Liquid Metal Soft Machines

Making soft robots that can flexibly transform among different morphologies has long been a dream in both science and engineering areas. With outstanding versatile capabilities, liquid metals are opening breakthrough strategies for molding future smart soft robots that had never been anticipated before or hardly achievable by a rigid metal or conventional material. All the evidences collected so far pointed out that liquid metal machine is evolving via a rather quick way. The latest discoveries on a group of very fundamental phenomena of liquid metals and technological advances thus enabled significantly strengthened this endeavor. Clearly, combining allied components with the liquid metal systems is offering many brand new machine roles as well as incubating future highly advanced robots. In fact, capabilities as offered by liquid metals are far much profound than one can expect. There is plenty of space to explore in the area. This chapter gives a brief overview of soft robots and some unconventional opportunities that liquid metal could provide for innovating the soft machine science and technology.

Many capabilities of the room temperature liquid metal are enabled due to its unique attributes such as high thermal and electrical conductivities, excellent fluidity, high surface tension, extremely low evaporation, chemical stability, and nontoxicity. Over the past few years, intensive research efforts based on these versatile features have led to the development of a group of newly emerging applications such as microfluidics, stretchable and soft electronics, energy management and storage, thermal management, biomedical technology, regulation of chemical reaction, actuators and soft robotics, as well as functional materials. This chapter is dedicated to present a basic introduction about the major properties of liquid metal materials in view its application in developing soft machine.

The room temperature liquid metal (RTLM) is emerging as an ideal material for fabricating microdroplets owing to its strong surface tension and easy phase switching property. In this sense, a bottle of liquid metal can be easily transformed into a large amount of tiny liquid metal droplets. Given specific control via external fields such as mechanical, electrical, chemical, boiling, or acoustic, different sized droplets can be quickly obtained in large amount. Such transition mechanism between large pool of liquid metal and its small-sized objects can be applied for making discrete soft machine which will also be illustrated in later chapters. Along this direction, Yu et al. [1] found out a low-cost and technically simple way for preparing metal droplets. They demonstrated a channelless fabrication method based on stream jetting and self breaking up mechanisms of the RTLM when injected into and interact with the matching solution. The role of such method in the fabrication is rather diverse. For example, the injected assembly of liquid metal droplets can be directly applied to construct a three-dimensional porous metal block with foam structures inside. Apart from the above mechanical injection, electrical field was also disclosed for the automatic generation of liquid metal droplets. Overall, the injection strategy provides an extremely simple way for large-scale fabrication of liquid metal microdroplets and particles which have rather important practical values. It also suggests a highly efficient approach for visualizing and investigating the fundamental mechanisms of fluids interactions between RTLM and general solution. This chapter is dedicated to present the basic strategy to realize the injectable transformation of liquid metal when subject to various external forces and discuss their potential applications.

As an emerging multifunctional material, the room temperature liquid metals own many intriguing properties that had never been anticipated before. Over the long year’s exploration of liquid metal as a soft machine, the present lab (Sheng et al. in Adv Mater 26: 6036–6042, 2014 [1]) found for the first time that through applying an external electrical field on the liquid metals sprayed with or immersed in water, a group of very unusual transformation phenomena of liquid metal among different morphologies and configurations can be induced. These basic machine effects and roles include transformation from a large-sized liquid metal film into a tiny sphere (over one thousand times variation in specific surface area), quick mergence of separate metal droplets, controlled self-rotation, and planar locomotion under different conditions. Further, a series of novel phenomena were observed, such as the self-rotating liquid metal sphere induced accompanying water vortexes nearby, and liquid metal droplet moving across the channel bridge under programmable external electrical fields, etc. In addition, the shape, size, voltage, orientation, and geometry of the electrodes would play important roles in controlling the liquid metal morphologies and transformations. Such soft machine capabilities were hard to achieve on rigid metal or conventional liquid objects otherwise. These findings have both fundamental and practical significances which suggest a generalized way of making smart soft machine in the coming time, collecting discrete metal fluids, as well as flexibly manipulating liquid metal objects or machines. This chapter is dedicated to illustrate the basics about driving the liquid metal soft machine among different configurations.

