Chapter 2 Basic Properties of Liquid Metal and Soft Matter Abstract 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 non- toxicity. 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, actu- ators 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. Keywords Soft matter · Composite · Physical property · Chemical property Hydrodynamics 2.1 The Room Temperature Liquid Metals Liquid metal discussed here refer to those metal elements or metal alloy, which main- tain in liquid state at, or near, normal room temperature. Its melting temperature is generally lower than 100 °C, which means that the solid–liquid state transition is very facile and fast. There are four nonradioactive metal elements, mercury (−38.83 °C), caesium (28.65 °C), gallium (29.76 °C), and rubidium (38.89 °C), keep liquid state at near room temperature [1, 2]. Among these metal elements, gallium is the only one that is neither highly toxic (like mercury) nor highly reactive (such as rubidium and caesium), which is important for the practical applications, respectively, for making soft robotics in the biomedical engineering area. The most liquid metal with low melting temperature is in the form of alloy, which mainly includes gallium-based alloy and bismuth-based alloy. Gallium combined with other elemental metal such as indium, tin, and zinc could form a solid solution alloy with a lower melting tem- perature around even zero degree. The fusing temperature of most bismuth-based alloy arranges from 40 to 273 °C, which is often adopted as low-temperature weld- © Springer Nature Singapore Pte Ltd. 2019 13 J. Liu et al., Liquid Metal Soft Machines, Topics in Mining, Metallurgy and Materials Engineering, https://doi.org/10.1007/978-981-13-2709-4_2 14 2 Basic Properties of Liquid Metal and Soft Matter ing materials. Table 2.1 lists several typical gallium-based and bismuth-based alloys with a low melting temperature. The room temperature gallium-based alloy is the major liquid metal candidate for making liquid machine, which is chemically stable and nontoxic. It is noteworthy that the alloys consisted of Bi–In–Sn with eutectic melting temperature around 60 °C also presents chemically stable property and non- toxicity. This eutectic alloy could be used as stiffness regulation material through the fast solid–liquid transition. The element gallium is discovered in 1875, which is a soft, silvery blue metal at room temperature and standard pressure. Generally, the actual crystallization temper- ature T n of a kind of metal is lower than the equilibrium crystallization temperature T s. Such difference, i.e., T T s − T n, is called degree of subcooling. The room temperature liquid metal usually has a high degree of subcooling. For example, gal- lium can be kept in a liquid state at a temperature much lower than room temperature. Related theory and experiments show that one can obtain a higher degree of subcool- ing by dispersing bulk samples into small droplets. Studies show that Ga remains in liquid even up to −80 °C when encapsulated in carbon nanotubes [6]. And the sub- micrometer gallium droplets can even be subcooled to 150 K when they are confined in epoxy resin [7]. The large temperature difference between the solidifying point and the melting point of Ga guarantees it a perfect coolant for convective cooling. Elemental gallium is an important component of semiconductors for application in microelectronics and thin-film photovoltaic cells, such as GaAs, GaN, InGaN, and Cu(InGa)Se2. These compounds are widely used in microwave integrated circuits, solar cells, laser diodes, and light-emitting diodes. Gallium (III) has a similar bio- logical effect like ferric salt and has been used in some medical applications such as pharmaceuticals and radiopharmaceuticals. The primary gallium typically recovered at 99.9–99.99% as a by-product of processing bauxite and zinc ores, where about 90% of current primary gallium production is extracted from bauxite during the refin- ing of alumina. China, Germany, Japan, and Ukraine were the leading producers of gallium, and China is current major producer of primary and refined gallium. In 2016, world low-grade primary gallium production was estimated to be 375 tons—a decrease of 20% from 470 tons in 2015. Low-grade primary gallium producers out- side of China most likely restricted output owing to a large surplus of primary gal- lium (MCS-2017-Gallium [8]). Approximately, 70% of the gallium consumed in the United States was contained in GaAs and GaN wafers. In