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Metals are the most important and earth-abundant materials. Ninety-one of overall one hundred eighteen elements are metals. They generally exhibit good electrical and thermal conductivity, excellent mechanical properties and unique chemical properties, which can be widely used in applications ranging from conductive wires, thermal conductors, structural frames and pipes, to coatings, medicines and catalysts. Most of the metals are in the solid state at room temperature.
Exceptions include francium (Fr), caesium (Cs), rubidium (Rb), mercury (Hg), and gallium (Ga), which can be defined as liquid metals. Their melting points are either lower than or close to room temperature, which enable them to remain in the liquid state at room temperature. This brings them additional advantages in comparison with the other metals; for example, they are fluid, stretchable and reformable at room temperature.
Unfortunately, the intrinsic radioactivity of Cs, extreme instability of Fr and Rb and toxicity of Hg limit their applications to certain specific areas. Ga, on the other hand, is a metalloid element, but it still shows metallic properties when it is in solid phase and becomes a superconductor at extremely low temperature [critical temperature T c ≈ −272.06 °C (1.09 K)]. Its high boiling point allows it to remain in the liquid phase from near room temperature to approximately 2403 °C . Its melting point, however, 29.7 °C , is slightly higher than room temperature. In order to decrease its meting point, eutectic Ga alloys have been developed. Indium was the first element that was alloyed with Ga. The melting point of EGaIn can be tuned to as low as 15 °C when it is incorporated at 14 wt%.
Recently, it was found that the Ga-based eutectic alloys, such Ga–Indium (EGaIn), Ga–Tin (EGaSn) and Ga–In–Sn (EGaInSn, Galinstan) systems show tunable melting temperatures from −19 °C to far above room temperature, depending upon their component ratio. In addition, these alloys show typical metallic properties in contrast to the metalloid nature of Ga, even in the liquid phase. Owing to these unique properties, research on Ga-based liquid metals has attracted great attention and made several significant breakthroughs very recently.
Applications
The good electrical and thermal properties and the unique mechanical, fluidic and surface properties of Ga-based liquid metal offer great potential for applications in functional electronics, flexible devices, actuators and bio-devices.
Electrical properties and applications
The resistance of Ga is higher than that of copper, but the conductivity of Ga-based liquid metal is much higher than for other liquids.
Liquid metal Antennas
Modern aircraft requires variety of antennas for radar, communications and Electronic warfare and when these are integerated on the airframe can compromise its structural integrity or increasing drag and fuel consumption. With a diverse range of missions, aircraft require the reconfiguration of antennas to perform multiple functionalities. However, the space for antennas on aircraft is limited and the integration of antennas onto the airframe can result in compromising its structural integrity or increasing drag and fuel consumption. How do we ensure the latest technologies can be used without compromising the performance of our aircraft?
Researchers are now developing liquid metal alloy based antennas are used that can be moved around to meet specific needs and is embedded in the aircraft structure, without compromising the structural properties. LM antenna is made of radiating structures created by fluidic metal alloys, which are injected into microfluidic channels built in flexible polymers. This type of antenna allows for tight physical co-design and integration with the wearable sensor devices, e.g. embedded in flexible resin based material. The liquid state allows for the antennas to be reconfigured to provide tunable frequency and directional operation and go so far as being multi-operational. These liquid metal antennas reduce the structural alteration to the craft.
In 2015, Researchers at North Carolina State University reported to have developed liquid antenna that makes use of capillary action .The antenna uses the application of positive or negative voltage across an interface between the liquid metal reservoir and an electrolyte. Applying a positive voltage causes the metal to extend out toward the electrolyte, while negative voltage causes it to contract.
This is all due to a peculiar electrical effect which changes the surface tension of the metal. The positive charge introduces an oxide to the surface of the metal which lowers surface tension and allows it to flow. The negative charge does the opposite, removing the oxide to increase the surface tension and cause the liquid to contract.