Reversible transformation of a liquid metal machine holds enormous promise across many scientific areas ranging from mechanical engineering to applied physics. So far, such capabilities are still hard to achieve through conventional rigid materials or depending mainly on elastomeric materials, which however imply somewhat limited performances and require complicated manipulations. The present lab (Zhang et al. in Synthetically chemical–electrical mechanism for controlling large-scale reversible deformation of liquid metal objects, 4:7116, 2014, [1]) established a basic strategy which is fundamentally different from the existing ones to realize large-scale reversible deformation through controlling the working materials of liquid metal. The method is termed as synthetically chemical–electrical mechanism and abbreviated as SCHEME. Such activity incorporates an object of liquid metal whose surface area could spread up to five times of its original size and vice versa under low energy consumption. Particularly, the alterable surface tension based on combination of chemical dissolution and electrochemical oxidation is ascribed to the reversible shape transformation, which works much more flexible than many former deformation principles through converting electrical energy into mechanical movement. A series of very unusual phenomena regarding the reversible configurational shifts are disclosed with dominant factors clarified. Such finding suggests a generalized way to combine the liquid metal serving as shape-variable element with the SCHEME to compose functional soft machines or devices, which implies the big potential for developing future smart robots to fulfill various complicated tasks. This chapter illustrates the basic SCHEME to realize the reversible transformation of liquid metal machine.

Unlike using a single external electric field, simultaneous administration of both electrical and magnetic fields would induce rather complex transformation behaviors of liquid metal. Along this direction, the present lab (Wang and Liu in Electromagnetic rotation of a liquid metal sphere or pool within a solution, 471:20150177, 2015, [1]) demonstrated a group of transformational behaviors of liquid metal electric motors. The machine system is composed of a pair of concentric ring electrodes, permanent magnet, and electrolyte solution. A liquid metal galinstan sphere, along with NaOH solution, is stimulated to rotate centrifugally around the central electrode and the rotating speed increases with the voltage. The NaOH solution serves to remove the oxide on the liquid metal surface in time, reduce the motion friction, and provide impetus to the liquid metal. As the liquid metal is added to 12.16 g to form a kidney like body, its rotating speed appears more controllable and the effect of the electrolytic action in the NaOH solution becomes weak in the range of 0–1.82 V. As the liquid metal is increased to 18.20 g to form a circular ring-shaped body, the ideal voltage range for controlling the rotating motion of the liquid metal is 0–0.81 V. The metal fluid rotates at a speed of 1.9 rpm even at an extremely low voltage of 0.03 V. Further, with the administrated increased electrical field, a variety of surface folding patterns of rotational liquid metal like wheel shape, dual concentric ring shape, and so on were found to occur (Wang and Liu in Liquid metal patterns induced by electric capillary force, 108:161602–161605, 2016, [2]), which refreshes the basic understanding of classical fluid kinematics. The knowledge obtained and the liquid metal electric motor, thus, established can find important applications in realizing certain future rotating soft machine. This chapter presents the typical phenomena of the electromagnetic field induced transformation of liquid metal.