fact, the alloys of K78Na22 (−11 °C) and Cs77K23 (−37.5 °C) have a much low melting temperature. These alloys own some properties (e.g., high thermal conduc- tivity) which are good for use. But their most prominent drawback is that they are chemically active due to easy loss of the outermost electrons outside the nucleus. Unlike the alkali metal alloys, Ga–In alloys are the ideal materials on account of their operational safety, as well as their satisfactory physical and chemical proper- ties. The disadvantage of this kind of materials is their market price. Both Ga and In are scarce. They are sparsely dispersed on the earth surface with a small content percentage. If certain cheap metals are added to Ga–In alloys to form new composite whose properties could fulfill people’s need, the mole fractions of Ga and In will be 2.1 The Room Temperature Liquid Metals 15 Table 2.1 Gallium-based and bismuth-based alloys [3–5] Alloy Melting Eutectic? Gallium Bismuth Tin (%) Indium (%) Lead (%) Cadmium Zinc (%) Thallium temperature (%) (%) (%) (%) (°C) GaInSnZn 7.6 Yes 61.0 – 1.0 25.0 – – 13.0 – Galinstan 10.5 Yes 67.0 – 12.5 20.5 – – – – GaIn 15.5 Yes 78.6 – – 21.4 – – – – GaSn 20.5 Yes 86.2 – 13.8 – – – – – GaZn 25.2 Yes 96.3 – – – – – 3.7 – Ga 29.76 No 100.0 – – – – – – – BiPb 41.5 Yes – 40.3 10.7 17.7 22.2 8.1 – 1.1 Cerrolow 47.2 Yes – 44.7 8.3 19.1 22.6 5.3 – – 117 Cerrolow 58 Yes – 49.0 12.0 21.0 18.0 – – – 136 InBiSn 60 Yes – 33.1 15.6 51.3 – – – – Wood’s 70 Yes – 50.0 13.3 – 26.7 10.0 – – metal BiIn 72 No – 66.3 33.7 – – – – – Cerrosafe 74 No – 42.5 11.3 – 37.7 8.5 – – BiInSn 79 Yes – 57.5 17.3 25.2 – – – – Rose’s 98 No – 50.0 25.0 – 25.0 – – – metal 16 2 Basic Properties of Liquid Metal and Soft Matter reduced. Many ways can then be figured out to modify as desired the existing liquid metal materials. Taking the multicomponent liquid metals whose natures are already familiar to us as the center, a variety of multicomponent liquid alloys can be obtained by adding other kinds of metals. Galinstan, for example, is a set of eutectic alloys mainly com- posed of Ga, In, and Sn. This kind of material is liquid at room temperature, typically melting at much lower temperature [9]. Galinstan is significantly advantageous over mercury because of its nontoxicity. It is, therefore, a promising alternative to mer- cury on many occasions. The research on experimental and application for such liquid metal has thus been made [10]. And based on such material, some thermo- dynamic parameters (electrical conductivity, viscosity, melting point, etc.) can be altered through the addition of some other nanoparticles [11] or elements such as zinc, aluminum, and so on. Full use can be made of the CALPHAD method to find out more optional liquid metal materials. Compared with the traditional alloy development method, the CAL- PHAD method can provide a clearer guideline for such selections and help avoid large-scale experiments with less promising alloys [12]. Thus, it is a powerful tool to cut down cost and time during the making of different metal alloys combined with key experiments. CALPHAD is based on the thermodynamic principle that the Gibbs free energy changes with the temperature, pressure, and compositions. In other words, the Gibbs free energy-composition diagram needs to be studied under con- stant temperature and pressure. According to the principle of minimum Gibbs free energy, to any system with a certain composition range, the phase with minimum free energy is the most stable phase. And there will be two phase coexistence regions between two stable phase regions. Therefore, the phase diagram can be obtained with the Gibbs free energy-composition diagrams of different composition ranges. If the thermodynamic model parameters of lower component (two-component and three- component) systems were known, the Gibbs free energy function of each phase of the multicomponent system can be obtained through the extrapolation method, and the phase diagram of the multicomponent system can thus be calculated. Figure 2.1 shows the phase diagrams of Ga–In–Sn [13] and Bi–In–Sn alloys [14], respectively. 2.2 The Physical and Chemical Properties of Liquid Metal Alloy Here we focused on discussing the basic properties of gallium-based liquid metal due to its great potential applications in developing soft robotics. Solid gallium has a complex crystal structure. Its stable phase under standard temperature and pressure includes orthorhombic with eight atoms in the conventional unit cell.