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 (234.3 K, −37.9 °F). Others include caesium, which has a melting point of 28.5 °C (83.3 °F), rubidium (39 °C [102 °F]), francium (estimated at 27 °C [81 °F]), and gallium (30 °C [86 °F]); bromine is also liquid at room temperature (melting at −7.2 °C, 19 °F), but it is a halogen, not a metal.
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.
Wetting to metallic and non-metallic surfaces
Once oxides have been removed from the substrate surface, most liquid metals will wet to most metallic surfaces. Specifically though, room-temperature liquid metal can be very reactive with certain metals. Liquid metal can dissolve most metals; however, at moderate temperatures, only some are slightly soluble, such as sodium, potassium, gold, magnesium, lead, nickel and mercury. 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 (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 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.
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.
Liquid metal is used extensively by overclockers and computer enthusiasts to replace the original thermal interface on CPU and/or GPU dies to improve cooling efficiency and performance.
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