Preparations,Characteristics and Applications of the Functional Liquid Metal Materials

As new generation functional materials, the recently emerging low‐melting liquid metals have displayed many unconventional properties superior to traditional materials. Various methods, such as alloying, oxidizing, adding metals, or non‐metallic materials and so on, have been developed to prepare desirable functional materials based on the gallium or more other metals. These methods could not only change the form of the materials, but also endow the original liquid metals with rather diversified performances, which have further expanded the application range of the low‐melting liquid metals to meet various needs. This article aims to review and summarize on the fabrication methods, characteristics, and applications of the functional liquid metal materials. Furthermore, the future outlook in this field, including challenges, routes, and related efforts, has also been illustrated and interpreted.

     

Magnetic Shape Memory Polymers with Integrated Multifunctional Shape Manipulation

Shape-programmable soft materials that exhibit integrated multifunctional shape manipulations, including reprogrammable, untethered, fast, and reversible shape transformation and locking, are highly desirable for a plethora of applications, including soft robotics, morphing structures, and biomedical devices. Despite recent progress, it remains challenging to achieve multiple shape manipulations in one material system. Here, a novel magnetic shape memory polymer composite is reported to achieve this. The composite consists of two types of magnetic particles in an amorphous shape memory polymer matrix. The matrix softens via magnetic inductive heating of low-coercivity particles, and high-remanence particles with reprogrammable magnetization profiles drive the rapid and reversible shape change under actuation magnetic fields. Once cooled, the actuated shape can be locked. Additionally, varying the particle loadings for heating enables sequential actuation. The integrated multifunctional shape manipulations are further exploited for applications including soft magnetic grippers with large grabbing force, reconfigurable antennas, and sequential logic for computing.     

Liquid Metal–Based Soft Microfluidics

Motivated by the increasing demand of wearable and soft electronics, liquid metal (LM)‐based microfluidics has been subjected to tremendous development in the past decade, especially in electronics, robotics, and related fields, due to the unique advantages of LMs that combines the conductivity and deformability all‐in‐one. LMs can be integrated as the core component into microfluidic systems in the form of either droplets/marbles or composites embedded by polymer materials with isotropic and anisotropic distribution. The LM microfluidic systems are found to have broad applications in deformable antennas, soft diodes, biomedical sensing chips, transient circuits, mechanically adaptive materials, etc. Herein, the recent progress in the development of LM‐based microfluidics and their potential applications are summarized. The current challenges toward industrial applications and future research orientation of this field are also summarized and discussed.     

Extreme Toughening of Soft Materials with Liquid Metal

Soft and tough materials are critical for engineering applications in medical devices, stretchable and wearable electronics, and soft robotics. Toughness in synthetic materials is mostly accomplished by increasing energy dissipation near the crack tip with various energy dissipation techniques. However, bio‐materials exhibit extreme toughness by combining multi‐scale energy dissipation with the ability to deflect and blunt an advancing crack tip. Here, we demonstrate a synthetic materials architecture that also exhibits multi‐modal toughening, whereby embedding a suspension of micron sized and highly deformable liquid metal (LM) droplets inside a soft elastomer, the fracture energy dramatically increases by up to 50x (from 250 ± 50 J m^‐2 to 11,900 ± 2600 J m^‐2) over an unfilled polymer. For some LM‐embedded elastomer (LMEE) compositions, the toughness is measured to be 33,500 ± 4300 J m^‐2, which far exceeds the highest value previously reported for a soft elastic material. This extreme toughening is achieved by (i) increasing energy dissipation, (ii) adaptive crack movement, and (iii) effective elimination of the crack tip. Such properties arise from the deformability of the LM inclusions during loading, providing a new mechanism to not only prevent crack initiation, but also resist the propagation of existing tears for ultra tough, soft materials.     

Al-NaOH-Composited Liquid Metal: A Fast-Response Water-Triggered Material with Thermal and Pneumatic Properties

Water-triggered materials are receiving increasing attentions due to their diverse capabilities such as easy operation, soft actuation, low cost, environmental friendliness, and many more other advantages. However, most of such materials generally have a long reaction time and require strict preservation conditions, which limit their adaptability in practice. In this study, a novel water-triggered material based on Al-NaOH-composited eutectic gallium–indium (eGaIn) alloys was proposed and demonstrated, which is rather fast-responsive and deformable. Once water is applied, the material thus fabricated would achieve a temperature rise of 40 °C in just several seconds along with gas production, indicating its big potential to be used as a thermal and pneumatic actuator. Further, the new material’s reusability and degradation ability were also tested. Following that, a double-layer-structure smart bandage was designed, whose bulk was filled with Al-NaOH-composited eGaIn while BiInSn served as outer supporting material. According to the experiments, a sheet structure with a thickness of 2 mm would support a weight of 1.8 kg after it was subjected to a cooling process, which is much better than the weight-bearing capability of fiberglass. In addition, a prototype of a water-triggered sphere robot was also fabricated using Al-NaOH-eGaIn, which realized rolling and bouncing behaviors under specific external stimulation. These findings indicate the potential value of the present material in developing future wearable devices, soft actuators, and soft robotics.     

