Abstract:
Soft robotic systems typically operate through the use of soft actuators constructed from highly deformable materials or liquids. Because of their intrinsic compliance, these actuators can achieve elastic resilience and adaptability similar to their biological counterparts. One challenge with engineering these artificial muscles is the selection of soft materials and activation methods while maintaining desired static and dynamic performance. Liquid metals (LM) provide great opportunities for creating a next-generation artificial muscle that generates forces with scaling advantage due to dominating surface tension at small length scales (L), has high work density with smaller L, and does not have voltage or efficiency issues like other soft actuators such as dielectric elastomer (DEA) and shape memory alloy (SMA). The surface energy of liquid metals can be electrochemically controlled over a wide range of values – from near zero to 500 mJ/m2 – using low voltage potential (∼1V), a phenomenon that goes beyond classical electrocapillarity. This enables the ability to create soft-matter actuators that exhibit high work density at small scales.

Here, we introduce a muscle-inspired soft actuator that is composed of an axis-symmetric capillary bridge of eutectic gallium-indium (EGaIn), which linearly contracts or expands in response to electrical stimuli. In order to analyze the shape change and hence the force-length relationship of a LM actuator, we present free energy minimization models where the equilibrium shape can be predicted at different length scales, volumes, and loading conditions. We demonstrate that theoretical results by a simplified toroidal approximation show reasonable agreement with exact solution by the Delaunay (or constant-mean-curvature surface) model and numerical results by Surface Evolver. We theorize the operation region of the loading cycle based on the force-length curves. We show experimentally that a mm-scale LM actuator is capable of an axial strain of 31.5% and a work density of 2.1 kJ/m3 at an actuation frequency of 1 Hz, which is consistent with the theoretical estimation. We highlight a comparison to natural muscles and other artificial muscles, in terms of static and dynamic performance, which shows that the electrochemical LM actuator has a unique combination of high wor density at small scales, biologically-relevant activation frequency, and low operational voltage that stands out from other classes of soft-matter actuators.

Committee:
Carmel Majidi (Chair)
Sarah Bergbreiter
Zeynep Temel
Jaskaran Grover