Liquid Metal
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A liquid metal is a
metal A metal () is a material that, when polished or fractured, shows a lustrous appearance, and conducts electrical resistivity and conductivity, electricity and thermal conductivity, heat relatively well. These properties are all associated wit ...
or a
metal alloy An alloy is a mixture of chemical elements of which in most cases at least one is a metallic element, although it is also sometimes used for mixtures of elements; herein only metallic alloys are described. Metallic alloys often have properties ...
which is
liquid Liquid is a state of matter with a definite volume but no fixed shape. Liquids adapt to the shape of their container and are nearly incompressible, maintaining their volume even under pressure. The density of a liquid is usually close to th ...
at or near
room temperature Room temperature, colloquially, denotes the range of air temperatures most people find comfortable indoors while dressed in typical clothing. Comfortable temperatures can be extended beyond this range depending on humidity, air circulation, and ...
. The only stable liquid elemental metal at room temperature is mercury (Hg), which is molten above −38.8 °C (234.3 K, −37.9 °F). Three more stable elemental metals melt just above room temperature:
caesium Caesium (IUPAC spelling; also spelled cesium in American English) is a chemical element; it has Symbol (chemistry), symbol Cs and atomic number 55. It is a soft, silvery-golden alkali metal with a melting point of , which makes it one of only f ...
(Cs), which has a melting point of 28.5 °C (83.3 °F);
gallium Gallium is a chemical element; it has Chemical symbol, symbol Ga and atomic number 31. Discovered by the French chemist Paul-Émile Lecoq de Boisbaudran in 1875, elemental gallium is a soft, silvery metal at standard temperature and pressure. ...
(Ga) (30 °C 6 °F; and
rubidium Rubidium is a chemical element; it has Symbol (chemistry), symbol Rb and atomic number 37. It is a very soft, whitish-grey solid in the alkali metal group, similar to potassium and caesium. Rubidium is the first alkali metal in the group to have ...
(Rb) (39 °C 02 °F. The radioactive metal
francium Francium is a chemical element; it has symbol Fr and atomic number 87. It is extremely radioactive; its most stable isotope, francium-223 (originally called '' actinium K'' after the natural decay chain in which it appears), has a half-l ...
(Fr) is probably liquid close to room temperature as well. Calculations predict that the radioactive metals
copernicium Copernicium is a synthetic chemical element; it has symbol Cn and atomic number 112. Its known isotopes are extremely radioactive, and have only been created in a laboratory. The most stable known isotope, copernicium-285, has a half-life of ap ...
(Cn) and
flerovium Flerovium is a synthetic chemical element; it has symbol Fl and atomic number 114. It is an extremely radioactive, superheavy element, named after the Flerov Laboratory of Nuclear Reactions of the Joint Institute for Nuclear Research in Du ...
(Fl) should also be liquid at room temperature. Alloys can be liquid if they form a eutectic, meaning that the alloy's melting point is lower than any of the alloy's constituent metals. The standard metal for creating liquid alloys used to be mercury, but
gallium Gallium is a chemical element; it has Chemical symbol, symbol Ga and atomic number 31. Discovered by the French chemist Paul-Émile Lecoq de Boisbaudran in 1875, elemental gallium is a soft, silvery metal at standard temperature and pressure. ...
-based alloys, which are lower both in their
vapor pressure Vapor pressure or equilibrium vapor pressure is the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system. The equilibrium vapor pressure is an indicat ...
at room temperature and toxicity, are being used as a replacement in various applications.


Thermal and electrical conductivity

Alloy systems that are liquid at room
temperature Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...
have
thermal A thermal column (or thermal) is a rising mass of buoyant air, a convective current in the atmosphere, that transfers heat energy vertically. Thermals are created by the uneven heating of Earth's surface from solar radiation, and are an example ...
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 more efficiently, by 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 Viscosity is a measure of a fluid's rate-dependent drag (physics), resistance to a change in shape or to movement of its neighboring portions relative to one another. For liquids, it corresponds to the informal concept of ''thickness''; for e ...
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 In physical chemistry, the Arrhenius equation is a formula for the temperature dependence of reaction rates. The equation was proposed by Svante Arrhenius in 1889, based on the work of Dutch chemist Jacobus Henricus van 't Hoff who had noted in 188 ...
dependence, to a much steeper (non-Arrhenius) dependence such as that given empirically by the
Vogel–Fulcher–Tammann equation The Vogel–Fulcher–Tammann equation, also known as Vogel–Fulcher–Tammann–Hesse equation or Vogel–Fulcher equation (abbreviated: VFT equation), is used to describe the viscosity of liquids as a function of temperature, and especially it ...
. A physical model for the viscosity of liquid metals, which captures this great variability in terms of the underlying interatomic interactions, was also developed. 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 In condensed matter physics and crystallography, the static structure factor (or structure factor for short) is a mathematical description of how a material scatters incident radiation. The structure factor is a critical tool in the interpretation ...
of the liquid as can be determined by
neutron The neutron is a subatomic particle, symbol or , that has no electric charge, and a mass slightly greater than that of a proton. The Discovery of the neutron, neutron was discovered by James Chadwick in 1932, leading to the discovery of nucle ...
or
X-ray scattering X-ray scattering techniques are a family of analytical techniques which reveal information about the crystal structure, chemical composition, and physical properties of materials and thin films. These techniques are based on observing the scatte ...
measurements.


