| Inspired by soft-bodied animals and human muscles,self-adaptive,agile,reconfigurable and multi-functional soft actuators have been developed for a variety of applications,such as soft grippers,as well as wearables,haptic and medical devices.Soft actuators,however,suffer from several issues such as insufficient actuating force,large hysteresis,and low efficiency,which seriously limit their futher applications.As an emerging soft material,room temperature gallium-based liquid metal combines metallic and fluidic properties and possesses excellent fluidity and selfhealing ability,water-like bulk viscosity,high electrical/thermal conductivities,jointly with extremely low toxicity compared to mercury and effectively zero vapor pressure.Despite possessing a surface tension larger than any other liquid,gallium-based liquid metal can tune its surface tension by rapidly forming a thin,solid native oxide layer in oxygen.More important,liquid metal has become vital member of soft actuator family by virtue of its excellent stimulus-response characteristics in a variety of physical fields.In current research,Gallium-based liquid metal has applications at multiple scales,such as micro-nano-scale drug delivery,macro-scale heat dissipation.On the other hand,actuation of liquid metal droplets becomes a key technology that plays an important role in flexible electronics,energy harvesting and especially soft robotic systems by determining the controllability and dexterity of these systems.However,to date,the actuation of liquid metal relies on either the formation of surface tension gradient between the two sides of droplets or chemical modification of liquid metal.These actuating methods may inevitably generate bubbles,or sacrifice some inherent properties of liquid metal,such as decreasing the fluidity.In addition,these methods can only act in a relatively narrow spatial sacle range and difficuit to form actuating force towards external systems,and further limits the real-world application of liquid metal.As such,this thesis focus on the common problems in actuation of liquid metal droplets,proposes four actuating methods based on multiple physical fields,analyzes the actuating mechanism,explores the influences of key parameters on actuating performance,and extends its application.The main research contents and results of the thesis are as follows:(1)To solve the problem of bubbles generation and inherent properties sacrifice in liquid metal actuation,we introduce an innovative method for controlling the locomotion of liquid metal droplets using alternating magnetic fields.The rotating magnetic field induces an eddy current and further Lorentz force inside the droplet which drives the droplets to locomote.We discover that the existence of a slip layer for liquid metal droplets distinguishes their actuation behaviors from solid metallic such as copper ball.Numerical simulations and experiments invertigations have verified the actuating mechanism.We further utilize the electrochemical oxidation/reduction of liquid metal to control the friction coefficient between it and the substrate,and realize the start and stop control of the magnetic actuation.This developed method is simple and chemical reaction-free,and has the potential to realize the remote and contactless actuation of liquid metal droplets.(2)To extend the magnetically actuated locomotion of liquid metal,we propose a magnetically actuated liquid metal spin method.This is achieved by spinning a magnetic field around a liquid metal droplet.The relative movement of the magnetic fields and liquid metal droplets induces eddy current,and further Lorentz torque inside the droplet,which can actuate the droplet to spin.Based on the finite element simulation model,the magnetic-force coupling simulation of the liquid metal spin has been carried out.In addition,the influences of magnetic field spinning speed,magnetic flud density and other key parameters on the actuating performance have been investigated.We also design and build a magnetic actuation platform based on permanet magnets,further optimizing the actuating performance by experimental means.This method avoids the generation of bubbles,and the spin is stable and steady,and as well as can be applied in accelerating fluid heat dissipation and mixing,verifying its application potential in microfluidics and mircroelectromechanical systems.(3)To solve the problem of narrow action scale of liquid metal actuation,we propose a capillary force-based actuating method for the actuation of liquid metal at micro-scale.We prepare particles-based porous materials(PBPM),which has excellent wettability with liquid metal.The PBPM provides sufficient equivalent capillaries for the diffusion of liquid metal.and the extraordinary wettability between the PBPM and liquid metal provides capillary force;thus liquid metal can overcome their large surface tension and diffuse spontaneously and rapidly in the PBPM.Hydrochloric acid(HCl)can further improve the wettability and accelerate the diffusion.The microstructure characterization,composition analysis and inter-component wettability exploration validated the liquid metal capillary actuating theory.The diffusion direction is controllable,and it can diffuse in channels of complicated shapes,and even diffuse along complex three-dimensional(3D)surfaces against gravity.These methods has the potential to be applied in the fields of self-healing electronics,rapid prototyping and additive manufacturing.(4)To solve the problem that it is difficuit to form actuating force in current liquid metal actuation,we propose a liquid metal artificial muscle,based on the alternating electric field.In use of electrochemically reaction,we can tunable tension of liquid metal to mimic the contraction and extension of muscles and output actuating force and displacement outward.The liquid metal artificial muscle only needs a driving voltage as low as 1 V,0.5 V in contraction and extension,respectively.Liquid metal artificial muscles can work in different solutions with a wide pH(0-14)and concentration range,generating actuating strain up to 87%at a maximum extension speed of 15 mm/s with a hysteresis less than 0.1 s.We also investigate the effects of parameters such as actuating voltage and frequency,solution type and concentration on the actuating performance,obtaining the optimum parameters,and further improve the performance by connecting the units in series and parallel.Proof-of-concept applications including controllable displays,cargo deliveries,reconfigurable optics reflectors,and bioinspired robots demonstrate the potential of liquid metal artificial muscles to extend the performance space of soft actuators for applications from engineering to biomedical fields. |
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