Synthetic self-fuelled motors, which can spontaneously convert chemical energy into mechanical activity to induce autonomous locomotion, are excellent candidates for making self-powered machines, detectors/sensors, and novel robots. The present lab (Zhang et al. in Adv Mater 27:2648–2655, 2004 [1]). discovered an extraordinary self-propulsion mechanism of synthetic motors based on liquid metal objects. Such motors could swim in a circular Petri dish or different structured channels containing aqueous solution with a pretty high velocity on the order of centimeters per second, and surprisingly long lifetime lasting for more than one hour without any assistance of external energy. The soft material liquid metal enables the motors to self-deform, which makes them highly adaptable for accomplishing tough missions in special environment. Interestingly, the motors work just like biomimetic mollusk since they closely resemble the nature by “eating” aluminum as “food”, and can change shape by closely conforming to the geometrical space it voyages in. From practical aspect, one can thus develop a self-powered pump based on the actuation of the liquid metal enabled motor. Further, such pump can also be conceived to work as a cooler. Apart from different geometrical channels, several dominating factors, including the volume of the motor, the amount of aluminum, the property of the solution and the material of the substrate etc., have been disclosed to influence the performance of the autonomous locomotion evidently. This artificial mollusk system suggests an exciting platform for molding the liquid metal science to fundamentally advance the field of self-driven soft machine design, microfluidic systems, and eventually lead to the envisioned dynamically reconfigurable intelligent soft robots in the near future. In this chapter, the typical behaviors and fundamental phenomena of the self fuelled transformable liquid metal machines were illustrated.

Liquid metal machine can also be made as tiny motors. In fact, the micro- or even nanomotors that could run in a liquid environment is very important for a variety of practices such as serving as pipeline robot, soft machine, drug delivery, microfluidics system, etc. However, fabrication of such tiny motors is generally rather time and cost consumptive and has been a tough issue due to the involvement of too many complicated procedures and tools. This lab had discovered a straightforward injectable way for spontaneously generating autonomously running soft motors in large quantity Yao et al (Injectable spontaneous generation of tremendous self-fuelled liquid metal droplet motors in a moment, 2015 [1]). It was demonstrated that injecting the GaIn alloy pre-fuelled with aluminum into electrolyte would automatically split in seconds into tremendous droplet motors swiftly running here and there. The driving force originated from the galvanic cell reaction among alloy, aluminum, and surrounding electrolyte, which offers interior electricity and hydrogen gas as motion power. This finding opens the possibility to develop injectable tiny-robots, droplet machines, or microfluidic elements. It also raised important scientific issues regarding characterizing the complicated fluid mechanics stimulated by the quick running of the soft metal droplet and the gases it generated during the traveling. Our lab Yuan et al (Sci Bull 60:1203–1210, 2014 [2]) made further efforts to disclose that the self-powered liquid metal motors takes interiorly driven macroscopic Brownian motion behavior. Such tiny motors in millimeter-scale move randomly at a velocity magnitude of centimeters per second in aqueous alkaline solution, well resembling the classical Brownian motion. However, unlike the existing phenomena where the particle motions were caused by collisions from the surrounding molecules, the random liquid metal motions are internally enabled and self-powered, along with the colliding among neighboring motors, the substrate, and the surrounding electrolyte molecules. This chapter illustrates the typical behaviors of the self-powered tiny liquid metal motors.

Internally triggered motion of an object owns important potential in diverse application areas ranging from micromachines, actuator, or sensor, to self-assembly of superstructures. This lab (Sheng et al. in Small 11:5253–5261, 2015 [1]) proposed and demonstrated a new conceptual liquid metal machine style, the transient state machine that can work as either a large size robot, partial running elements, or just divided spontaneously running swarm of tiny motors. According to the need, the discrete droplet machines as quickly generated through injecting the stream of a large liquid metal machine can combine back again to the original one. Over the process, each tiny machine just keeps its running, colliding, bouncing, or adhesion states until finally assembles into a single machine. Unlike the commonly encountered rigid machines, such transient state system can be reversible in working shapes. Depending on their surface tension, the autonomously traveling droplet motors can experience bouncing and colliding before undergoing total coalescence, arrested coalescence, or total bounce. Further, the liquid metal machine can also be made as color changeable. In this sense, some transformable biomimetic soft machine which was termed as liquid metal “chameleon” has now been possible. As it was demonstrated, the fluorescent liquid metal can be manipulated into various appearances like fluorescent orange, blue, pink, or even send out fluorescence in the dark. It can realize transformation and discoloration by splitting and merging among different colors. If subject to electrical stimulation, such colors would transform from fluorescence into metallic color. These findings are expected to help design future color changeable machines and mold unconventional robot that could automatically transform among different geometries such as a single or swarm, small or large size, assembling and interaction including color, etc. It refreshes people’s basic understandings of machines, smart materials, fluid mechanics, and micromotors. In this chapter, the basic concept of the liquid metal transient state and color changeable machines will be illustrated.

