Liquid metal consists of alloys with very low melting points which form a eutectic that is liquid at room temperature. The standard metal used to be mercury, but gallium-based alloys, which are lower both in their vapor pressure at room temperature and toxicity, are being used as a replacement in various applications. A few elemental metals are liquid at or near room temperature. The most well known is mercury, which is molten above −38.8 °C. Others include caesium, which has a melting point of 28.5 °C, rubidium, francium, and gallium.
Thermal and electrical conductivity
Alloy systems that are liquid at room temperature have thermal conductivity far superior to ordinary non-metallic liquids, allowing liquid metal to efficiently transfer energy from the heat source to the liquid. They also have a higher electrical conductivity that allows the liquid to be pumped by more efficient, electromagnetic pumps. This results in the use of these materials for specific heat conducting and/or dissipation applications. Another advantage of liquid alloy systems is their inherent high densities.
Viscosity
The viscosity of liquid metals can vary greatly depending on the atomic composition of the liquid, especially in the case of alloys. In particular, the temperature dependence of the viscosity of liquid metals may range from the standard Arrhenius law dependence, to a much steeper dependence such as that given empirically by the Vogel-Fulcher-Tammann equation. A physical model for the viscosity of liquid metals, which captures this great variability in terms of the underlying interatomic interactions, is provided by the Krausser-Samwer-Zaccone equation. The electrical resistance of a liquid metal can be estimated by means of the Ziman formula, which gives the resistance in terms of the static structure factor of the liquid as can be determined by neutron or X-ray scattering measurements.
Wetting to metallic and non-metallic surfaces
Once oxides have been removed from the substrate surface, most liquid metals will wet most metallic surfaces. At room-temperature liquid metals are often reactive and soluble to metallic surfaces, though some solid metals are resistant to attack by the common liquid metals. For example gallium is corrosive to all metals except tungsten and tantalum, which have a high resistance to corrosion, more so than niobium, titanium and molybdenum. Similar to indium, gallium and gallium-containing alloys have the ability to wet to many non-metallic surfaces such as glass and quartz. Gently rubbing the alloy into the surface may help induce wetting. However, this observation of "wetting by rubbing into glass surface" has created a widely spread misconception that the gallium-based liquid metals wet glass surfaces, as if the liquid breaks free of the oxide skin and wets the surface. The reality is the opposite: the oxide makes the liquid wet the glass. In more details: as the liquid is rubbed into and spread onto the glass surface, the liquid oxidizes and coats the glass with a thin layer of oxide residues, on which the liquid metal wets. In other words, what is seen is a gallium-based liquid metal wetting its solid oxide, not glass. Apparently, the above misconception was caused by the super-fast oxidation of the liquid gallium in even a trace amount of oxygen, i.e., nobody observed the true behavior of a liquid gallium on glass, until research at the UCLA debunked the above myth by testing Galinstan, a gallium-based alloy that is liquid at room temperature, in an oxygen-free environment. Note: These alloys form a thin dull looking oxide skin that is easily dispersed with mild agitation. The oxide-free surfaces are bright and lustrous.
Applications
Because of the excellent characteristics and manufacturing methods, liquid metals are often used in wearable devices, medical devices, interconnected devices and so on. Typical uses of liquid metals include thermostats, switches, barometers, heat transfer systems, and thermal cooling and heating designs. Uniquely, they can be used to conduct heat and/or electricity between non-metallic and metallic surfaces.
Thermal Interfaces
Liquid metal is used by some computer enthusiasts who use it as a thermal interface between coolers and processors or to replace the default thermal interface used inside processors and video cards as liquid metal conducts heat much better than the used defaults.
Also, liquid metal can be used for wearable devices. Emerging IoT applications require reliable and effective wireless connectivity. Therefore, it is necessary to make a small flexible antenna. In Mathieu's paper, a method for implementing a 3D flexible antenna is proposed. The method uses a Fused Deposition Modeling technique to fabricate a dielectric radome in a NinjaFlex flexible plastic. This technique can also be used to design fully flexible antennas. The main advantage of this method is that it can realize 3D structures with large thickness values. Antenna material and structural parameters like this:
the Galinstan liquid metal, which is used to realize the radiating element;
the NinjaFlex flexible plastic used to realize, through a 3D FDM printing process, the dielectric substrate encapsulating the liquid metal;
the electro-textile copper constituting the antenna ground plane.
Prospect
In 2015, Jie Zhang's team found that the liquid metal placed in the electrolyte can provide energy by “ingesting” aluminum as a food or fuel, enabling high-speed, high-efficiency long-term operation. A small piece of aluminum can drive liquid metal balls with a diameter of about 5 mm to achieve long-term operation. More than 1 hour of continuous exercise, speed up to 5 cm/s. This flexible machine can move in free space and in various structural channels; surprisingly, it can also make its own deformation adjustment according to the width of the path along the path. The dynamic mechanism comes from two aspects. According to Rebinder's effect, liquid metal alloy EGaIn or Galinstan would penetrate into aluminum, destroying the oxide skin on Al surface and leading to the activation of Al. In NaOH solution, Al is the cathode and liquid metal works as the anode. the dissolution of Al on the cathode renders Al to lose electrons, Under the equilibrium state, gallium reacts with the alkali solution slowly, producing gallates like −, which makes the surface of gallium negatively charged and cations accumulated nearby, and thus the electrically charged interface forms into a uniform diffuse electrical double layer. The electrons flowing internally from Al to the liquid metal preferentially deoxidate the oxidized gallium near the Al, which alters the distribution of the charges across the EDL, leading to the generation of a potential gradient along the liquid metal surface. According to Lippmann's equation, the alteration of the EDL induces an imbalance of the surface tension on the liquid metal. Following the Young-Laplace equation, the imbalance of the surface tension induces a pressure difference between the rear and the head, which produces a thrust to drive the droplet toward the head; at the same time, the hydrogen produced in the electrochemical reaction process also further raises the thrust. The discovery of this phenomenon laid the technical foundation for the development of intelligent motors, vascular robots, fluid pumping systems, flexible actuators and even more complex liquid metal robots.
