsulants. They also have large capacitance (~40 μF cm^-2). Mechanically varying the interfacial contact area, and thus the capacitance, disrupts equilibrium and generates a driving force for charge movement through an external circuit. Because the design is entirely soft, it is possible to vary the area of the electrode itself. This new approach is attractive because the entirely soft device can work under water and can use various modes of mechanical input such as pressing, stretching, bending, and twisting. In addition, the talk will discuss the use of liquid metal as electrodes for more established modes of energy harvesting, including thermal (thermoelectric), mechanical (tribocharging), and electromagnetic. Finally, the talk will discuss the use of liquid metals for actuation via the use of interfacial tension. Liquid metals have the largest surface tension of any liquid at room temperature. The interfacial tension can be modulated significantly using electrochemical reactions. These changes in tension can generate forces to create actuators, muscles, and jumping droplets. This work is interesting because it provides a route to actuate devices using entirely soft materials that are controlled electrically.