Self-propelled motors have inspired enormous potential in accomplishing various tasks when navigating themselves in aqueous environment due to the virtue of external energy-free feature and autonomous locomotion. However, without reliable controllability, their potential value will be heavily discounted. In fact, the motion control of small motors in solution is of significance to applications ranging from microfluidics, smart machine, to tiny robot, etc. To realize such object, the present lab had ever proposed an electrical controlling method to flexibly manipulate liquid metal droplet motors powered with aluminum (Tan et al. in Electrical method to control the running direction and speed of self-powered tiny liquid metal motors, 471:20150297–20150306, 2015, [1]). It was discovered that adding aluminum to liquid metal droplets would significantly magnify its electric controlling capability, which provides dozens of times’ driving force compared to pure GaIn₁₀ droplets. If switching on the electrical field, the aluminum powered liquid metal droplet would accelerate its running speed to an extremely high magnitude like 40 cm/s under 20 V, as measured in a channel with 1 cm width and filled with aqueous solution. In addition, it was also observed that the motion trajectories of the motors in the free space of a Petri dish nearly reflect the electric field lines in the electrolyte which suggests an important way to visualize such complex physical property. Lastly, the oscillating motion behavior of the motor under small electrical voltage and its continuous running modality can be experimentally discriminated. The findings on the Al–Ga–In motors would have profound impact on developing future controllable microfluidic systems or tiny soft robot that are hard to achieve through conventional methods otherwise. In addition, through coating magnetic material such as nickel cap on the surface of a liquid metal droplet, such self-propelled motor can also be flexibly controlled and steered by external magnet or electric field, which successfully enables the motor to avoid out-of-control trajectory encountered in former approach. This chapter is dedicated to illustrate several feasible methods to realize directional control of self-fuelled liquid metal machine.

Simulating nature to manufacture self-powered device or motor has been an important goal in science and engineering. Conventional spontaneous motions of an abject were generally achieved through the Marangoni flow of organic liquid or water solution. Regarding the metallic materials, mercury had ever been developed as beating heart as a kind of self-propulsion example. However, the serious safety concern of such medium restricts its extensive application. From an alternative, Yi et al. (Breathing to harvest energy as a mechanism toward making a liquid metal beating heart, 6:94692–94698, 2016 [1]) discovered an important mechanism to realize gallium-based liquid metal beating heart through introducing a breathing mechanism in analogy to living organisms. With the unique configuration of a semi-submerged liquid metal droplet partially immersed in alkaline solution, such a system would produce a surface tension gradient perpendicular to the three-phase contact line which subsequently leads to the oscillation of the droplet and the surrounding solution. This finding suggests a feasible way to fabricate self-oscillating liquid metal motors without external electricity or fuels input. Except for such air driving mechanism, more environmental factors such as heat can also be applied to power the liquid metal system given appropriate designing. The present chapter illustrates the basic behaviors of such environment enabled liquid metal machines and their self-actuating driving mechanisms.