液态金属基油墨的印刷适性调控与可拉伸电子应用

目的 调整液态金属基油墨的印刷适性,用于可拉伸电极的印刷法构建。方法 通过超声破碎法减小液态金属微粒直径,降低其表面张力;探究聚氨酯种类和含量对液态金属微粒的分散性、油墨流变性、电极的印刷适性和可拉伸性的影响。结果 引入PU1185制备的液态金属油墨,丝印电极分辨率达58 µm;印制电极展现了良好的导电性和可拉伸性,预拉伸稳定后的电极在100%的应变下拉伸1 000次,电阻变化不超2倍。结论 液态金属基油墨能够用于高分辨电路的加工,在可拉伸电子领域具有可预见的应用前景。     

A Magnetically and Thermally Controlled Liquid Metal Variable Stiffness Material

Smart materials that can actively tune their stiffness are of great interest to many fields, including the construction industry, medical devices, industrial machines, and soft robotics. However, developing a material that can offer a large range of stiffness change and rapid tuning remains a challenge. Herein, a liquid metal variable stiffness material (LMVSM) that can actively and rapidly tune its stiffness by applying an external magnetic field or by changing the temperature is developed. The LMVSM is composed of three layers: a gallium–iron magnetorheological fluid (Ga–Fe MRF) layer for providing variable stiffness, a nickel–chromium wire layer for Joule heating, and a soft heat dissipation layer for accelerating heating and rapid cooling. The stiffness can be rapidly increased by 4 times upon the application of a magnetic field or 10 times by solidifying the Ga–Fe MRF. Finally, the LMVSM is attached to a pneumatically controlled soft robotic gripper to actively tune its load capacity, demonstrating its potential to be further developed into smart components that can be widely adopted by smart devices.     

A Highly Stretchable Liquid Metal Polymer as Reversible Transitional Insulator and Conductor

Materials with a temperature‐controlled reversible electrical transition between insulator and conductor are attracting huge attention due to their promising applications in many fields. However, most of them are intrinsically rigid and require complicated fabrication processes. Here, a highly stretchable (680% strain) liquid metal polymer composite as a reversible transitional insulator and conductor (TIC), which is accompanied with huge resistivity changes (more than 4 × 10⁹ times) reversibly through a tuning temperature in a few seconds is introduced. When frozen, the insulated TIC becomes conductive and recovers after warming. Both the phase change of the liquid metal droplets and the rigidity change of the polymer contribute directly to transition between insulator and conductor. A simplified model is established to predict the expansion and connection of liquid metal droplets. Along with high stretchability, straightforward fabrication methods, rapid triggering time, large switching ratio, good repeatability, the TIC offers tremendous possibilities for numerous applications, like stretchable switches, semiconductors, temperature sensors, and resistive random‐access memory. Accordingly, a system that can display numbers and letters via converting alternative TIC temperature to a binary signal on a computer is conceived and demonstrated. The present discovery suggests a general strategy for fabricating and stimulating a stretchable transitional insulator and conductor based on liquid metal and allied polymers.     

Stretchable and Soft Electronics using Liquid Metals

The use of liquid metals based on gallium for soft and stretchable electronics is discussed. This emerging class of electronics is motivated, in part, by the new opportunities that arise from devices that have mechanical properties similar to those encountered in the human experience, such as skin, tissue, textiles, and clothing. These types of electronics (e.g., wearable or implantable electronics, sensors for soft robotics, e‐skin) must operate during deformation. Liquid metals are compelling materials for these applications because, in principle, they are infinitely deformable while retaining metallic conductivity. Liquid metals have been used for stretchable wires and interconnects, reconfigurable antennas, soft sensors, self‐healing circuits, and conformal electrodes. In contrast to Hg, liquid metals based on gallium have low toxicity and essentially no vapor pressure and are therefore considered safe to handle. Whereas most liquids bead up to minimize surface energy, the presence of a surface oxide on these metals makes it possible to pattern them into useful shapes using a variety of techniques, including fluidic injection and 3D printing. In addition to forming excellent conductors, these metals can be used actively to form memory devices, sensors, and diodes that are completely built from soft materials. The properties of these materials, their applications within soft and stretchable electronics, and future opportunities and challenges are considered.     

液相制备纳米金属粉的粒度及形貌控制研究进展

纳米金属粉的粒度和颗粒形状决定其性能和应用,粒度和形貌的控制对纳米金属粉的制备至关重要.主要从纳米金属粉的粒度可控制备、形貌可控制备和复合结构纳米金属粉的制备3方面阐述了液相制备纳米金属粉的粒度和形貌控制研究现状;并总结了液相制备纳米金属粉的粒度和形貌控制作用机理.     