Dickey et al. reported a procedure to fabricate reversibly tunable fluidic antennas . They utilised photolithography to make a pattern for the dipole on (polydimethylsiloxane) (PDMS) layer on a silicon slide. After exposure of this substrate to oxygen, the substrate was sealed to generate micro-channel, and then liquid metal was injected into the microfluidic channels to form the antenna. The resonance of antenna is approximately 1962 MHz, and the radiation efficiency is 90% in far field measurements. (Efficiency of 100% means that the antenna has no losses.) Thus, the electrical loss of EGaIn antennas is acceptable in applications. The novel antennas provide a simple way to fabricate components, including electronic fabric
The most immediate benefit will probably be the reduction in the size of tunable antennas. The researchers speculate that for millimeter-wave frequencies, the liquid metal antennas can be integrated directly onto microchips in many devices. At the moment, the technique simply allows the metal to extend and contract on one axis. Eventually, however, the team hopes to make use of the electrical capillary action to draw the metal out into arbitrary two-dimensional surfaces to alter the shape as well as the length. This would introduce the possibility of directional liquid metal antennas
HOW IT WORKS Using gallium, and other metals, a liquid metal alloy is formed. This is then placed within the structure of the aircraft through channels confined within the skin of the airframe providing superior aerodynamic performance. The liquid metal is integrated into aerospace grade epoxy composites with additive manufacturing and is physically reconfigured to provide tunable frequency and directional operation. Because it is liquid it becomes possible to reconfigure and change the antenna for mission specific systems.
This is a revolutionary technology and a novel approach to tunable and reconfigurable radio frequency (RF) antennas. It allows for cost savings through flexible/integrated electronics, multi-functional components, and damage resistant electronics. Further, the structural integrity of the aircraft is not compromised with this advancement in technology.
IMPACT FOR THE FUTURE As the liquid metal technology advances, it will be integrated into more electronic processes and the ability to reconfigure antennas based on missions will increase. Next steps for furthering this technology include chemically altering the reactions when the liquid metal comes in contact with other metals as well as impeding the liquid from solidifying at high altitudes.
Superconductive electronics at low temperature
A superconductor is a type of material that shows zero electrical resistance and the Meissner effect, when the temperature is below a certain critical temperature (T c ). Conventional superconductors have some drawbacks, for example, they are fragile and subject to processing difficulties. The superconducting transition temperature of pure Ga is lower than −268.95 °C (4.2 K), the boiling temperature of helium. This is a barrier to practical applications of this material as a superconductor. Ren et al. found that the superconducting transition temperature for GaInSn alloy can be tuned by the component ratio. The highest T c in this family is −266.55 °C (6.6 K), which is higher than liquid helium temperature. It was found that liquid metal nanoparticles also retained the same superconductive properties as their bulk material.
These nanoparticles were dispersed into aqueous solution with organic dispersants (surfactants). A superconducting ink can be then developed by using such the dispersive nanoparticles, which can be directly used for inkjet printing. The micro-/nano-circuits have been printed by such inks, which displays mechanical flexibility and superconducting properties. This work indicates that liquid metal superconductor may be used for microscale nuclear magnetic resonance (NMR), micro-/nano-superconducting coils, flexible superconducting electronic components and other applications
Thermal properties and applications
Similar to most metallic materials, Ga and its alloys also show high thermal conductivity
Coolant
Heat dissipation management is important for cooling the compact electronic packages including central processing unit (CPU) in computers. The evolution of micro/nano-devices has significantly enhanced the capability of liquid cooling systems. Traditionally, water and aqueous solutions are utilised as coolant, but their low thermal conductivity reduces the effectiveness of heat dissipation. As an alternative, Ga-based liquid metal coolants were developed in recent years, owing to their intrinsic liquidity and high thermal conductivity.
Mechanical and fluidic properties and applications
Ga and its alloys show passivation behaviour with the formation of a surface oxide layer when exposed in air. The oxide skin (0.5–3 nm thick) arising from intrinsic passivation further promotes the chemical and mechanical stability of the liquid metals. The mechanical properties cannot be changed because the Ga-based liquid metals are in the liquid phase around room temperature
Flexible and stretchable electronics
Flexible and stretchable electronics have components and circuits that can retain their function while being deformed. Conventional flexible electronics are made of rigid materials, for example aluminium (Al) and copper (Cu). They could be rendered flexible by making them sufficiently thin [8]. The flexibility of these classical solid antennas electronics is not adequate, however, since they could be still damaged by metal fatigue and the conductivity of the electronic systems might be influenced.