Jumping is a special, however, hard to achieve capability by a liquid metal machine. But under certain assistance, such behavior can also be partially realized. In a recently found electron discharge effect due to point contact between liquid metal and solid metal particles in electrolyte (Tang et al in Appl Phys Lett 108: 223901–223905, 2016 [1]), it was disclosed that adding nickel particles induces drastic hydrogen generation and intermittent jumping of sub-millimeter EGaIn droplet in NaOH solution. Observations from different orientations indicated that such jumping is triggered by pressurized bubble under assistance of interfacial interactions. Hydrogen evolution around particle provides clear evidence that such electric instability is originated from the varied electric potential and morphology between the two metallic materials. The point-contact-induced charge significantly enhances the near-surface electric field intensity at the particle tips and thus causes electric breakdown of the electrolyte. Further, with a particle raft, electrohydrodynamic liquid metal surface convection can even be triggered. Such phenomenon would be very useful to trace the flow of the liquid metal which is hard to do otherwise. This chapter is dedicated to present a basic understanding on the fundamental route to realize particles stimulated liquid metal motions which can help develop potential machine thus involved.

Although liquid metals such as eutectic gallium–indium and gallium–indium–tin have been found extremely important in making various kinds of soft machines, there however always exists a big challenge to flexibly and stably control the shape of liquid metal due to its extremely high surface tension. Along this direction, the present lab (Hu et al. in Adv Mater 28:9210–9217, 2016 [1]) made a fundamental discovery that the bouncing bright liquid metal droplet in alkaline electrolyte can be transformed to a flat and dull puddle when placed on graphite surface. Through the intrinsic interactions between liquid metal and graphite, the liquid metal puddle on graphite can be manipulated as desired into various stable shapes with sharp angles in semi-open space via a simple and highly feasible way. Moreover, it was also disclosed that the electric field can be flexibly applied to control the transformation, locomotion even anti-gravity behavior of liquid metal puddle on graphite. Such phenomena are fundamentally different from those observed before when placing liquid metal on glass substrate. Further, if the liquid metal was fed with aluminum in advance, the graphite-like substrate would induce a group of very unusual amoeba-like behaviors for such self-driven liquid metal machines. With basic science value and practical significance, these finding suggests a pivotal strategy for liquid metal patterning as well as developing future soft mobile machine owning three-dimensional locomotion capability. It also adds new knowledge for understanding the liquid metal science. This chapter presents the typical strategies and mechanisms in manipulating the liquid metal and the allied machine systems inside the electrolyte environment.

If well designed, chemicals can help in realizing very complicated liquid metal machine styles. In a large sense, such strategy resembles that of bionics and suggests an important way for the manufacture of soft robots which are not possible otherwise. Among the many different robots, the realization of serpentine locomotion has been a core goal pursued for decades among worldwide researchers. However, there still remains tough challenges in the area due to the complexity of the systems involved. Recently, a straightforward approach was discovered to generate the discretized self-growing and serpentine motion behaviors of liquid metal mollusk based on a new phenomenon observed on liquid metal (LM: Ga₆₇In₂₁Sn₁₂) immersed in specific solutions [1]. The dynamic process that liquid metal can automatically produce and move like tremendous slim snakes in acidic copper salt solution was revealed and the underlying mechanisms were clarified and interpreted. It was revealed that the self-growing serpentine locomotion of liquid metal is driven by the localized surface pressure difference related to the surface tension imbalance originating from the numerous tiny Cu–Ga galvanic couples through the electrocapillary mechanism. Particularly, the significant effect of the acids on inducing the continuous serpentine locomotion of liquid metal was disclosed and comparatively evaluated. The discretely self-growing serpentine locomotion of liquid metal induced by copper ions is very different in its dynamics and configurations from the formerly discovered integral large-scale shape transformation of the liquid metal. This chapter illustrates some basic insights and forms in developing future autonomous soft systems and bionic multifunctional robots with complicated capabilities.