Wetting to metallic and non-metallic surfaces

Once
oxide An oxide () is a chemical compound containing at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion (anion bearing a net charge of −2) of oxygen, an O2− ion with oxygen in the oxidation st ...
s 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 Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engine ...
to all metals except
tungsten Tungsten (also called wolfram) is a chemical element; it has symbol W and atomic number 74. It is a metal found naturally on Earth almost exclusively in compounds with other elements. It was identified as a distinct element in 1781 and first ...
and
tantalum Tantalum is a chemical element; it has Symbol (chemistry), symbol Ta and atomic number 73. It is named after Tantalus, a figure in Greek mythology. Tantalum is a very hard, ductility, ductile, lustre (mineralogy), lustrous, blue-gray transition ...
, which have a high resistance to corrosion, more so than
niobium Niobium is a chemical element; it has chemical symbol, symbol Nb (formerly columbium, Cb) and atomic number 41. It is a light grey, crystalline, and Ductility, ductile transition metal. Pure niobium has a Mohs scale of mineral hardness, Mohs h ...
,
titanium Titanium is a chemical element; it has symbol Ti and atomic number 22. Found in nature only as an oxide, it can be reduced to produce a lustrous transition metal with a silver color, low density, and high strength, resistant to corrosion in ...
and
molybdenum Molybdenum is a chemical element; it has Symbol (chemistry), symbol Mo (from Neo-Latin ''molybdaenum'') and atomic number 42. The name derived from Ancient Greek ', meaning lead, since its ores were confused with lead ores. Molybdenum minerals hav ...
. Similar to
indium Indium is a chemical element; it has Symbol (chemistry), symbol In and atomic number 49. It is a silvery-white post-transition metal and one of the softest elements. Chemically, indium is similar to gallium and thallium, and its properties are la ...
, gallium and gallium-containing alloys have the ability to wet to many non-metallic surfaces such as
glass Glass is an amorphous (non-crystalline solid, non-crystalline) solid. Because it is often transparency and translucency, transparent and chemically inert, glass has found widespread practical, technological, and decorative use in window pane ...
and
quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...
. 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 (solid) 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 The University of California, Los Angeles (UCLA) is a public land-grant research university in Los Angeles, California, United States. Its academic roots were established in 1881 as a normal school then known as the southern branch of the C ...
debunked the above myth by testing
Galinstan Galinstan is a brand name for an alloy composed of gallium, indium, and tin which melts at and is thus liquid at room temperature. In scientific literature, galinstan is also used to denote the eutectic alloy of gallium, indium, and tin, which ...
, 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

