Liquid metal

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 (non-Arrhenius) 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, 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 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 (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, this misconception was caused by the super-fast oxidation of the liquid gallium in even a trace amount of oxygen, which meant that nobody observed the true behavior of a liquid gallium on glass until research at the UCLA debunked the myth by testing Galinstan, a gallium-based alloy that is liquid at room temperature, in an oxygen-free environment.

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, switches, barometers, heat transfer 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 between coolers and processors because of its high thermal conductivity. The PlayStation 5 video game console 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, such as 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 and fluidic deformability, liquid metals are useful for creating soft actuators. 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 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 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 of a reflecting telescope.

The Spallation Neutron Source employs liquid metals as targets for generating pulsed neutron beams.