Exploring applications of liquid metals in robotic systems

Cole, Timothy (2025). Exploring applications of liquid metals in robotic systems. University of Birmingham. Ph.D.

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Abstract

Current robotic systems are typically made using all rigid materials. This causes them to have precise, fast control. However, all-rigid robots face difficulty handling delicate objects and interacting with people, who may get injured by them. Additionally, if the robot gets unexpectedly caught on an obstruction, it can get stuck and damage its motors. The introduction of controllable soft elements using low melting point alloys can eliminate these problems. For example, the phase change of liquid metal (between liquid and solid) can provide an extreme change in stiffness to help a robot pass by an obstruction. For grasping delicate objects and interacting with people, soft robots can be used instead of rigid. Typically, soft robots utilise a pump to change the pressure in a liquid, or high voltage to compress a fluid enclosed within a shell. However, the use of external pumps and high voltages limits their usefulness. Using liquid metal as the fluid, it is possible to create soft robots actuated using low voltages without using external pumps. This thesis therefore aims to solve problems in robotics using the unique properties of liquid metals.
Firstly, a smart stiffness changing elastomer made using a non-toxic low melting point alloy − Field’s metal (melting point 62 °C) − is created. This elastomer composite exhibits unconventional and tuneable mechanical and electrical properties that change with temperature and strain. Its resistance decreases by orders of magnitude when compressed or stretched. The electrical and mechanical properties are first investigated with variations in applied stress and temperature. Afterwards, its smart ability to change stiffness and resistance upon a combination of mechanical and electrical stimuli is demonstrated. It is then used in two proof of concept demonstrations − to make a variable stiffness compliance module for robotic grippers, and a resettable fuse.
Secondly, a capillary liquid metal muscle is created, which utilises the giant, switchable change in interfacial tension of eutectic gallium indium liquid metal to generate force and movement. The theory behind the working of the muscle is first explored, and the estimated force output for capillary size and overall diameter is calculated. The design is then optimised for maximum force and stroke. Tests were done on force change for various frequencies, movement of the muscle for different mass payloads, and the effect of different electrolytes on the performance of the muscle. The force output of the muscle and its position during movement is able to be controlled using feedback control. Finally, designs with extra small capillary slit widths are fabricated and tested, showing further increased force change.
Overall, this thesis explores two different ways that liquid metals can be used to improve robotic systems.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):