Owning to the intrinsic liquid property at room temperature, Ga-based liquid metal can be reshaped easily while remains metallic function. Thus, it can be injected into stretchable channels [30]. Hayes et al. developed a flexible multi-layer antenna, which was constructed by liquid metal (EGaIn) injecting into the microstrip. This antenna can be flexed without any great change in function. It could be also used to make a conductor with excellent mechanical performance.
Even though the conductivities of Ga and its alloys are lower than for Cu, these materials are acceptable due to their additional advantages, including stretchability, flexibility and deformation. Ga-based liquid metals have also demonstrated the potential to be utilised in diffraction gratings, metamaterials, etc.
Self-healing devices
Self-healing wires can improve the durability of many electronic devices, especially for stretchable electronics. In addition to that, self-healing wires create a novel approach to the rewiring of circuits, as they offer a simple way to reconfigure micro-channels, which be set in complex shapes and systems. Traditionally, conductive polymers have been used as self-healing conductive materials, although those materials could only self-heal at high-temperature (~200 °C). The conductivity of the polymers is much worse than for metals. Ga-based Liquid metals can solve these problems. Li et al. created a light emitting diode (LED) integrated EGaInSn circuit. In this self-healing device, the liquid metal wire can be cut by shears, and then heals by itself under ambient conditions without an additional force to reconnect the broken wire.
Palleau et al. also fabricated self-healing stretchable circuits by using Ga-based liquid metal. The wires are fabricated in self-healing polymer micro-channels, which are injected with liquid metal (EGaIn). The circuits can self-heal, not only mechanically, but also electrically after being cut.
Surface tension and wettability
The oxide skin of Ga allows Ga-based liquid metal to wet other surfaces. The oxide layer shows elastic layer which can support the maximum surface stress of 0.5 – 0.6 N•m−1. When it is broken, it reforms instantaneously and rapidly under oxygenated conditions. Moreover, the surface tension of Ga is large than 400 mN•m−1, the oxide layer not only lower the surface tension of Ga-based liquid metal, but also helps to remain the shape of Ga-based liquid metal and keep stable after injection. As the surface or interface tension and wettability can be changed or controlled by electric-induced effect (including electrocapillarity, continuous electrowetting (CEW) and electrowetting on dielectric (EWOD)), the Ga-based liquid metal has been manipulated by applied the external electric field in different liquid-related system.
Actuator and pump
Actuator and Pump is a type of devices that transfer non-mechanical external energy to mechanical energy.
Gough et al. studied about the electrocapillary actuation of EGaInSn marble. The external voltage was applied across the EGaInSn–electrolyte surface directly to create interfacial tension gradient making the pressure between electrolyte and droplets imbalance. This approach can be utilised to control the liquid metal flow in complex channels.
The transformation of liquid metal marble on graphite connecting an external voltage directly with an electrode. Both of the cathode and anode are fixed and installed relatively far away from liquid metal, the marble stretched and moved toward cathode, the shape of liquid metal likes a tail. The liquid metal is in the form of droplets when no electric potential is applied. When the bias is applied, the liquid metal changes to line shapes
Zavabeti et al. showed an novel approach that only utilises the ionic imbalance of aqueous electrolyte surrounding the liquid metal. The PH and ionic concentration gradients across liquid metal are significant factors for liquid metal actuation. Therefore, ionic properties for electrolytes contain enough energy to induce the liquid metal marble movement, the marble can move without electric field. They can be utilised to fabricate future autonomous low dimensional micromechanical components which are based on the changes of compositional of electrolytes.
