Liquid Metal Actuators

This thesis contributes to the field of soft actuators by introducing a generalized framework of actuators from liquid metals. The evolution of robotic actuators has enabled robots to achieve a diversity of motions. Like natural muscles, which converts chemical energy into mechanical work in response to electrical stimuli from the nervous system, actuators are engineered to generate forces and motion and to function as an essential component of a robot for locomotion or manipulation. Unlike natural muscles, however, force generation in traditional, rigid actuators, such as electric motors, does not typically involve intrinsic deformation in the materials in compliance with soft biological tissues, which often makes a robot unsafe to interact with humans. Another physical limitation arises from the scaling laws of forces, where many types of forces, such as magnetic forces in an electric motor, will become insignificant at mm and sub-mm length scales relative to other types of dominating forces such as surface tension.

The advances of small-scale soft actuators have been propelled by the exciting discovery of physical interactions that can be used to generate mechanical work. As an artificial muscle, many soft actuators have been demonstrated to surpass, by some metrics, performance of natural muscles in an ongoing evolution. However, the ability to intrinsically deform in response to an external stimulus is not limited to soft materials such as elastomers, gels, and liquid crystals. Liquid metals, in particular eutectic gallium-indium (EGaIn), also share certain characteristics commonly found in other soft condensed matters, including their ability to change shape freely. Unlike other soft materials, whose stresses often act throughout the volume of the body, liquids are predominantly shaped by the intermolecular forces on the surface rather than within the volume, particularly at a length scale less than the capillary length. This scaling advantage provides opportunities for building smallscale, high-performance actuators from liquid metals.

We present a series of theoretical and experimental studies on a diversity of liquid metal actuator designs, where the liquid metal droplets are configured with certain kinematic constraints to generate a wide range of modalities of robotic actuation, including linear contraction by a classic liquid bridge, contraction inspired by the natural muscle hierarchy, bending by asymmetric liquid bridges, and rotation by pairwise bistability. In all cases, surface tension of the liquid metal is modulated by electrochemical oxidation at low voltages ∼1 volts, making the liquid metal actuators stand out from many existing soft actuators that require higher voltages. Our theoretical models validated by the mm-scale experiments, predict that the liquid metal actuators have a unique scaling advantage of higher work density at smaller length scales due to the dominance of surface tension. Finally, we demonstrate a fully encapsulated and self-contained form factor of liquid metal actuator that is capable of dry-air operation and interaction with external objects and linkages for the first time in a practical robotic setting.