Clearly, liquid metal alone cannot solve everything. However, if combining with other materials or structures, liquid metal would aid to make complicated system, which is not possible otherwise. This chapter illustrates a typical way to fabricate a kind of oscillating system made of hybrid liquid metal and rigid needle. As is known to all, oscillation is a widely seen dynamic phenomenon (Jenkins in Phys Rep 525:167–222, 2013 [1]; Cross and Hohenberg in Rev Mod Phys 65:851–1112, 1993 [2]; PikovskyjÜ and Kurths in Phys Rev Lett 78:775–778, 1997 [3]) in mechanical (Hendricks in Science 3:775–776, 1884 [4]), electrical (Tesla in Proc IEEE 87(7):1282, 1999 [5]), biological (Aschoff in Science 148:1427–1432, 1965 [6]), chemical (Petrov et al. in Nature 388:655–657, 1997 [7]) systems, etc. Oscillatory chemical reactions such as Belousov–Zhabotinsky (BZ) reaction (Petrov et al. in Nature 361:240–243, 1993 [8]), mercury beating heart (Lin et al. in Proc Natl Acad Sci U S A 71:4477–4481, 1974 [9]) are classical examples of nonequilibrium thermodynamics switching between different patterns. This lab (Yuan et al. Adv Sci 3:1600212, 2016 [10]) found the first ever oscillation phenomenon of a copper wire embraced inside the liquid metal machine via chemical and mechanical coupling. Previously, it was revealed that gallium-based liquid metal owns rather important value to serve as shape transformable material (Zhang et al. in Adv Mater 27:2648–2655, 2015 [11]; Sheng et al. Adv Mater 26:5889–5889, 2014 [12]) due to its unique property of high electrical conductivity, excellent fluidity, and low melting point. In addition, feeding the liquid metal with aluminum would lead to self-powered motors which could keep long-term actuation performance in alkaline solution (Zhang et al. in Adv Mater 27:2648–2655, 2015 [11]) due to surface tension gradient and H₂ propulsion mechanism. The study discovered even more unusual effects that apart from self-actuation, such liquid metal machine would trigger a copper wire to reciprocally move back and forth across the liquid metal body. When contacting a copper wire to the liquid metal motor, it will be wetted and swallowed and then oscillates horizontally like a violin bow at the frequency of about 1.2 Hz. Moreover, the oscillation could be easily regulated and speeded up by touching a steel needle on the liquid metal motor surface. This fundamental phenomenon can be explained by the wetting behavior difference due to chemical reaction. Given appropriate designing, such autonomous oscillator composed of hybrid solid and liquid metal structures can be developed as a core switch element in periodically regulating devices to realize various particular fluidic, electrical, mechanical, and optical functions. The present finding refreshes the basic understanding of the soft machines commonly conceived in textbook as well as add new knowledge to the wetting science (Gennes et al. in Phys Today 57:66–67, 2004 [13]). It also opens a basic way to fabricate self-powered wire oscillator using liquid metal as the main machine body. Further, through combining with other external material, substrate, and environmental factors, more hybrid liquid metal machines can still be made. Some of such typical strategies will be discussed thereafter.

Liquid metal as smart soft material can serve as powerful core elements in innovating unconventional dynamic system. For illustrating purpose, this chapter presents a basic strategy to develop the liquid metal driven vehicle and the related soft machines it may involve. Miniaturized vehicles are witnessing an increasing demand in many areas such as lab-on-chip, flexible fabrication, microfluidics, and small object manipulation. Lots of efforts had therefore been made to build a small-scale, controllable, robust, and adaptable carrying vehicle. To explore an alternative way, this lab (Yao and Liu in RSC Adv 6:56482–56488 [1]) demonstrated a new conceptual vehicle driven by liquid metal droplet “wheels” with geometric size in millimeter scale. Unlike former trials, this vehicle is a movable structure composed of soft wheels and rigid body. Such a hybrid construction could adapt to multiple electrolytes especially NaOH solution. Under variable conditions of electrical voltages and channels, the vehicle can be controlled precisely to achieve progressing, steering, and more complex locomotion. With a boat-like core body, the vehicle can take burdens up to 0.4 g at a speed of about 25 mm/s. More sophisticated vehicles with integrated manipulators and power supply could still be built based on such attempt. This kind of vehicle design realizes complex and accurate controlling as well as driving of miniaturized robotic structure. The finding may shed light for the construction of further complex miniaturized machine or robot in the coming time.