Liquid metal machine is evolving to soft robotics

Abstract

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 materials [1,2]. 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 [3][4][5][6][7] of liquid metals further strengthen this endeavor. Unlike former trials on pure liquid metal, the new findings [3][4][5][6] revealed that hybrid structures with both liquid and solid elements together would enable more complicated functions. As it reveals [3], when contacting a copper wire to the liquid metal fueled with aluminum, the wire will be swallowed and then moves back and forth, just like a violin bow. Such oscillating motion could be regulated and speeded up by touching a steel needle on the liquid metal surface. This autonomous oscillator can be developed as a switch element in periodically controlling fluidic, electrical, mechanical and optical functions.In fact, combining solid components with liquid metal can produce more machine roles. For example, contacting nickel particles to sub-millimeter EGaIn droplet in NaOH solution would induce intermittent jumping of the metal droplet [4]. The reason comes from the electron discharge effect due to point contact between liquid metal and solid metal particles in electrolyte. It is the short circuit effect that significantly enhances the near-surface electric field at the particle tips which then causes electric breakdown of the electrolyte and thus drastic hydrogen generation leading to strong propelling force.Alternatively, if intentionally electroplating more other materials like a magnetic Ni layer on the liquid metal, the autonomous Ni/Al/EGaIn machine fueled with aluminum could be manipulated by external magnet or electric field which can realize diverse motion control such as steering, changing direction, start, and stop [5]. Through further coating with soft biomedical hydrogel, the liquid metal motor can even be utilized to deliver drugs.Moreover, liquid metal can be integrated with solid structure to construct various machines. In this side, metal droplet was demonstrated to be used as "wheels" to drive 3D printed vehicle or boat [6]. The hybrid machine thus made adapts to multiple electrolytes especially NaOH solution. Under variable electrical voltages and configurations, the vehicle can be controlled to achieve various progressing, steering, carrying freight and more locomotion states.In earlier efforts, several pivotal findings on shape changeable liquid metal are on the electrically induced liquid metal transformers. It was disclosed that through applying electrical field on the liquid metals immersed in or sprayed with water, very unusual phenomena would occur such as transformation from ...

Characteristics for Gallium-Based Liquid Alloys of Low Melting Temperature

“…More specifically, gallium alloys are ideal base carriers for temperature-sensitive ferrofluid (magnetic fluid) and energy convection. They can also be used in various fields such as "soft robotics [3], microchannel [4], magnetorheological fluid [5], 3D printing [6], catalysis, energy storage [7], chemical sensors [8], etc." Our group studied gallium-based ferrofluid [9].…”

“…The liquid metal gallium and its liquid alloy are ideal for thermosensitive ferrofluid carrier liquids [14]. Gallium (density 5.91 g/cm 3 , thermal conductivity 40.6 W•m −1 k −1 ) has a melting point of 29.8 • C. Therefore, it has the advantage of remaining liquid when used at room temperature. It has been reported that when silicon nanoparticles are dispersed in metal gallium with a melting point of 30 • C, they remain liquid even at 0 • C due to the super-cooling effect [15].…”

Desktop-level small automatic guided vehicle driven by a liquid metal droplet

“…Gallium-based LMs, such as eutectic gallium indium alloy (EGaIn, 75.5 wt% gallium and 24.5 wt% indium) and Galinstan (68.5 wt% gallium, 21.5 wt% indium, and 10 wt% tin), are a new kind of intelligent material that combines the characteristics of metals and fluids, such as excellent thermal conductivity, 9 electrical conductivity, 10–12 fluidity, 13 biocompatibility, 14 and low toxicity. 15 The LM applications in flexible robots, 16,17 flexible electronics, 18,19 micromotors, 20,21 neural probes, 22 drug delivery, 23–25 chip cooling, 26 3D printing, 27–29 liquid metal pumps 30 and other fields 31–34 have attracted much attention. In addition, LM droplets can move under the action of ultrasound, 35 optical, 36,37 electrical, 38,39 magnetic, 40–42 and chemical mechanisms, 20 which makes LMs have great applications in microfluidics and robotics.…”

Coloration of Liquid-Metal Soft Robots: From Silver-White to Iridescent

“…Soft robotics is a nascent field that has gained increasing interest ranging from mechanics and physics to biology. − Conventional materials with rigid counterparts can hardly achieve flexibly transformation into diverse morphologies. To achieve highly deformable metals with superior thermal and electrical conductivities, low viscosity, favorable flexibility, and biocompatibility, − the use of intrinsically liquid-state metals (LMs) has been considered as a principal solution to problems encountered in soft intelligent robots. ,− In particular, the natural physicochemical properties of LMs enable the deformation of LMs in electrolytes, allowing conversion between different shape and motion modes. ,− This revolutionary discovery has changed our inherent understanding of metals and has had a huge impact in areas such as soft materials, complex fluids, and even soft robotics. An external alternating magnetic field to LMs induces various physical phenomena, such as exothermic behavior, controlled locomotion, electromagnetic levitation, and transformations of the LMs between different morphologies and configurations in a noncontact manner .…”