Applications of liquid metals include
thermostats A thermostat is a regulating device component which senses the temperature of a physical system and performs actions so that the system's temperature is maintained near a desired setpoint. Thermostats are used in any device or system tha ...
,
switches In electrical engineering, a switch is an electrical component that can disconnect or connect the conducting path in an electrical circuit, interrupting the electric current or diverting it from one conductor to another. The most common type o ...
,
barometers A barometer is a scientific instrument that is used to measure air pressure in a certain environment. Pressure tendency can forecast short term changes in the weather. Many measurements of air pressure are used within surface weather analysis ...
,
heat transfer Heat transfer is a discipline of thermal engineering that concerns the generation, use, conversion, and exchange of thermal energy (heat) between physical systems. Heat transfer is classified into various mechanisms, such as thermal conduction, ...
systems, and thermal cooling and heating designs. They can also be used to conduct heat and electricity between non-metallic and metallic surfaces. Due to their free-flowing nature, another potential application is wearable and medical devices, where material deformability is important. Liquid metal is sometimes used as a
thermal interface material A thermal interface material (shortened to TIM) is any material that is inserted between two components in order to enhance the thermal coupling between them. A common use is heat dissipation, in which the TIM is inserted between a heat-producing de ...
between
coolers A cooler, portable ice chest, ice box, cool box, chilly bin (in New Zealand), or esky (Australia) is an insulated box used to keep food or drink cool. Ice cubes are most commonly placed in it to help the contents inside stay cool. Ice packs ...
and
processors Processor may refer to: Computing Hardware * Processor (computing) ** Central processing unit (CPU), the hardware within a computer that executes a program *** Microprocessor, a central processing unit contained on a single integrated circuit ( ...
because of its high thermal conductivity. The
PlayStation 5 The PlayStation 5 (PS5) is a home video game console developed by Sony Interactive Entertainment. It was announced as the successor to the PlayStation 4 in April 2019, was launched on November 12, 2020, in Australia, Japan, New Zealand, North ...
video game console A video game console is an electronic device that Input/output, outputs a video signal or image to display a video game that can typically be played with a game controller. These may be home video game console, home consoles, which are generally ...
uses liquid metal to cool components inside the console. Liquid metal cooled nuclear reactors also use them. Liquid metal can sometimes be used for biological applications, i.e., making interconnects that flex without fatigue. As Galinstan is not particularly toxic, wires made from silicone with a core of liquid metal would be ideal for intracardiac pacemakers and neural implants where delicate brain tissue cannot tolerate a conventional solid implant. In fact, a wire constructed of this material can be stretched to 3 or even 5 times its length and still conduct electricity, returning to its original size and shape with no loss. Due to their unique combination of high
surface tension Surface tension is the tendency of liquid surfaces at rest to shrink into the minimum surface area possible. Surface tension (physics), tension is what allows objects with a higher density than water such as razor blades and insects (e.g. Ge ...
and fluidic deformability, liquid metals are useful for creating soft
actuator An actuator is a machine element, component of a machine that produces force, torque, or Displacement (geometry), displacement, when an electrical, Pneumatics, pneumatic or Hydraulic fluid, hydraulic input is supplied to it in a system (called an ...
s. The force-generating mechanisms in liquid metal actuators are typically achieved by modulation of their surface tension. For instance, a liquid metal droplet can be designed to
bridge A bridge is a structure built to Span (engineering), span a physical obstacle (such as a body of water, valley, road, or railway) without blocking the path underneath. It is constructed for the purpose of providing passage over the obstacle, whi ...
two moving parts (e.g., in robotic systems) in such a way to generate contraction when the surface tension increases. The principles of muscle-like contraction in liquid metal actuators have been studied for their potential as a next-generation
artificial muscle Artificial muscles, also known as muscle-like actuators, are materials or devices that mimic natural muscle and can change their stiffness, reversibly contract, expand, or rotate within one component due to an external stimulus (such as voltage, c ...
that offers several liquid-specific advantages over other solid materials. Liquid-mirror telescopes can use liquid metals formed into a parabola through a spinning tank to serve as the
primary mirror A primary mirror (or primary) is the principal light-gathering surface (the objective) of a reflecting telescope. Description The primary mirror of a reflecting telescope is a spherical, parabolic, or hyperbolic shaped disks of polished ...
of a
reflecting telescope A reflecting telescope (also called a reflector) is a telescope that uses a single or a combination of curved mirrors that reflect light and form an image. The reflecting telescope was invented in the 17th century by Isaac Newton as an alternati ...
. The
Spallation Neutron Source The Spallation Neutron Source (SNS) is an accelerator-based neutron source facility in the U.S. that provides the most intense pulsed neutron beams in the world for scientific research and industrial development.In 2007, SNS was entered into th ...
employs liquid metals as targets for generating pulsed neutron beams.


See also

*
Electromagnetic pump An electromagnetic pump is a pump that moves liquid metal, molten salt, brine, or other electrically conductive liquid using electromagnetism. A magnetic field is set at right angles to the direction the liquid moves in, and a current is passed t ...
* NaK *
Fusible alloy A fusible alloy is a metal alloy capable of being easily fused, i.e. easily meltable, at relatively low temperatures. Fusible alloys are commonly, but not necessarily, eutectic alloys. Sometimes the term "fusible alloy" is used to describe alloy ...
*
Galinstan Galinstan is a brand name for an alloy composed of gallium, indium, and tin which melts at and is thus liquid at room temperature. In scientific literature, galinstan is also used to denote the eutectic alloy of gallium, indium, and tin, which ...


References

{{Authority control Fusible alloys Brazing and soldering Alloys Amorphous metals