Modulation of Metal and Insulator States in 2D Ferromagnetic VS2 by van der Waals Interaction Engineering

2D transition‐metal dichalcogenides (TMDCs) are currently the key to the development of nanoelectronics. However, TMDCs are predominantly nonmagnetic, greatly hindering the advancement of their spintronic applications. Here, an experimental realization of intrinsic magnetic ordering in a pristine TMDC lattice is reported, bringing a new class of ferromagnetic semiconductors among TMDCs. Through van der Waals (vdW) interaction engineering of 2D vanadium disulfide (VS₂), dual regulation of spin properties and bandgap brings about intrinsic ferromagnetism along with a small bandgap, unravelling the decisive role of vdW gaps in determining the electronic states in 2D VS₂. An overall control of the electronic states of VS₂ is also demonstrated: bond‐enlarging triggering a metal‐to‐semiconductor electronic transition and bond‐compression inducing metallization in 2D VS₂. The pristine VS₂ lattice thus provides a new platform for precise manipulation of both charge and spin degrees of freedom in 2D TMDCs availing spintronic applications.     

Metal–Matrix Composites from High-Pressure Torsion with Functionalized Material Behavior

In composites, outstanding properties of two materials can be combined. In particular, metal–matrix composites (MMCs) can combine the properties of a high-strength ductile metallic matrix with special properties of embedded ceramic particles. This hybrid can be used to create a functional material. However, during consolidation, the thermal load of most common MMC-processing routes is an obstacle for such functionalization, because the unique properties of the ceramic phases most likely degrade. Mechanical alloying, in this case, by high-pressure torsion (HPT), can overcome this challenge. Herein, the attempt to obtain smart materials through HPT processing is aimed. For that purpose, Cu-MMCs are produced from mixed powders with ${\text{ZrO}}_2$ and ${\text{BaTiO}}_3$ (BTO) with the challenge to incorporate their functional phase. BTO can provide a sensing ability for internal stress and ${\text{ZrO}}_2$ can provide a fatigue lifetime by a retarded crack growth. The amount of the stabilized phase is evaluated by X-ray diffraction. Cu–BTO–MMCs exhibit a local piezoelectric effect when strained, shown by in situ scanning Kelvin probe force microscopy. Cu–${\text{ZrO}}_2$–MMCs feature a retarded fatigue crack initiation and an earlier crack closure during fatigue crack growth due to the volume expansion once ${\text{ZrO}}_2$ transforms.     

Light‐Induced Shape Morphing of Liquid Metal Nanodroplets Enabled by Polydopamine Coating

Shape morphing nanosystems have recently attracted much attention and a number of applications are developed, spanning from autonomous robotics to drug delivery. However, the fabrication of such nanosystems remains at an early stage owing to limited choices of strategies and materials. This work reports a facile method to fabricate liquid metal (LM) nanodroplets by sonication of bulk LM in an aqueous dopamine hydrochloride solution and their application in light‐induced shape morphing at the nanoscale. In this method, dopamine acts as a surfactant, which stabilizes the LM nanodroplets dispersion during the sonication, and results in downsizing of the nanodroplets. Furthermore, by adding 2‐amino‐2‐(hydroxymethyl)‐1,3‐propanediol to the suspension, self‐polymerization of dopamine molecules occurs, resulting in the formation of polydopamine (PDA)‐coated LM nanodroplets. Owing to the high photothermal conversion of the PDA, PDA‐coated LM nanodroplets are transformed from spherical shapes to ellipsoids by NIR laser irradiation. This study paves a simple and reliable pathway for the preparation of functional LM nanodroplets and their application as shape‐morphing nanosystems.     

Oxide‐Mediated Formation of Chemically Stable Tungsten–Liquid Metal Mixtures for Enhanced Thermal Interfaces

Modern microelectronics and emerging technologies such as wearable devices and soft robotics require conformable and thermally conductive thermal interface materials to improve their performance and longevity. Gallium‐based liquid metals (LMs) are promising candidates for these applications yet are limited by their moderate thermal conductivity, difficulty in surface‐spreading, and pump‐out issues. Incorporation of metallic particles into the LM can address these problems, but observed alloying processes shift the LM melting point and lead to undesirable formation of additional surface roughness. Here, these problems are addressed by introducing a mixture of tungsten microparticles dispersed within a LM matrix (LM‐W) that exhibits two‐ to threefold enhanced thermal conductivity (62 ± 2.28 W m^?1 K^?1 for gallium and 57 ± 2.08 W m^?1 K^?1 for EGaInSn at a 40% filler volume mixing ratio) and liquid‐to‐paste transition for better surface application. It is shown that the formation of a nanometer‐scale LM oxide in oxygen‐rich environments allows highly nonwetting tungsten particles to mix into LMs. Using in situ imaging and particle dipping experimentation within a focused ion beam and scanning electron microscopy system, the oxide‐assisted mechanism behind this wetting process is revealed. Furthermore, since tungsten does not undergo room‐temperature alloying with gallium, it is shown that LM‐W remains a chemically stable mixture.