Transformation
As Ga-based liquid metals have mechanical stability, they can be transformed into different shapes around room temperature by utilising various applied forces. Khan et al. introduced the oxide layer of liquid metal can be formed and withdrawn by low voltages. The electrocapillarity-induced liquid metal removal from micro channels. They demonstrated that the oxide skin is an excellent surfactant for metals, and liquid metal can be removed and transformed quickly and reversibly. Based on these merits, a wide range of applications can be developed, such as microelectromechanical systems (MEMS) switches and conductive microcomponents.
Biocompatibility and its applications
The toxicity of mercury limited its clinical application, however, unlike Hg, Ga and its alloys show a wide range of benefits, such as low toxicity and biocompatibility. Compared with conventional biomaterials, liquid metal can provide novel capabilities and solutions due to its high conductivity and liquid feature.
Movement disorders mainly caused by Peripheral nerve injury (PNI) are a serious problem worldwide. Traditionally, nerve auto-grafting has been a common approach to repair nerves, but it is limited by the problems of donor grafts and matching dimensions. Recently, Ga-based liquid metal has been found to provide an effective method for the regeneration of peripheral nerve functional channels. It is simple to fabricate, and convenient in surgical operation, and can be deformed in vivo. In addition, the electrical conductivity is much higher than in non-metallic materials. Therefore, it is a relative ideally biomaterial for PNI treatment.
Drug delivery
As inorganic nanoparticles display numerous advantages in drug delivery for the treatment of diseases, several inorganic nanocarriers have been created for targeted treatment. These approaches are limited by certain factors, however, such as toxicity and lack of biodegradability. Lu et al. reported a method to use Ga-based liquid metal nanoparticles for drug delivery, owing to the low toxicity of Ga and its alloys. They use EGaIn-based drug nanocarriers that are assembled with thiolated ligands on the surfaces of these nanoparticles through ultrasonication around room temperature.
Liquid Metal Enables Switchable Mirrors
Mirrors and other reflective optical components are typically created through the use of optical coatings or polishing processes. The researchers’ approach, developed by a team led by Yuji Oki of Kysuhu University in collaboration with a team from North Carolina State University led by Michael Dickey, used an electrically driven reversible chemical reaction to create a reflective surface on the liquid metal.
Switching between the reflective and scattering states can be done with just 1.4 V, about the same voltage used to light a typical LED, and at ambient temperatures.
In the work, the researchers created a reservoir using an embedded flow channel. They then used a “push-pull method” to form optical surfaces by either pumping gallium-based liquid metal into the reservoir or sucking it out. This process was used to create convex, flat, or concave surfaces, each with different optical characteristics.
From the application of electricity, the team induced a reversible chemical reaction, which oxidizes the liquid metal in a process that changes the liquid’s volume in such a way that many small scratches on the surface are created, which causes light to scatter. When electricity is applied in the opposite direction, the liquid metal returns to its original state. The liquid metal’s surface tension removes the scratches, returning it to a clean reflective mirror state.
“Our intention was to use oxidation to change the surface tension and reinforce the surface of the liquid metal,” Oki said. “However, we found that under certain conditions the surface would spontaneously change into a scattering surface. Instead of considering this a failure, we optimized the conditions and verified the phenomenon.”
Tests showed that changing the voltage on the surface from −800 mV to +800 mV would decrease the light intensity as the surface changed from reflective to scattering. The electrochemical measurements revealed that a voltage change of 1.4 V was sufficient to create redox reactions with good reproducibility.
“We also found that under certain conditions the surface can be slightly oxidized and still maintain a smooth reflective surface,” Oki said. “By controlling this, it might be possible to create even more diverse optical surfaces using this approach that could lead to applications in advanced devices such as biochemical chips or be used to make 3D-printed optical elements.”
“In the immediate future, this technology could be used to create tools for entertainment and artistic expression that have never been available before,” Oki said. “With more development it might be possible to expand this technology into something that works much like 3D printing for producing electronically controlled optics made of liquid metals. This could allow the optics used in light-based health testing devices to be easily and inexpensively fabricated in areas of the world that lack medical laboratory facilities.”
References and Resources also include:
https://www.tandfonline.com/doi/full/10.1080/23746149.2018.1446359
https://www.photonics.com/Articles/Liquid_Metal_Enables_Switchable_Mirrors/a67085
